The number of common shares to be outstanding after this offering is based upon 68,984,192 shares outstanding as of September 14, 2011. Unless otherwise stated, the number of common shares to be outstanding after this offering does not include:

•

approximately 31.5 million common shares we expect to issue in exchange for CPILP units in connection with the Plan of Arrangement;



•

409,295 unvested notional units granted under the terms of our Long Term Incentive Plan; and



•

13,643,645 shares issuable upon conversion, redemption, purchase for cancellation or maturity of our outstanding convertible debentures.

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Summary Historical Consolidated Financial Data of Atlantic Power

              The following table presents summary consolidated financial information for Atlantic Power. The annual historical information as of December 31, 2010 and 2009 and for the years ended December 31, 2010, 2009 and 2008 has been derived from the audited consolidated financial statements appearing in Atlantic Power's Annual Report on Form 10-K for the year ended December 31, 2010, incorporated by reference into this prospectus. The annual historical information as of December 31, 2008, 2007 and 2006 and for the years ended December 31, 2007 and 2006 has been derived from historical financial statements not incorporated by reference into this prospectus. The historical information as of, and for the six-month periods ended June 30, 2011 and 2010 has been derived from the unaudited consolidated financial statements appearing in Atlantic Power's Quarterly Report on Form 10-Q for the quarter ended June 30, 2011, incorporated by reference into this prospectus. Data for all periods have been prepared under U.S. GAAP. You should read the following selected consolidated financial information together with Atlantic Power's consolidated financial statements and the notes thereto and the discussion under "Management's Discussion and Analysis of Financial Condition and Results of Operations" included as part of Atlantic Power's Annual Report on Form 10-K for the year ended December 31, 2010 and Quarterly Report on Form 10-Q for the quarter ended June 30, 2011, each of which is incorporated by reference into this prospectus. See "Where you can find more information" beginning on page 85 of this prospectus.

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Summary Historical Consolidated Financial Information of CPILP

              The following table presents summary consolidated financial information for CPILP. The selected historical financial data as of, and for the years ended, December 31, 2010, 2009 and 2008 has been derived from CPILP's audited consolidated financial statements for those periods prepared under Canadian generally accepted accounting principles and appearing elsewhere in this prospectus. Data as of, and for the years ended, December 31, 2010 and 2009 has been reconciled to U.S. generally accepted accounting principles as noted below. The selected historical financial data as of, and for the years ended, December 31, 2007 and 2006 has been derived from the audited consolidated financial statements of CPILP prepared under Canadian generally accepted accounting principles not appearing in this prospectus. The selected historical financial data as of, and for the six-month periods ended June 30, 2011 and 2010 has been derived from CPILP's unaudited consolidated financial statements for those periods prepared using International Financial Reporting Standards and appearing elsewhere in this prospectus.

              Data for all periods presented below is reported in Canadian dollars. You should read the following selected consolidated financial data together with CPILP's consolidated financial statements and the notes thereto included elsewhere in this prospectus.

              Under U.S. GAAP, the following differences are noted for the years indicated below:

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Summary Unaudited Pro Forma Condensed Combined Consolidated Financial Information

              The following table sets forth selected information about the pro forma financial condition and results of operations, including per share data, of Atlantic Power after giving effect to the completion of the Plan of Arrangement with CPILP. The table sets forth selected unaudited pro forma condensed combined consolidated statements of operations for the six months ended June 30, 2011 and the year ended December 31, 2010, as if the Plan of Arrangement had been completed on January 1, 2010, and the selected unaudited pro forma condensed combined consolidated balance sheet data as of June 30, 2011, as if the Plan of Arrangement had been completed on that date. The information presented below was derived from Atlantic Power's and CPILP's consolidated historical financial statements, and should be read in conjunction with these financial statements and the notes thereto, included elsewhere or incorporated by reference into this prospectus and the other unaudited pro forma financial data, including related notes, included elsewhere in this prospectus. CPILP's historical consolidated financial statements have been prepared in accordance with Canadian GAAP (or, in the case of the six months ended June 30, 2011, International Financial Reporting Standards) and include a discussion of the significant differences between Canadian GAAP and U.S. GAAP in Note 27 to the CPILP audited consolidated financial statements for the year ended December 31, 2010. For purposes of the unaudited pro forma condensed combined consolidated financial data, CPILP's balance sheet financial data has been translated from Canadian dollars into U.S. dollars using a C$/$ exchange rate of C$0.9643 to $1.00 and is presented in accordance with U.S. GAAP. CPILP's statement of operations financial data has been translated from Canadian dollars into U.S. dollars using an average C$/$ exchange rate of C$0.9766 to $1.00 and C$1.0295 to $1.00 for the six months ended June 30, 2011 and the year ended December 31, 2010, respectively, and is presented in accordance with U.S. GAAP.

              The unaudited pro forma financial data is based on estimates and assumptions that are preliminary and does not purport to represent the financial position or results of operations that would actually have occurred had the Plan of Arrangement been completed as of the dates or at the beginning of the periods presented or what the combined company's results will be for any future date or any future period. See the sections entitled "Cautionary statements regarding forward-looking statements" and "Risk factors."

Unaudited Pro Forma Condensed Combined Consolidated Financial Information

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RISK FACTORS

              Investing in our common shares involves a high degree of risk. In addition to other information contained in this prospectus you should carefully consider the risks described below in evaluating our company and our business before making a decision to invest in our common shares. These risks are not the only ones faced by us. Additional risks not presently known or that we currently deem immaterial could also materially and adversely affect our financial condition, results of operations, business and prospects. The trading price of our common shares could decline due to any of these risks, and you may lose all or part of your investment. This prospectus also contains forward-looking statements that involve risks and uncertainties. Actual results could differ materially from those anticipated in these forward-looking statements as a result of certain factors, including the risks faced by us described below and elsewhere in this prospectus. Please refer to the section entitled "Cautionary statements regarding forward-looking statements" in this prospectus.

Risks Related to Our Business and Our Projects

Our revenue may be reduced upon the expiration or termination of our power purchase agreements.

              Power generated by our projects, in most cases, is sold under PPAs that expire at various times. For example, the PPA at our Badger Creek project expires in 2012 and represents 23 MWs of our net generating capacity. PPAs at our Auburndale, Lake and Gregory projects expire by the end of 2013 and represent 335 MWs of our net generating capacity. The table on page 11 contains details about our projects' PPAs. In addition, these PPAs may be subject to termination in certain circumstances, including default by the project. When a PPA expires or is terminated, it is possible that the price received by the project for power under subsequent arrangements may be reduced significantly. It is possible that subsequent PPAs may not be available at prices that permit the operation of the project on a profitable basis. If this occurs, the affected project may temporarily or permanently cease operations.

Our projects depend on their electricity, thermal energy and transmission services customers.

              Each of our projects rely on one or more PPAs, steam sales agreements or other agreements with one or more utilities or other customers for a substantial portion of its revenue. The largest customers of our power generation projects, including projects recorded under the equity method of accounting, are Progress Energy Florida, Inc. ("PEF"), Tampa Electric Company ("TECO"), and Atlantic City Electric ("ACE"), which purchase approximately 37%, 14% and 10%, respectively, of the net electric generation capacity of our projects. The amount of cash available to pay dividends to shareholders is highly dependent upon customers under such agreements fulfilling their contractual obligations. There is no assurance that these customers will perform their obligations or make required payments.

Certain of our projects are exposed to fluctuations in the price of electricity.

              Those of our projects with no PPA or PPAs based on spot market pricing will be exposed to fluctuations in the wholesale price of electricity. In addition, should any of the long-term PPAs expire or terminate, the relevant project will be required to either negotiate a new PPA or sell into the electricity wholesale market, in which case the prices for electricity will depend on market conditions at the time.

              Our most significant exposure to market power prices is at the Selkirk and Chambers projects. At Chambers, the project has the right to sell a portion of the plant's output to our utility customer at spot market prices if it is economical to do so and the Chambers project shares in the profits from these sales. In addition, during periods of low spot electricity prices the customer takes less generation, which negatively affects the project's profitability. At Selkirk, approximately 23% of the capacity of the

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facility is not contracted and is sold at market prices or not sold at all if market prices do not support the profitable operation of that portion of the facility.

Our projects may not operate as planned.

              The revenue generated by our power generation projects is dependent, in whole or in part, on their availability, performance and the amount of electric energy and steam generated by them. The ability of our projects to meet availability requirements and generate the required amount of power to be sold to customers under the PPAs are primary determinants of the amount of cash that will be distributed from the projects to us, and that will in turn be available for dividends paid to our shareholders. There is a risk of equipment failure due to wear and tear, latent defect, design error or operator error, or force majeure events among other things, which could adversely affect revenues and cash flow. To the extent that our projects' equipment requires more frequent and/or longer than forecasted down times for maintenance and repair, or suffers disruptions of plant availability and power generation for other reasons, the amount of cash available for dividends may be adversely affected.

              In general, our power generation projects transmit electric power to the transmission grid for purchase under the PPAs through a single step up transformer. As a result, the transformer represents a single point of vulnerability and may exhibit no abnormal behavior in advance of a catastrophic failure that could cause a temporary shutdown of the facility until a replacement transformer can be found or manufactured.

              If the reason for a shutdown is outside of the control of the operator, a power generation project may be able to make a force majeure claim for temporary relief of its obligations under the project contracts such as the PPA, fuel supply, steam sales agreement, or otherwise mitigate impacts through business interruption insurance policies maintenance and debt service reserves. If successful, such insurance claims may prevent a default or reduce monetary losses under such contracts. However, a force majeure claim may be challenged by the contract counterparty and, to the extent the challenge is successful, the outage may still have a materially adverse effect on the project.

              We provide letters of credit under our senior credit facility for contractual credit support at some of our projects. If the projects fail to perform under the related project-level agreements, the letters of credit could be drawn and the Company would be required to reimburse our senior lenders for the amounts drawn.

Our projects depend on suppliers under fuel supply agreements and increases in fuel costs may adversely affect the profitability of the projects.

              Revenues earned by our projects may be affected by the availability, or lack of availability, of a stable supply of fuel at reasonable or predictable prices. To the extent possible, the projects attempt to match fuel cost setting mechanisms in supply agreements to energy payment formulas in the PPA. To the extent that fuel costs are not matched well to PPA energy payments, increases in fuel costs may adversely affect the profitability of the projects.

              The amount of energy generated at the projects is highly dependent on suppliers under certain fuel supply agreements fulfilling their contractual obligations. The loss of significant fuel supply agreements or an inability or failure by any supplier to meet its contractual commitments may adversely affect our results.

              Upon the expiration or termination of existing fuel supply agreements, we or our project operators will have to renegotiate these agreements or may need to source fuel from other suppliers. Our project operators may not be able to renegotiate these agreements or enter into new agreements on similar terms. Furthermore, there can be no assurance as to availability of the supply or pricing of fuel under new arrangements and it can be very difficult to accurately predict the future prices of fuel.

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For example, a portion of the required natural gas at our Auburndale project and all of the natural gas required at our Lake project is purchased at market prices, but the projects' PPAs that expire in 2013 do not effectively pass through changes in natural gas prices.

              The amount of energy generated at the projects is dependent upon the availability of natural gas, coal, oil or biomass. The long-term availability of such resources could change in the future.

Generation from windpower projects may be less than anticipated.

              We now own an interest in a windpower project, which is exposed to the risk of its wind resource having unfavorable characteristics, which could result in unfavorable financial impacts to its generation and revenues.

Our operations are subject to the provisions of various energy laws and regulations.

              Generally, in the United States, our projects are subject to regulation by the Federal Energy Regulatory Commission, or "FERC," regarding the terms and conditions of wholesale service and rates, as well as by state agencies regarding PPAs entered into by qualifying facility projects and the siting of the generation facilities. The majority of our generation is sold by qualifying facility projects under PPAs that required approval by state authorities.

              In August 2005, the Energy Policy Act of 2005 was enacted, which removed certain regulatory constraints on investment in utility power producers. The Energy Policy Act of 2005 also limited the requirement that electric utilities buy electricity from qualifying facilities to certain markets that lack competitive characteristics, potentially making it more difficult for our current and future projects to negotiate favorable PPAs with these utilities. Finally, the Energy Policy Act of 2005 amended and expanded the reach of the FERC's merger approval authority.

              If any project that is a qualifying facility were to lose its status as a qualifying facility, then such project may no longer be entitled to exemption from provisions of the Public Utility Holding Company Act of 2005 or from provisions of the Federal Power Act of 1935 and state law and regulations. Such project may be able to obtain exempt wholesale generator status to maintain its exemption from the provisions of the Public Utility Holding Company Act of 2005; however, our projects may not be able to obtain such exemptions. Loss of qualifying facility status could trigger defaults under covenants to maintain qualifying facility status in the PPAs and project-level debt agreements and if not cured within allowed cure periods, could result in termination of agreements, penalties or acceleration of indebtedness under such agreements, plus interest.

              Our projects require licenses, permits and approvals which can be in addition to any required environmental permits. No assurance can be provided that we will be able to obtain, comply with and renew, as required, all necessary licenses, permits and approvals for these facilities. If we cannot comply with and renew as required all applicable licenses, permits and approvals, our business, results of operations and financial condition could be adversely affected.

              The Energy Policy Act of 2005 provides incentives for various forms of electric generation technologies, which may subsidize our competitors. In addition, pursuant to the Energy Policy Act of 2005, the FERC selected an electric reliability organization to impose mandatory reliability rules and standards. Among other things, the FERC's rules implementing these provisions allow such reliability organizations to impose sanctions on generators that violate their new reliability rules.

              The introductions of new laws, or other future regulatory developments, may have a material adverse impact on our business, operations or financial condition.

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Future FERC rate determinations could negatively impact Path 15's cash flows.

              The stability of Path 15's cash flows will continue to be subject to the risk of the FERC's adjusting the expected formulation of revenues upon its rate review every three years. Such a rate review has commenced in February 2011. The cost-of-service methodology currently applied by the FERC is well established and transparent; however, certain inputs in the FERC's determination of rates are subject to its discretion, including its response to protests from intervenors in such rate cases, which include return on equity and the recovery of certain extraordinary expenses. Unfavorable decisions on these matters could adversely affect Path 15 and our cash flow, financial position and results of operations, and could adversely affect our cash available for dividends.

Noncompliance with federal reliability standards may subject us and our projects to penalties.

              Our operations are subject to the regulations of the North American Electric Reliability Corporation ("NERC"), a self-regulatory non-governmental organization which has statutory responsibility, granted by the FERC, to regulate bulk power system users, owners and operators. NERC groups the users, owners, and operators of the bulk power system into 15 categories, known as functional entities—e.g., Generator Owner, Generator Operator, Purchasing-Selling Entity, etc.—according to the tasks they perform. The NERC Compliance Registry lists the users, owners and operators of the bulk power system and the functional entity type(s) under which they are registered. All registered entities are responsible for complying with the applicable mandatory reliability standards and the FERC, or NERC or a regional reliability organization, with FERC's approval, may assess financial penalties or other enforcement actions against any responsible entity found to be in noncompliance. Violations may be discovered through self-certification, compliance audits, spot checking, self-reporting, compliance investigations by NERC (or a regional reliability organization) and the FERC, periodic data submittals, exception reporting, and complaints. The penalty that might be imposed for violating the requirements of the standards is a function of the Violation Risk Factor, a pre-violation assessment of the potential risk to the reliability of the bulk power system that a violation of a particular reliability standard requirement presents, and the Violation Security Level, a post-violation measurement of the degree to which a reliability standard requirement was violated. Penalties for the most severe violations can reach as high as $1 million per violation, per day, and our projects could be exposed to these penalties and other enforcement actions if violations occur.

Our projects are subject to significant environmental and other regulations.

              Our projects are subject to numerous and significant federal, state and local laws, including statutes, regulations, by-laws, guidelines, policies, directives and other requirements governing or relating to, among other things: air emissions; discharges into water; ash disposal; the storage, handling, use, disposal, transportation and distribution of dangerous goods and hazardous, residual and other regulated materials, such as chemicals; the prevention of releases of hazardous materials into the environment; the presence and remediation of hazardous materials in soil and groundwater, both on and off site; the protection of natural resources; land use and zoning matters; and workers' health and safety matters. As such, the operation of our projects carries an inherent risk of environmental, health and safety liabilities (including potential civil actions, compliance or remediation orders, fines and other penalties), and may result in the projects being involved from time to time in administrative and judicial proceedings relating to such matters.

              The Clean Air Act and related regulations and programs of the Environmental Protection Agency extensively regulate the air emissions of sulfur dioxide, nitrogen oxides, mercury and other compounds by power plants. Environmental laws and regulations have generally become more stringent over time, and this trend may continue. In particular, the Environmental Protection Agency promulgated regulations under the federal Clean Air Interstate Rule ("CAIR") requiring additional reductions in nitrogen oxides, or "NOX," and sulphur dioxide, or "SO₂," emissions, beginning in 2009

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and 2010 respectively, and also promulgated regulations requiring reductions in mercury emissions from coal-fired electric generating units, beginning in 2010 with more substantial reductions expected in 2018. Moreover, certain of the states in which we operate have promulgated air pollution control regulations which are more stringent than existing and proposed federal regulations.

              While CAIR was set aside by a court decision in 2008, that decision allowed the CAIR requirements to remain in place pending further rulemaking by the Environmental Protection Agency. On August 8, 2011, the Environmental Protection Agency promulgated the final Cross-State Air Pollution Rule (which will replace CAIR as of October 7, 2011) which requires 27 states and the District of Columbia to curb emissions of sulfur dioxide and nitrogen oxides from power plants through more aggressive state-by-state emissions limits for nitrogen oxides and sulfur dioxide. The first phase of compliance would begin on January 1, 2012 and the second (and more restrictive) phase would begin on January 1, 2014. Compliance with the new rule may have a material adverse impact on our business, operations or financial condition.

              The Environmental Protection Agency proposed new mercury emissions standards for power plants on March 16, 2011 and is expected to have new standards in place by November 2011. Meeting these new standards at our coal-fired facility may have a material adverse impact on our business, operations or financial condition.

              The Resource Conservation and Recovery Act has historically exempted fossil fuel combustion wastes from hazardous waste regulation. However, in June 2010 the Environmental Protection Agency proposed two alternative sets of regulations governing coal ash. One set of proposed regulations would designate coal ash as "special waste" and bring ash impoundments at coal-fired power plants under federal regulations governing hazardous solid waste under Subtitle C of the Resource Conservation and Recovery Act. Another set of proposed regulations would regulate coal ash as a non-hazardous solid waste. In March 2011, the Environmental Protection Agency announced that, due to the volume of public comments received on the two alternative sets of proposed coal ash regulations, it would not issue final regulations governing coal ash in 2011. If the Environmental Protection Agency ultimately decides to regulate coal ash as a hazardous waste, our coal-fired facility may be subject to increased compliance obligations and costs that may have a material adverse impact on our business, operations or financial condition.

              Significant expenditures may be required for either capital improvements or the purchase of allowances under any or all of these programs to keep the projects compliant with environmental laws and regulations. The projects' PPAs do not allow for the pass through of emissions allowance or emission reduction capital expenditure costs, with the exception of Pasco. If it is not economical to make those expenditures it may be necessary to retire or mothball facilities, or restrict or modify our operations to comply with more stringent standards.

              Our projects have obtained environmental permits and other approvals that are required for their operations. Compliance with applicable environmental laws, regulations, permits and approvals and material future changes to them could materially impact our businesses. Although we believe the operations of the projects are currently in material compliance with applicable environmental laws, licenses, permits and other authorizations required for the operation of the projects and although there are environmental monitoring and reporting systems in place with respect to all the projects, there is no guarantee that more stringent laws will not be imposed, that there will not be more stringent enforcement of applicable laws or that such systems may not fail, which may result in material expenditures. Failure by the projects to comply with any environmental, health or safety requirements, or increases in the cost of such compliance, including as a result of unanticipated liabilities or expenditures for investigation, assessment, remediation or prevention, could result in additional expense, capital expenditures, restrictions and delays in the projects' activities, the extent of which cannot be predicted.

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Our projects are subject to regulation of CO₂ and other greenhouse gases.

              Ongoing public concerns about emissions of CO₂ and other greenhouse gases have resulted in the enactment of, and proposals for, laws and regulations at the federal, state and regional levels, some of which do or could apply to some of our project operations. For example, the multi-state CO₂ cap-and-trade program known as the Regional Greenhouse Gas Initiative applies to our fossil fuel facilities in the Northeast region. The Regional Greenhouse Gas Initiative program went into effect on January 1, 2009. CO₂ allowances are auctioned quarterly, with a clearing price for the June 2011 auction of $1.89/ton, and can be traded within a secondary market at a similar price in 2011. The State of Florida has conducted stakeholder meetings and workshops as part of the process of developing greenhouse gas emissions regulations, the most recent of which was in 2009. In October 2010, the Florida DEP issued its "Comparative Study of Selected Offset Protocols for Greenhouse Gas Reduction and Reporting Programs." These regulatory efforts could result in stringent limits on CO₂ emissions in that state.

              California, New Mexico, Washington and other states and Canadian Provinces are part of the Western Climate Initiative, which is developing a regional cap-and-trade program to reduce greenhouse gas emissions in the region to 15% below 2005 levels by 2020.

              In 2006, the State of California passed legislation initiating two programs to control/reduce the creation of greenhouse gases. The two laws, more commonly known as AB 32 and SB 1368, are currently in the regulatory rulemaking phase which will involve public comment and negotiations over specific provisions. Development towards the implementation of these programs continues.

              Under AB 32 (the California Global Warming Solutions Act of 2006) the California Air Resources Board ("CARB") is required to adopt a greenhouse gas emissions cap on all major sources (not limited to the electric sector). In order to do so, it must adopt regulations for the mandatory reporting and verification of greenhouse gas emissions and to reduce state-wide emissions of greenhouse gases to 1990 levels by 2020. This will most likely require that electric generating facilities reduce their emissions of greenhouse gases or pay for the right to emit by the implementation date of January 1, 2012. The program has yet to be finalized and the decision as to whether allocations will be distributed or auctioned will be determined in the rulemaking process that is currently underway. Discussion to date favors an auction-based allocation program.

              The legality of AB 32 has been challenged by several parties. On January 21, 2011, the San Francisco Superior Court issued a proposed decision that could significantly delay the implementation of AB 32. In Association of Irritated Residents, et al. v. California Air Resources Board, Case No. CPF-09-509562, the Court held that the CARB failed to comply with the California Environmental Quality Act. The Court found the CARB to have neglected to conduct a sufficient environmental impact review prior to adopting the AB 32 Scoping Plan. Specifically, CARB failed to adequately analyze all potential alternatives and prematurely adopted the Scoping Plan prior to fully responding to public comment. CARB has appealed that decision but, in the interim, has revised its alternatives analysis on which public comment was accepted until July 28, 2011.

              SB 1368 required the California Energy Commission, in consultation with the California Public Utilities Commission (the "CPUC") and the CARB to establish greenhouse gas emission performance standards and implement regulations for power purchase agreements that exceed five years entered into prospectively by publicly-owned electric utilities. The regulations have been finalized and establish a performance standard for greenhouse gas emissions from power plants not exceeding the rate of greenhouse gas emitted per megawatt-hour associated with combined-cycle, gas turbine baseload generation, such as our Badger Creek project.

              In addition to the regional initiatives, legislation for the reduction of greenhouse gases has been introduced at the federal level and if passed, may eventually override the regional efforts with a

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national cap and trade program. Federal bills to create both a cap-and-trade allowance system and a renewable/efficiency portfolio standard have been introduced in both the House and Senate. Separately, the Environmental Protection Agency has taken several recent actions proposing possible regulation of greenhouse gas emissions.

              The Environmental Protection Agency's actions include its finding of "endangerment" to public health and welfare from greenhouse gases, its issuance in September 2009 of the Final Mandatory Reporting of Greenhouse Gases Rule which requires large sources, including power plants, to monitor and report greenhouse gas emissions to the Environmental Protection Agency annually starting in 2011, and its publication in May 2010 of its final Prevention of Significant Deterioration and Title V Greenhouse Gas Tailoring Rule, effective January 2, 2011, which requires large industrial facilities, including power plants, to obtain permits to emit, and to use best available control technology to curb emissions of, greenhouse gases.

              The implementation of existing CO₂ and other greenhouse gas legislation or regulation, the introduction of new regulation, or other future regulatory developments may subject the Company to increased compliance obligations and costs that could have a material adverse impact on our business, operations or financial condition.

              All of our generating facilities met the March 31, 2011 requirement to submit 40 CFR Part 98 Mandatory Greenhouse Gas reporting for the emission of eligible site generated greenhouse gases in 2010.

Increasing competition could adversely affect our performance and the performance of our projects.

              The power generation industry is characterized by intense competition, and our projects encounter competition from utilities, industrial companies and other independent power producers, in particular with respect to uncontracted output. In recent years, there has been increasing competition among generators for power sales agreements, and this has contributed to a reduction in electricity prices in certain markets where supply has surpassed demand plus appropriate reserve margins. In addition, many states have implemented or are considering regulatory initiatives designed to increase competition in the U.S. power industry. Increasing competition among participants in the power generation industry may adversely affect our performance and the performance of our projects.

We have limited control over management decisions at certain projects.

              In a number of cases, our projects are not wholly-owned by us or we have contracted for their operations and maintenance, and in some cases we have limited control over the operation of the projects. Although we generally prefer to acquire projects where we have control, we may make acquisitions in non-control situations to the extent that we consider it advantageous to do so and consistent with regulatory requirements and restrictions, including the Investment Company Act of 1940. Third-party operators (such as Caithness, PPMS and Western) operate many of the projects. As such, we must rely on the technical and management expertise of these third-party operators, although typically we are represented on a management or operating committee if we do not own 100% of a project. To the extent that such third-party operators do not fulfill their obligations to manage the operations of the projects or are not effective in doing so, the amount of cash available to pay dividends may be adversely affected.

We may face significant competition for acquisitions and may not successfully integrate acquisitions.

              Our business plan includes growth through identifying suitable acquisition opportunities, pursuing such opportunities, consummating acquisitions and effectively integrating them with our business. We may be unable to identify attractive acquisition candidates in the power industry in the future, and we may not be able to make acquisitions on an accretive basis or be sure that acquisitions

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will be successfully integrated into our existing operations, any of which could negatively impact our ability to continue paying dividends in the future at current rates.

              Although electricity demand is expected to grow, creating the need for more generation, and the U.S. power industry is continuing to undergo consolidation and may offer attractive acquisition opportunities, we are likely to confront significant competition for those opportunities and, to the extent that any opportunities are identified, we may be unable to effect acquisitions or investments.

              Any acquisition or investment may involve potential risks, including an increase in indebtedness, the inability to successfully integrate operations, the potential disruption of our ongoing business, the diversion of management's attention from other business concerns and the possibility that we pay more than the acquired company or interest is worth. There may also be liabilities that we fail to discover, or are unable to discover, in our due diligence prior to the consummation of an acquisition, and we may not be indemnified for some or all these liabilities. In addition, our funding requirements associated with acquisitions and integration costs may reduce the funds available to us to make dividend payments.

Insurance may not be sufficient to cover all losses.

              Our business involves significant operating hazards related to the generation of electricity. While we believe that the projects' insurance coverage addresses all material insurable risks, provides coverage that is similar to what would be maintained by a prudent owner/operator of similar facilities, and are subject to deductibles, limits and exclusions which are customary or reasonable given the cost of procuring insurance, current operating conditions and insurance market conditions, there can be no assurance that such insurance will continue to be offered on an economically feasible basis, nor that all events that could give rise to a loss or liability are insurable, nor that the amounts of insurance will at all times be sufficient to cover each and every loss or claim that may occur involving our assets or operations of our projects. Any losses in excess of those covered by insurance, which may include a significant judgment against any project or project operator, the loss of a significant permit or other approval or the imposition of a significant fine or penalty, could have a material adverse effect on our business, financial condition and future prospects and could adversely affect dividends to our shareholders.

Financing arrangements could negatively impact our business.

              Our current or future borrowings could increase the level of financial risk to us and, to the extent that the interest rates are not fixed and rise, or that borrowings are refinanced at higher rates, then cash available for dividends could be adversely affected. As of June 30, 2011, we had no borrowings outstanding under our revolving credit facility, $209.7 million of outstanding convertible debentures, and $285.1 million of outstanding non-recourse project-level debt. Covenants in these borrowings may also adversely affect cash available for dividends. In addition, most of the projects currently have non-recourse term loans or other financing arrangements in place with various lenders. These financing arrangements are typically secured by all of the project assets and contracts as well as our equity interests in the project. The terms of these financing arrangements generally impose many covenants and obligations on the part of the borrower. For example, some agreements contain requirements to maintain specified debt service coverage ratios before cash may be distributed from the relevant project to us, which is currently the case with a number of our projects. In many cases, a default by any party under key project agreements (such as a PPA or a fuel supply agreement) will also constitute a default under the project's term loan or other financing arrangement. Failure to comply with the terms of these term loans or other financing arrangements, or events of default thereunder, may prevent cash distributions by the project to us and may entitle the lenders to demand repayment and/or enforce their security interests.

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              Our failure to refinance or repay any indebtedness when due could constitute a default under such indebtedness. Under such circumstances, it is expected that dividends to our shareholders would not be permitted until such indebtedness was refinanced or repaid and we may be required to sell assets or take other actions, including the initiation of bankruptcy proceedings or the commencement of an out-of-court debt restructuring.

Our equity interests in our projects may be subject to transfer restrictions.

              The partnership or other agreements governing some of the projects may limit a partner's ability to sell its interest. Specifically, these agreements may prohibit any sale, pledge, transfer, assignment or other conveyance of the interest in a project without the consent of the other partners. In some cases, other partners may have rights of first offer or rights of first refusal in the event of a proposed sale or transfer of our interest. These restrictions may limit or prevent us from managing our interests in the projects in the manner we see fit, and may have an adverse effect on our ability to sell our interests in these projects at the prices we desire.

The projects are exposed to risks inherent in the use of derivative instruments.

              We and the projects may use derivative instruments, including futures, forwards, options and swaps, to manage commodity and financial market risks. In the future, the project operators could recognize financial losses on these arrangements as a result of volatility in the market values of the underlying commodities or if a counterparty fails to perform under a contract. If actively quoted market prices and pricing information from external sources are not available, the valuation of these contracts would involve judgment or use of estimates. As a result, changes in the underlying assumptions or use of alternative valuation methods could affect the reported fair value of these contracts.

              Most of these contracts are recorded at fair value with changes in fair value recorded currently in earnings, resulting in significant volatility in our income (as calculated in accordance with U.S. GAAP) that does not significantly affect current period cash flows or the underlying risk management purpose of the derivative instruments. As a result, we may be unable to accurately predict the impact that our risk management decisions may have on our quarterly and annual income (as calculated in accordance with U.S. GAAP).

              If the values of these financial contracts change in a manner that we do not anticipate, or if a counterparty fails to perform under a contract, it could harm our financial condition, results of operations and cash flows. We have executed natural gas swaps to reduce our risks to changes in the market price of natural gas, which is the fuel consumed at many of our projects. Due to declining natural gas prices, we have incurred losses on these natural gas swaps. We execute these swaps only for the purpose of managing risks and not for speculative trading.

Our Piedmont project is subject to construction risk.

              The Piedmont project commenced construction in November 2010 and is expected to be completed in late 2012. In any construction project, there is a risk that circumstances occur which prevent the timely completion of a project, cause construction costs to exceed the level budgeted, or result in operating performance standards not being met.

              In the event a power project does not achieve commercial operation by its expected date, the project may be subject to increased construction costs associated with the continuing accrual of interest on the project's construction loan, which customarily matures at the start of commercial operation and converts to a term loan. A delay in completion of construction may also impact a project under its PPA which may include penalty provisions for a delay in commercial operation date or in situations of extreme delay, termination of the PPA.

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              Construction cost overruns which exceed the project's construction contingency amount may require that the project owner infuse additional funds in order to complete construction.

              At the completion of construction, the power project may not meet its expected operating performance levels. Adverse circumstances may impact the design, construction, and commissioning of the project that could result in reduced output, increased heat rate or excessive air emissions.

Risks Related to Our Structure

We are dependent on our projects for virtually all cash available for dividends.

              We are dependent on the operations and assets of the projects through our indirect ownership of interests in the projects. The actual amount of cash available for dividends to our shareholders depends upon numerous factors, including profitability, changes in revenues, fluctuations in working capital, availability under existing credit facilities, capital expenditure levels, applicable laws, compliance with contracts and contractual restrictive covenants contained in any debt documentation.

Distribution of available cash may restrict our potential growth.

              A payout of a significant portion of substantially all of our operating cash flow will make additional capital and operating expenditures dependent on increased cash flow or additional financing in the future. Lack of these funds could limit our future growth and cash flow. In addition, we may be precluded from pursuing otherwise attractive acquisitions or investments if the projected short-term cash flow from the acquisition or investment are not adequate to service the capital raised to fund the acquisition or investment.

Future dividends are not guaranteed.

              Dividends to shareholders are paid at the discretion of our board of directors. Future dividends, if any, will depend on, among other things, the results of operations, working capital requirements, financial condition, restrictive covenants, business opportunities, provisions of applicable law and other factors that our board of directors may deem relevant. Our board of directors may decrease the level of or entirely discontinue payment of dividends.

Exchange rate fluctuations may impact the amount of cash available for dividends.

              Our payments to shareholders and convertible debenture holders are denominated in Canadian dollars. Conversely, all of our projects' revenues and expenses are denominated in U.S. dollars. As a result, we are exposed to currency exchange rate risks. Despite our hedges against this risk through 2013, any arrangements to mitigate this exchange rate risk may not be sufficient to fully protect against this risk. If hedging transactions do not fully protect against this risk, changes in the currency exchange rate between U.S. and Canadian dollars could adversely affect our cash available for distribution.

Our indebtedness could negatively impact our business and our projects.

              The degree to which we are leveraged on a consolidated basis could increase and have important consequences for our shareholders, including:

•

our ability in the future to obtain additional financing for working capital, capital expenditures, acquisitions or other purposes may be limited;



•

our ability to refinance indebtedness on terms acceptable to us or at all; and



•

our ability to react to competitive pressures.

              As of June 30, 2011, our consolidated long-term debt and our share of the debt of our unconsolidated affiliates represented approximately 53.4% of our total capitalization, comprised of debt and balance sheet equity.

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Changes in our creditworthiness may affect the value of our common shares.

              Changes to our perceived creditworthiness may affect the market price or value and the liquidity of our common shares. The interest rate we pay on our credit facility may increase if certain credit ratios deteriorate.

Future issuances of our common shares could result in dilution.

              Our articles of incorporation authorize the issuance of an unlimited number of common shares for such consideration and on such terms and conditions as are established by our board of directors without the approval of any of our shareholders. We may issue additional common shares in connection with a future financing or acquisition. The issuance of additional common shares may dilute an investor's investment in us and reduce cash available for distribution per common share.

Investment eligibility.

              There can be no assurance that our common shares will continue to be qualified investments under relevant Canadian tax laws for trusts governed by registered retirement savings plans, registered retirement income funds, deferred profit sharing plans, registered education savings plans, registered disability savings plans and tax-free savings accounts.

We are subject to Canadian tax.

              As a Canadian corporation, we are generally subject to Canadian federal, provincial and other taxes, and dividends that we pay are generally subject to Canadian withholding tax if paid to a shareholder that is not a resident of Canada. In connection with our conversion from an IPS structure to a traditional common share structure in 2009 and the related reorganization of our organizational structure, we received a note from our primary U.S. holding company (the "Intercompany Note"). We are required to include in computing our taxable income interest on the Intercompany Note. We expect that our existing tax attributes initially will be available to offset this income inclusion such that it will not result in an immediate material increase to our liability for Canadian taxes. However, once we fully utilize our existing tax attributes (or if, for any reason, these attributes were not available), our Canadian tax liability would materially increase. Although we intend to explore potential opportunities in the future to preserve the tax efficiency of our structure, no assurances can be given that our Canadian tax liability will not materially increase at that time.

Our prior and current structure may be subject to additional U.S. federal income tax liability.

              Under our prior IPS structure, we treated the subordinated note component of each IPS as debt for U.S. federal income tax purposes. Accordingly, we deducted the interest payments on the subordinated notes and reduced our net taxable income treated as "effectively connected income" for U.S. federal income tax purposes. Under our current structure, our subsidiaries that are incorporated in the United States are subject to U.S. federal income tax on their income at regular corporate rates (currently as high as 35%, plus state and local taxes), and our primary U.S. holding company will claim interest deductions with respect to the Intercompany Note in computing its income for U.S. federal income tax purposes. To the extent this interest expense is disallowed or is otherwise not deductible, the U.S. federal income tax liability of our primary U.S. holding company will increase, which could materially affect our after-tax cash available for distribution. While we received advice from our U.S. tax counsel, based on certain representations by us and our primary U.S. holding company and determinations made by our independent advisors, as applicable, that the subordinated notes and the Intercompany Note should be treated as debt for U.S. federal income tax purposes, it is possible that the Internal Revenue Service ("IRS") could successfully challenge those positions and assert that the subordinated notes or the Intercompany Note should be treated as equity rather than debt for U.S. federal income tax purposes. In this case, the otherwise deductible interest on the subordinated notes or the Intercompany Note would be treated as non-deductible distributions, and interest payments on

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the subordinated notes would be subject to branch profits tax to the extent we had effectively connected earnings and profits, and interest payments on the Intercompany Note would be subject to U.S. withholding tax to the extent that our primary U.S. holding company had current or accumulated earnings and profits. The determination of whether the subordinated notes and the Intercompany Note are debt or equity for U.S. federal income tax purposes is based on an analysis of the facts and circumstances. There is no clear statutory definition of debt for U.S. federal income tax purposes, and its characterization is governed by principles developed in case law, which analyzes numerous factors that are intended to identify the nature of the purported creditor's interest in the borrower.

              Furthermore, not all courts have applied this analysis in the same manner, and some courts have placed more emphasis on certain factors than other courts have. To the extent it were ultimately determined that the subordinated notes or the Intercompany Note were not debt, our U.S. federal income tax liability for the applicable open tax years would materially increase, which could materially affect our after-tax cash available for distribution. Alternatively, the IRS could argue that the interest on the subordinated notes or the Intercompany Note exceeded or exceeds an arm's length rate, in which case only the portion of the interest expense that does not exceed an arm's length rate may be deductible and, the remainder would be treated as an equity distribution and potentially subject to withholding tax as described above. We have received advice from independent advisors that the interest rates on the subordinated notes and the Intercompany Note were, when issued, commercially reasonable under the circumstances, but the advice is not binding on the IRS.

              Furthermore, pursuant to the U.S. "earnings stripping" limitations, our primary U.S. holding company's deductions attributable to the interest expense on the Intercompany Note may be limited by the amount by which its net interest expense (the interest paid by the U.S. holding company on all debt, including the Intercompany Note, less its interest income) exceeds 50% of its adjusted taxable income (generally, U.S. federal taxable income before net interest expense, net operating loss carryovers, depreciation and amortization). Any disallowed interest expense may currently be carried forward to future years. Moreover, proposed legislation has been introduced, though not enacted, several times in recent years that would further limit the 50% of adjusted taxable income cap described above to 25% of adjusted taxable income. Furthermore, if our primary U.S. holding company does not make regular interest payments as required under the Intercompany Note, other limitations on the deductibility of interest under U.S. federal income tax laws could apply to defer and/or eliminate all or a portion of the interest deduction that the U.S. holding company would otherwise be entitled to with respect to the Intercompany Note.

Passive foreign investment company treatment.

              We do not believe that we are a passive foreign investment company, and we do not expect to become a passive foreign investment company. However, if we were a passive foreign investment company while a taxable U.S. holder held common shares, such U.S. holder could be subject to an interest charge on any deferred taxation and the treatment of gain upon the sale of our stock as ordinary income.

Risks Related to the Plan of Arrangement

There can be no assurance that the Plan of Arrangement will be completed and this offering is not conditioned upon the completion of the Plan of Arrangement.

              The Arrangement Agreement provides that we will acquire, directly or indirectly, all of the issued and outstanding CPILP units pursuant to a Plan of Arrangement under the Canada Business Corporations Act. Completion of the Plan of Arrangement is conditioned upon the receipt of certain governmental authorizations, consents, orders or other approvals, including but not limited to approval under the Investment Canada Act, the Competition Act (Canada), the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended (United States) and the United States Federal Power Act. The Plan of Arrangement must also be approved by the Court of Queen's Bench of Alberta and both our

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shareholders and CPILP's unitholders. No assurance can be given that the required approvals will be obtained, and, even if such approvals are obtained, no assurance can be given as to the terms, conditions and timing of the approvals or that they will satisfy the terms of the Arrangement Agreement.

              This offering is not conditioned on the completion of the Plan of Arrangement and there can be no assurance that the Plan of Arrangement will be completed. The common shares offered hereby will remain outstanding whether or not the Plan of Arrangement is completed.

If the financing for the transactions contemplated by the Arrangement Agreement becomes unavailable, the Plan of Arrangement may not be completed.

              We intend to use the net proceeds from this offering to finance a portion of the cash purchase price necessary to complete the Plan of Arrangement. We plan to fund the remainder of the cash consideration payable by us under the Plan of Arrangement, including related fees and expenses, with the net proceeds from a senior unsecured notes offering and/or drawings under a $625 million senior secured credit facility, for which we have received the written commitment of a Canadian chartered bank and another financial institution. However, funding under this facility is subject to certain conditions, including, without limitation, that there shall not have occurred a material adverse effect with respect to Atlantic Power, CPILP, CPI Income Services Ltd. and CPI Investments Inc. taken as a whole. In the event that the lenders under the facility fail to provide funding for this or any other reason, we may not be able to complete the Plan of Arrangement and may be subject to a termination fee of C$35.0 million.

If the Arrangement Agreement is terminated, we may be required to pay a termination fee or make an expense reimbursement payment.

              The Arrangement Agreement provides that we will pay a C$35.0 million termination fee to CPILP if CPILP terminates the Arrangement Agreement on account of certain actions or events, including the failure of our board of directors to maintain its favorable recommendation for the issuance of Atlantic Power common shares in connection with the Plan of Arrangement, failure of financing or a breach of our obligations under the Arrangement Agreement. If the Arrangement Agreement is terminated under circumstances where no termination fee is payable, we may still be required to make an expense reimbursement payment to CPILP, up to a maximum of C$8.0 million, in certain circumstances, including, but not limited to, if our shareholders do not approve issuance of our common shares in exchange for CPILP units pursuant to the Plan of Arrangement.

Failure to complete the Plan of Arrangement could negatively impact our share price and our future business and financial results.

              If the Plan of Arrangement is not completed for any reason, our ongoing business may be adversely affected and we will be subject to a number of risks, including the following:

•

we may be required, under certain circumstances, to pay a termination fee in the amount of C$35.0 million to CPILP or make a reimbursement of expenses payment to CPILP of up to C$8 million in connection with a termination of the Arrangement Agreement in certain circumstances;



•

we will be required to pay certain other costs relating to the Arrangement Agreement and Plan of Agreement, whether or not completed, such as legal, accounting, financial advisor and printing fees; and



•

matters relating to the Arrangement Agreement and Plan of Arrangement (including integration planning) require substantial commitments of time and resources by our management, whether or not the Plan of Arrangement is completed, which could otherwise

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have been devoted to conducting our business and pursuing other opportunities that may have been beneficial to us.

              If the Plan of Arrangement is not completed, these risks may materialize and may adversely affect our business, operations, financial results and financial condition, as well as the trading price of our common shares.

We have not identified any specific use of the net proceeds of this offering in the event the Plan of Arrangement is not completed.

              Consummation of the Plan of Arrangement is subject to a number of conditions and approvals, and, if the Plan of Arrangement is not completed for any reason, our board of directors and management will have broad discretion over the use of the net proceeds we receive following this offering and might not apply the net proceeds in ways that increase the trading price of our common shares. Since the primary purpose of this offering is to provide funds to pay a portion of the cash amount we are required to pay in connection with the Plan of Arrangement, we have not identified a specific use for the net proceeds in the event the Plan of Arrangement is not completed. Any funds received may be used by us for any corporate purpose, which may include pursuit of other business combinations, expansion of our operations, share repurchases or other uses. The failure of our management to use the net proceeds from this offering effectively could have an adverse effect on our business and may have an adverse effect on our earnings per share.

The failure to integrate successfully the businesses of Atlantic Power and CPILP in the expected timeframe would adversely affect the combined company's future results.

              The success of the Plan of Arrangement will depend, in large part, on our ability to realize the anticipated benefits, including cost savings, from combining the businesses of Atlantic Power and CPILP. To realize these anticipated benefits, the businesses of Atlantic Power and CPILP must be successfully integrated. This integration will be complex and time-consuming. The failure to integrate successfully and to manage successfully the challenges presented by the integration process may result in the combined company not fully achieving the anticipated benefits of the Plan of Arrangement.

              Potential difficulties that may be encountered in the integration process include the following:

•

challenges and difficulties associated with managing the larger, more complex, combined business;



•

conforming standards, controls, procedures and policies, business cultures and compensation structures between the entities;



•

integrating personnel from the two entities while maintaining focus on developing, producing and delivering consistent, high quality services;



•

consolidating corporate and administrative infrastructures;



•

coordinating geographically dispersed organizations;



•

potential unknown liabilities and unforeseen expenses, delays or regulatory conditions associated with the Plan of Arrangement;



•

performance shortfalls at one or both of the entities as a result of the diversion of management's attention caused by completing the Plan of Arrangement and integrating the entities' operations; and



•

the ability of the combined company to deliver on its strategy going forward.

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Our growth plans upon completion of the Plan of Arrangement are dependent on future acquisitions and growth opportunities that may not be realized.

              The ability to expand through acquisitions and growth opportunities is integral to our business strategy following completion of the Plan of Arrangement and requires that we identify and consummate suitable acquisition or investment opportunities that meet our investment criteria and are compatible with our growth strategy. We may not be successful in identifying and consummating acquisitions or investments that meet our investment criteria on satisfactory terms or at all. The failure to identify and consummate suitable acquisitions, to take advantage of other investment opportunities, or to integrate successfully any acquisitions without substantial expense, delay or other operational or financial problems, would impede our growth and negatively affect our results of operations and cash available for distribution to our shareholders.

We may be adversely affected by increased debt and debt service obligations.

              We plan to fund a significant portion of the cash consideration payable by us under the Plan of Arrangement, including related fees and expenses, with the net proceeds from a senior unsecured notes offering and/or drawings under a $625 million senior secured credit facility, for which we have received the written commitment of a Canadian chartered bank and another financial institution. Such facility will be guaranteed by us and each of our existing and subsequently acquired or organized direct or indirect subsidiaries (excluding CPILP and each of its subsidiaries), and will contain covenants restricting certain actions by us and our subsidiaries (including CPILP and its subsidiaries), including financial, affirmative and negative covenants, which may include limitations on the ability to incur indebtedness, create liens and merge and consolidate with other companies, in each case, subject to exceptions and baskets that may be mutually agreed upon by us and the lender parties thereto, the exact terms of which will be negotiated before the effective time for the Plan of Arrangement.

              Assuming completion of the Plan of Arrangement, we will have an increased amount of indebtedness. On a pro forma basis assuming the completion of this offering and that the Plan of Arrangement was consummated on                    , we would have had $            of indebtedness. We may also obtain additional long-term debt and working capital lines of credit to meet future financing needs, subject to certain restrictions under our existing indebtedness, which would further increase our total debt.

              The potential significant negative consequences on our financial condition and results of operations that could result from our increased amount of debt include:

•

limitations on our ability to obtain additional debt or equity financing;



•

instances in which we are unable to meet the financial covenants contained in our debt agreements or to generate cash sufficient to make required debt payments, which circumstances would have the potential of accelerating the maturity of some or all of our outstanding indebtedness;



•

the allocation of a material portion of our cash flow from operations to service our debt, thus reducing the amount of our cash flow available for other purposes, including funding operating costs and capital expenditures that could improve our competitive position, results of operations or share price;



•

requiring us to sell debt or equity securities or to sell some of our core assets, possibly on unfavorable terms, to meet payment obligations;



•

compromising our flexibility to plan for, or react to, competitive challenges in our business and the power industry;

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•

the possibility of us being put at a competitive disadvantage with our competitors that do not have as much debt as we have, and competitors that may be in a more favorable position to access additional capital resources in a timely manner; and



•

limitations on our ability to execute business development activities to support our strategies.

A downgrade in Atlantic Power's or CPILP's credit ratings or any deterioration in their credit quality could negatively affect our ability to access capital and our ability to hedge and could trigger termination rights under certain contracts.

              A downgrade in Atlantic Power's or CPILP's credit ratings or deterioration in their credit quality could adversely affect our ability to renew existing, or obtain access to new, credit facilities and could increase the cost of such facilities and trigger termination rights or enhanced disclosure requirements under certain contracts to which CPILP is a party. Any downgrade of CPILP's corporate credit rating could cause counterparties and financial derivative markets to require CPILP to post letters of credit or other collateral, make cash prepayments, obtain a guarantee agreement or provide other security, all of which would expose CPILP to additional costs.

Our results of operations may differ significantly from the unaudited pro forma condensed combined consolidated financial information included in this prospectus.

              This prospectus includes unaudited pro forma condensed combined consolidated financial statements to illustrate the effects of the Plan of Arrangement on our historical financial position and operating results. The unaudited pro forma condensed combined consolidated statements of operations for the fiscal year ended December 31, 2010 and for the six months ended June 30, 2011 combine the historical consolidated statements of operations of Atlantic Power and CPILP, giving effect to the Plan of Arrangement, as if it had occurred on January 1, 2010. The unaudited pro forma condensed combined consolidated balance sheet as of June 30, 2011 combines the historical consolidated balance sheets of Atlantic Power and CPILP, giving effect to the Plan of Arrangement as if it had occurred on June 30, 2011. This unaudited pro forma financial information is presented for illustrative purposes only and does not necessarily indicate the results of operations or the combined financial position that would have resulted had the Plan of Arrangement been completed as of the dates or at the beginning of the periods presented, as applicable, nor is it indicative of the results of operations in future periods or the future financial position of the combined company.

We expect to incur significant expenses related to the integration of Atlantic Power and CPILP.

              We have incurred significant expenses in connection with the Plan of Arrangement and expect to incur additional significant expenses in connection with the integration of Atlantic Power and CPILP. There are a large number of processes, policies, procedures, operations, technologies and systems that must be integrated. While we have assumed that a certain level of expenses will be incurred, there are many factors beyond our control that could affect the total amount or the timing of the integration expenses. Moreover, many of the expenses that will be incurred are, by their nature, difficult to estimate accurately. These integration expenses likely will result in our taking significant charges against earnings following the completion of the Plan of Arrangement, and the amount and timing of such charges are uncertain at present.

If goodwill or other intangible assets that we record in connection with the Plan of Arrangement become impaired, we could have to take significant charges against earnings.

              In connection with the accounting for the Plan of Arrangement, we expect to record a significant amount of goodwill and other intangible assets. Under U.S. GAAP, we must assess, at least annually and potentially more frequently, whether the value of goodwill and other indefinite-lived intangible assets has been impaired. Amortizing intangible assets will be assessed for impairment in the event of an impairment indicator. Any reduction or impairment of the value of goodwill or other

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intangible assets will result in a charge against earnings, which could materially adversely affect our results of operations and shareholders' equity in future periods.

We must continue to retain, motivate and recruit executives and other key employees, which may be difficult in light of the uncertainty regarding the Plan of Arrangement, and failure to do so could negatively affect us.

              We must be successful at retaining, recruiting and motivating key employees following the completion of the Plan of Arrangement. Experienced employees in the power industry are in high demand and competition for their talents can be intense. Employees of both Atlantic Power and CPILP may experience uncertainty about their future role with the combined company until, or even after, strategies with regard to the combined company are announced or executed. These potential distractions of the Plan of Arrangement may adversely affect our ability to attract, motivate and retain executives and other key employees and keep them focused on applicable strategies and goals. A failure to retain and motivate executives and other key employees during the period prior to or after the completion of the Plan of Arrangement could have an adverse impact on our business.

There are factors that could cause the Plan of Arrangement not to be accretive and could cause dilution to our distributable cash flow per share, which may negatively affect the market price of our common shares.

              We could encounter transaction and integration-related costs or other factors such as the failure to realize benefits anticipated in the Plan of Arrangement. All of these factors could cause dilution to our distributable cash flow per share or decrease or delay the expected accretive effect of the Plan of Arrangement and cause a decrease in the market price of our common shares. Accordingly, we may not be able to increase our dividends following completion of the Plan of Arrangement as currently planned.

CPI Preferred Equity Ltd. is subject to Canadian tax, as is Atlantic Power's income from CPILP.

              As a Canadian corporation, we are generally subject to Canadian federal, provincial and other taxes. See "Risk factors—Risks related to our structure—We are subject to Canadian tax". Following completion of the Plan of Arrangement, we will be required to include in computing our taxable income any income earned by CPILP. In addition, CPI Preferred Equity Ltd., a subsidiary of CPILP, is also a Canadian corporation and is generally subject to Canadian federal, provincial and other taxes. CPI Preferred Equity Ltd. is, and following the completion of the Plan of Arrangement will continue to be, liable to pay material Canadian cash taxes.

Our incorporation of the CPILP structure following the Plan of Arrangement may be subject to additional U.S. federal income tax liability.

              CPILP's U.S. structure has in place certain intercompany financing arrangements (the "CPILP Financing Arrangements"). While CPILP has received advice from its U.S. accountants, based on certain representations by its holding companies, that the payments on the CPILP Financing Arrangements should be deductible for U.S. federal income tax purposes, it is possible that the IRS could successfully challenge the deductibility of these payments. If the IRS were to succeed in characterizing these payments as non-deductible, the adverse consequences discussed above with respect to the Intercompany Loan could apply in connection with the CPILP Financing Arrangements. See "Risk factors—Risks related to our structure—Our prior and current structure may be subject to additional U.S. federal income tax liability." In addition, even if the payments are respected as interest, the deduction thereof could nevertheless be limited by the earnings stripping limitations, as discussed above with respect to our current structure. The earnings stripping limitations will also apply to other indebtedness of CPILP's U.S. group that is guaranteed by CPILP or Atlantic Power. Finally, the applicability of recent changes to the U.S.-Canada Income Tax Treaty to the structure associated with certain of the CPILP Financing Arrangements may result in distributions from CPILP's U.S. group to

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its Canadian parent being subject to a 30% rate of withholding tax instead of the 5% rate that would otherwise have applied.

Risks Related to Our Common Shares

Market conditions and other factors may affect the value of our common shares.

              The trading price of our common shares will depend on many factors, which may change from time to time, including:

•

conditions in the power generation markets and the energy markets generally;



•

interest rates;



•

the market for similar securities;



•

government action or regulation;



•

general economic conditions or conditions in the financial markets;



•

our past and future dividend practice; and



•

our financial condition, performance, creditworthiness and prospects.

              Accordingly, the common shares that an investor purchases, whether in this offering or in the secondary market, may trade at a price lower than that at which they were purchased.

The market price and trading volume of our common shares may be volatile.

              The market price of our common shares may be volatile, particularly given the current economic environment. In addition, the trading volume in our common shares may fluctuate and cause significant price variations to occur. If the market price of our common shares declines significantly, you may be unable to resell your shares at or above the purchase price. The market price of our common shares may fluctuate or decline significantly in the future.

              Some of the factors that could negatively affect our share price or result in fluctuations in the price or trading volume of our common shares include:

•

quarterly variations in our operating results or the quality of our assets;



•

changes in applicable regulations or government action;



•

operating results that vary from the expectations of management, securities analysts and investors;



•

changes in expectations as to our future financial performance;



•

announcements of innovations, new products, strategic developments, significant contracts, acquisitions and other material events by us or our competitors;



•

changes in financial estimates or publication of research reports and recommendations by financial analysts or actions taken by rating agencies with respect to us or other companies in our industry;



•

the operating and securities price performance of other companies that investors believe are comparable to us;



•

changes in general market conditions, such as interest or foreign exchange rates, stock or commodity valuations, or volatility; and



•

actions by our current shareholders, including sales of our common shares by existing shareholders and/or directors and executive officers.

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              Stock markets in general have experienced significant volatility over the past two years, and continue to experience significant price and volume volatility. As a result, the market price of our common shares may continue to be subject to similar market fluctuations that may be unrelated to our operating performance or prospects. Increased volatility could result in a decline in the market price of our common shares.

Present and future offerings of debt or equity securities, ranking senior to our common shares, may adversely affect the market price of our common shares.

              If we decide to issue debt or equity securities ranking senior to our common shares in the future it is likely that they will be governed by an indenture or other instrument containing covenants restricting our operating flexibility. Additionally, any convertible or exchangeable securities that we issue in the future may have rights, preferences and privileges more favorable than those of holders of our common shares and may result in dilution to holders of our common shares. We and, indirectly, our shareholders, will bear the cost of issuing and servicing such securities. Because our decision to issue debt or equity securities in any future offering will depend on market conditions and other factors, we cannot predict or estimate the amount, timing or nature of our future offerings. Thus, holders of our common shares will bear the risk of our future offerings reducing the market price of our common shares and diluting the value of their share holdings in us.

The number of shares available for future sale could adversely affect the market price of our common shares.

              We cannot predict whether future issuances of our common shares or the availability of shares for resale in the open market will decrease the market price per common share. We may issue additional common shares, including securities that are convertible into or exchangeable for, or that represent the right to receive common shares. Sales of a substantial number of common shares in the public market or the perception that such sales might occur could materially adversely affect the market price of our common shares. Because our decision to issue securities in any future offering will depend on market conditions and other factors, we cannot predict or estimate the amount, timing or nature of our future offerings. Thus, our shareholders bear the risk of our future offerings reducing the market price of our common shares and diluting their share holdings in us.

              The exercise of the underwriters' option to purchase additional common shares, the exercise of any options granted to directors, executive officers and other employees under our stock compensation plans, and other issuances of our common shares could have an adverse effect on the market price of our common shares, and the existence of options may materially adversely affect the terms upon which we may be able to obtain additional capital through the sale of equity securities. In addition, future sales of our common shares may be dilutive to existing shareholders.

The redemption of our outstanding debentures for or repayment of principal by issuing common shares may cause common shareholders dilution.

              We may determine to redeem outstanding debentures for common shares or to repay outstanding principal amounts thereunder at maturity of the debentures by issuing additional common shares. The issuance of additional common shares may have a dilutive effect on shareholders and an adverse impact on the price of our common shares.

Provisions of our articles of continuance could discourage potential acquisition proposals and could deter or prevent a change in control.

              We are governed by the Business Corporations Act (British Columbia). Our articles of continuance contain provisions that could have the effect of delaying, deferring or discouraging another party from acquiring control of our company by means of a tender offer, a proxy contest or otherwise. These provisions may make it more difficult for other persons, without the approval of our board of directors, to make a tender offer or otherwise acquire a substantial number of our common shares or to launch other takeover attempts that a shareholder might consider to be in his or her best interest. These provisions could limit the price that some investors might be willing to pay in the future for our common shares.

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CAUTIONARY STATEMENTS REGARDING FORWARD-LOOKING STATEMENTS

              This prospectus and the documents incorporated by reference herein contain forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995 with respect to the financial condition, results of operations, business strategies, operating efficiencies, synergies, revenue enhancements, competitive positions, plans and objectives of management and growth opportunities of Atlantic Power and CPILP, and with respect to the Plan of Arrangement and the markets for CPILP units and Atlantic Power common shares and other matters. Statements in this prospectus and the documents incorporated by reference herein that are not historical facts are hereby identified as forward-looking statements for the purpose of the safe harbor provided by Section 27A of the Securities Act and Section 21E of the Exchange Act and forward-looking information within the meaning defined under applicable Canadian securities legislation (collectively, "forward-looking statements").

              These forward-looking statements relate to, among other things, the expected benefits of the Plan of Arrangement, such as accretion, the ability to pay increased dividends, enhanced cash flow, growth potential, liquidity and access to capital, market profile and financial strength, the position of the combined company and the expected timing of the completion of the Plan of Arrangement, if completed.

              Forward-looking statements can generally be identified by the use of words such as "should," "intend," "may," "expect," "believe," "anticipate," "estimate," "continue," "plan," "project," "will," "could," "would," "target," "potential" and other similar expressions. In addition, any statements that refer to expectations, projections or other characterizations of future events or circumstances are forward-looking statements. Although we believe that the expectations reflected in such forward-looking statements are reasonable, such statements involve risks and uncertainties, and undue reliance should not be placed on such statements. Certain material factors or assumptions are applied in making forward-looking statements, including, but not limited to, factors and assumptions regarding the items outlined above. Actual results may differ materially from those expressed or implied in such statements. Important factors that could cause actual results to differ materially from these expectations include, among other things:

•

the failure to receive, on a timely basis or otherwise, the required approvals by Atlantic Power shareholders, CPILP unitholders and government or regulatory agencies (including the terms of such approvals);



•

the risk that a condition to closing of the Plan of Arrangement may not be satisfied;



•

the possibility that the anticipated benefits and synergies from the Plan of Arrangement cannot be fully realized or may take longer to realize than expected;



•

the possibility that costs or difficulties related to the integration of Atlantic Power and CPILP operations will be greater than expected;



•

the ability of the combined company to retain and hire key personnel and maintain relationships with customers, suppliers or other business partners;



•

the impact of legislative, regulatory, competitive and technological changes;



•

the risk that the credit ratings of the combined company may be different from what the companies expect;



•

the amount of distributions expected to be received from our projects and our estimated net cash tax refunds;

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•

the expected use of proceeds from this offering and any private offering of senior unsecured notes; and



•

other risk factors relating to us and the power industry, as detailed from time to time in our filings with the SEC and the Canadian Securities Administrators (the "CSA").

              Additional information about these factors and about the material factors or assumptions underlying such forward-looking statements may be found in this prospectus under the section entitled "Risk factors," beginning on page 19.

              You are cautioned that any forward-looking statement speaks only as of the date of this prospectus or, if such statement is included in a document incorporated by reference into this prospectus, as of the date of such other document. We undertake no obligation to update or revise any forward-looking statement, whether as a result of new information, future events or otherwise, except as may be required by applicable law.

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EXCHANGE RATE INFORMATION

              The following table sets forth, for each period indicated, the high and low exchange rates for one U.S. dollar, expressed in Canadian dollars, the average of such exchange rates on the last day of each month during such period and the exchange rate at the end of such period, based on the noon buying rate as quoted by the Bank of Canada. On September 14, 2011, the noon buying rate was $1.00 = C$0.9913.

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USE OF PROCEEDS

              We expect to receive net proceeds from this offering of approximately $             million after deducting the underwriting discount and our estimated expenses (or approximately $             million if the underwriters exercise their over-allotment option to purchase additional shares in full). We intend to use the net proceeds from this offering to:

(i)

fund a portion of the cash consideration payable by us under the Plan of Arrangement; and

(ii)

to the extent that any proceeds remain thereafter, or the Plan of Arrangement is not completed, to fund additional growth opportunities and for general corporate purposes.

              This offering is not conditioned on the completion of the Plan of Arrangement and there can be no assurance that the Plan of Arrangement will be completed. The shares offered hereby will remain outstanding whether or not the Plan of Arrangement is completed.

              We intend to fund the remainder of the cash consideration payable by us under the Plan of Arrangement, including related fees and expenses, with the net proceeds from a senior unsecured notes offering and/or borrowings under a $625 million senior secured credit facility, for which we have received the written commitment of a Canadian chartered bank and another financial institution. This offering is not conditioned upon the successful completion of a private offering of senior unsecured notes and any private offering of senior unsecured notes will not be conditioned upon the successful completion of this offering.

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DIVIDENDS AND DIVIDEND POLICY

              On November 24, 2009, our shareholders approved our conversion to a common share structure. Subsequent to the conversion, we have continued to maintain our business strategy and our current distribution levels. Each IPS has been exchanged for one new common share. Our entire current monthly cash distribution of C$0.0912 per common share is being paid as a dividend on the new common shares on the last business day of each month for holders of record on the last business day of the immediately preceding month.

              The transactions contemplated by the Plan of Arrangement are expected to be immediately accretive to cash available for distribution following the effective date of the Plan of Arrangement. As a result, our management intends to recommend to our board of directors to increase our dividend by 5% from C$1.094 per share to C$1.15 per share on an annual basis following the effective date of the Plan of Arrangement. Upon completion of the Plan of Arrangement, our dividend will continue to be paid monthly. Future dividends are paid at the discretion of our board of directors subject to, among other things, our earnings and cash flow and are not guaranteed. The primary risk that impacts our ability to continue paying cash dividends at the current rate is the operating performance of our projects and their ability to distribute cash to us after satisfying project-level obligations.

              Dividends declared per common share (or distributions per IPS) for each of the monthly periods shown below were as follows (C$):

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MARKET PRICE OF THE COMMON SHARES

              The IPSs were listed and posted for trading on the TSX under the symbol "ATP.UN" from the time of our initial public offering in November 2004 through November 30, 2009. Following the closing of the exchange of IPSs for common shares, our new common shares commenced trading on the TSX on December 2, 2009 under the symbol "ATP." The following table sets forth the price ranges of the outstanding IPSs and common shares, as applicable, as reported by the TSX for the periods indicated:

              Our shares began trading on the NYSE under the symbol "AT" on July 23, 2010. The following table sets forth the price ranges of our outstanding common shares, as reported by the NYSE from the date on which our common shares were listed through December 31, 2010:

              On September 14, 2011, there were 68,984,192 of our common shares issued and outstanding, and the number of holders of our common shares was approximately 53,998.

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CAPITALIZATION

              The following table shows our capitalization as of June 30, 2011:

•

on an actual basis;



•

on an as adjusted basis to give effect to the sale of our common shares in this offering, at an assumed offering price of $14.71 per share, the last reported sale price of our common shares on the New York Stock Exchange September 14, 2011, after deducting underwriting discounts and estimated transaction expenses payable by us; and



•

on an as further adjusted basis to give effect to:



•

the issue of approximately 31.5 million common shares we expect to issue in exchange for CPILP units in connection with the Plan of Arrangement;



•

the incurrence of additional debt (in the form of one or more private issuances of senior notes, borrowings under our available credit facilities or a combination thereof) in the amount of $425 million; and



•

application of the net proceeds from this offering, any senior notes offerings and/or any borrowings under our credit facilities to pay the cash portion of the amounts payable by us pursuant to the Plan of Arrangement, including related fees, costs and expenses, and completion of the Plan of Arrangement as described elsewhere in this prospectus.

              This offering is not conditioned on the completion of the Plan of Arrangement and there can be no assurance that the Plan of Arrangement will be completed. The shares offered hereby will remain outstanding whether or not the Plan of Arrangement is completed.

              You should read this table in conjunction with the sections entitled "Use of proceeds" and "Unaudited pro forma condensed combined consolidated financial statements" included elsewhere in

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this prospectus, in addition to our consolidated financial statements and related notes incorporated by reference herein.