Atlantic Power Corporation — Proxy Solicitation & Information Statement 2021

Mar 2, 2021

ATLANTIC POWER CORPORATION 3 Allied Drive, Suite 155 Dedham, MA 02026

NOTICE OF SPECIAL MEETING OF COMMON SHAREHOLDERS To Be Held on April 7, 2021

Dear Shareholder:

PLEASE TAKE NOTICE that a special meeting of holders (the “ Common Shareholders ”) of common shares (the “ Common Shares ”) of Atlantic Power Corporation, a corporation existing under the laws of the Province of British Columbia (“ Atlantic Power ”, the “ Company ”, “ we ” or “ us ”), will be held on April 7, 2021, at 10:00 a.m., Eastern Daylight Time, virtually via the Internet at https://web.lumiagm.com/422322246 (the “ Common Shareholder Meeting ”), for the following purposes:

• (1) To consider, pursuant to an interim order of the Supreme Court of British Columbia, dated as of February 23, 2021 (the “ Interim Order ”) and, if deemed advisable, to pass, with or without variation, a special resolution (the “ Arrangement Resolution ”) to approve an arrangement (the “ Arrangement ”) in accordance with Division 5 of Part 9 of the Business Corporations Act (British Columbia) (the “ BCBCA ”) pursuant to the Arrangement Agreement (the “ Arrangement Agreement ”), dated as of January 14, 2021, by and among the Company, Atlantic Power Preferred Equity Ltd., a corporation existing under the laws of the Province of Alberta, Atlantic Power Limited Partnership, a limited partnership existing under the laws of the Province of Ontario, Tidal Power Holdings Limited, a private limited company existing under the laws of the United Kingdom, and Tidal Power Aggregator, L.P., a limited partnership existing under the laws of the Cayman Islands (together with Tidal Power Holdings Limited, the “ Purchasers ”), to effect among other things, the acquisition by the Purchasers of all of the outstanding Common Shares in exchange for US$3.03 in cash (less any applicable withholding taxes) per Common Share.

• (2) To consider and vote on a proposal to approve, by non-binding, advisory vote, certain compensation arrangements for the Company’s named executive officers in connection with the Arrangement (the “ NEO arrangement-related compensation proposal ”).

• (3) Subject to the provisions of the Arrangement Agreement, to consider and vote on the proposal to approve the adjournment or postponement of the Common Shareholder Meeting, if necessary, to solicit additional proxies if there are insufficient votes at the time of the meeting to approve the Arrangement Resolution.

• (4) To act upon other business as may properly come before the Common Shareholder Meeting and any and all adjourned or postponed sessions thereof.

The record date for the determination of Common Shareholders entitled to notice of and to vote at the Common Shareholder Meeting is February 16, 2021. Accordingly, only Common Shareholders of record as of that date will be entitled to notice of and to vote at the Common Shareholder Meeting or any adjournment or postponement thereof. A list of our Common Shareholders entitled to vote at the Common Shareholder Meeting will be available at our principal executive offices at 3 Allied Drive, Suite 155, Dedham, MA 02026 during ordinary business hours for ten days prior to the Common Shareholder Meeting. Requests to inspect the list prior to the Common Shareholder Meeting should be addressed to our Investor Relations department at our principal executive offices.

Please read the accompanying information circular and proxy statement carefully as it sets forth details of the proposed Arrangement and other important information related to the Arrangement.

Your vote is important, regardless of the number of Common Shares you own. The Arrangement Resolution must be approved by no less than two-thirds of the votes cast by Common Shareholders present virtually or represented by proxy at the Common Shareholder Meeting. In addition, the Arrangement Resolution must be approved by a majority of the votes cast at the Common Shareholder Meeting by Common Shareholders, excluding votes attached to Common Shares held by persons described in items (a) through (d) of Section 8.1(2) of Multilateral Instrument 61-101 Protection of Minority Security Holders in Special Transactions . The number of Common Shares to be excluded from this “majority of the minority” vote is immaterial. The NEO arrangement-related compensation proposal requires the affirmative vote of a majority of the votes cast by Common Shareholders present or represented by proxy at the Common Shareholder Meeting and entitled to vote thereon. The adjournment proposal requires the affirmative vote of a majority of the votes cast by Common Shareholders present or represented by proxy at the Common Shareholder Meeting and entitled to vote thereon. Even if you plan to attend the Common Shareholder Meeting virtually, we request that you complete, sign, date and return the enclosed proxy card by mail or facsimile or submit your proxy by telephone or the Internet prior to the Common Shareholder Meeting and thus ensure that your Common Shares will be represented at the Common Shareholder Meeting. If you fail to return your proxy card or fail to submit your proxy by telephone or the Internet, your Common Shares will not be counted for any purpose. Atlantic Power’s board of directors recommends that Common Shareholders vote FOR the approval of the Arrangement Resolution, FOR the NEO arrangementrelated compensation proposal and FOR the adjournment of the Common Shareholder Meeting, if necessary, to solicit additional proxies.

Pursuant to the Interim Order, registered holders of Common Shares will have a right to dissent in respect of the Arrangement Resolution and to be paid an amount equal to the fair value of their Common Shares. See the section entitled “Dissent Rights of Common Shareholders and Preferred Shareholders in Respect of the Arrangement,” beginning on page 113 in the accompanying information circular and proxy statement.

Whether or not you plan to attend the Common Shareholder Meeting virtually, please complete, sign and return, as promptly as possible, the enclosed proxy card in the accompanying reply envelope or by facsimile, or submit your proxy by telephone or the internet. Common Shareholders who attend the Common Shareholder Meeting virtually may revoke their proxies and vote in person.

By Order of the Board, /s/ “James J. Moore, Jr.” James J. Moore, Jr. President and Chief Executive Officer

Dedham, MA March 2, 2021