FIRST AMENDMENT TO COMMON STOCK PURCHASE AGREEMENT

This First Amendment to Common Stock Purchase Agreement (this “ First Amendment ”) is dated as of June 19, 2009, by and between Beacon Power Corporation, a Delaware corporation (the “ Company ”), and Seaside 88, LP, a Florida limited partnership (such investor, including its successors and assigns, “ Seaside ”).

WHEREAS, the parties entered into that certain Common Stock Purchase Agreement dated as of February 19, 2009 (the “ Agreement ”; capitalized terms used herein without definition shall have the meanings assigned to such terms in the Agreement), which Agreement provided for the sale by the Company to Seaside, and the purchase by Seaside from the Company, of up to $18,000,000 of shares of Common Stock of the Company on certain Closing Dates as set forth in the Agreement; and

WHEREAS, the parties now desire to modify the timing of the Closings pursuant to the Agreement and certain other provisions of the Agreement, as more specifically set forth below, while retaining the aggregate maximum of $18,000,000 of shares to be purchased and sold;

NOW, THEREFORE, in consideration of the mutual covenants contained in this Agreement, and for other good and valuable consideration the receipt and adequacy of which are hereby acknowledged, the Company and Seaside agree as follows:

1.           The following definitions as set forth in Section 1.1 of the Agreement shall be amended hereby by deleting each in its entirety and substituting therefor the following:

“ Per Share Purchase Price ” shall be an amount equal to the daily volume weighted average of actual trading prices measured in hundredths of cents of the Common Stock of the Company on the Trading Market for the ten consecutive trading days prior to a Closing Date multiplied by 0.86.”

“ Subsequent Closing Date ” means March 20, 2009, April 20, 2009, May 20, 2009 and June 20, 2009 and thereafter on the 5 ^th and 20 ^th of each month (or, if any such day is not a Trading Day, then the first day thereafter that is a Trading Day) during the term of this Agreement in accordance with Section 5.1 hereof, except that there shall be no Subsequent Closing Date during any Delay Period.”

2.           The parties acknowledge that (a) the Company has given Seaside notice of its election to extend this Agreement for the First Extended Term; (b) that the Subsequent Closings scheduled for June 20, 2009 and July 5, 2009 (or, if any such day is not a Trading Day, then the first day thereafter that is a Trading Day) will complete the Initial Term; and (c) that the Subsequent Closings scheduled for July 20, 2009, August 5, 2009, August 20, 2009, September 5, 2009, September 20, 2009 and October 5, 2009 (or, if any such day is not a Trading Day, then the first day thereafter that is a Trading Day) will comprise the First Extended Term, in each case subject to adjustment in the event of a Delay Period under Section 2.6 of the Agreement.

3.           Section 2.2 of the Agreement shall be amended hereby by deleting such section in its entirety and substituting therefor the following:

“2.2            Subsequent Closings .  On each Subsequent Closing Date, Seaside shall purchase from the Company, and the Company shall issue and sell to Seaside, One Million Five Hundred Thousand (1,500,000) Shares at the then-applicable Per Share Purchase Price; provided , however , that in no event shall the Company issue and sell pursuant to this Agreement at the Initial Closing and all Subsequent Closings Shares having an aggregate purchase price i