JOINDER

                                                                   EXHIBIT 10.3

                                     JOINDER


       THIS JOINDER AGREEMENT is entered into as of June 13, 2005 by Beacon
       Power Corporation ("Joining Stockholder"). Capitalized terms used herein
       without definition shall have the meaning assigned to such terms in that
       certain Investor Rights Agreement by and among NxtPhase T&D Corporation
       ("Company") and certain Stockholders of the Company, dated as of November
       12, 2004 ("Investor Rights Agreement").


1.   As of the date hereof, Joining Stockholder has become the owner of shares
     of the Company's capital stock that it purchased from the Company.
2.   By execution and delivery of this Joinder Agreement, the Joining
     Stockholder becomes a party to, and agrees to be bound by all the terms and
     conditions of, the Investor Rights Agreement, and shall have the same
     obligations and duties thereunder as the other Stockholders who are party
     thereto. Attached hereto is a duly executed counterpart signature page of
     the Investor Rights Agreement which Joining Stockholder agrees shall be
     attached to and become a part of the Investor Rights Agreement.
3.   Joining Stockholder acknowledges that it has read and understood the
     Investor Rights Agreement and the rights and obligations of a Stockholder
     thereunder.
4.   Joining Stockholder's address for notice under the Investor Rights
     Agreement is:

                                                     Beacon Power Corporation
                                                     000 Xxxxxxxxxxx Xxxxxx
                                                     Xxxxxxxxxx, XX 00000
                                                     Attn.: F. Xxxxxxx Xxxx
                                                               President and CEO
                                                     Facsimile:  (000) 000-0000

5.   THIS JOINDER AGREEMENT SHALL BE INTERPRETED IN ACCORDANCE WITH THE LAWS OF
     THE STATE OF NEW YORK, WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES.

This Joinder Agreement and the Investor Rights Agreement constitute the entire
agreement of Joining Stockholder with respect to the subject matter hereof.
Joining Stockholder and its permitted successors and assigns agree to execute
any instrument that may be necessary or proper to carry out the purposes and
intent of the Investor Rights Agreement and/or this Joinder Agreement, and to
evidence that such successors and assigns are and shall be bound by this Joinder
Agreement and the Investor Rights Agreement.

         IN WITNESS WHEREOF, this Joinder Agreement is executed as of the 13 day
of June, 2005.

                                       BEACON POWER CORPORATION

                                   By: /s/ Xxxxx X. Xxxxxxx
                                       ---------------------------
                                 Name:    Xxxxx X. Xxxxxxx
                                Title:   Chief Financial Officer