AMENDMENT NO. 13
                            TO AGREEMENT AND PLAN OF
                            REORGANIZATION AND MERGER
                     BY AND AMONG U.S. ENERGY SYSTEMS, INC.,
                            USE ACQUISITION CORP. AND
                      ZAHREN ALTERNATIVE POWER CORPORATION

            This Amendment No. 13 to Agreement and Plan of Reorganization and
Merger (the "Amendment") is made as of the 11th day of July, 2003 by and between
U.S. Energy Systems, Inc. ("Parent") and), and US Energy Biogas Corp. (formerly
known as Zahren Alternative Power Corporation (the "Surviving Corporation")), as
the surviving corporation of the merger with USE Acquisition Corp.("Merger
Sub"). Unless indicated otherwise, capitalized terms shall have the same
meanings herein as they have in the Merger Agreement (as defined below).

                               W I T N E S S E T H

            WHEREAS, Parent, Merger Sub and the Surviving Corporation previously
entered into that certain Agreement and Plan of Reorganization and Merger dated
as of November 28, 2000 as amended by Amendment No. 1, Amendment No. 2,
Amendment No. 3, Amendment No. 4, Amendment No. 5, Amendment No. 6, Amendment
No. 7, Amendment No. 8, Amendment No. 9, Amendment No. 10., Amendment No. 11 and
Amendment No. 12 (collectively the "Merger Agreement"); and

            WHEREAS, the Parent and the Surviving Corporation now wish to amend
the Agreement.

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            NOW, THEREFORE, in consideration of $10.00 and other consideration,
the receipt and sufficiency of which are hereby acknowledged, the parties hereto
agree as follows:

            1. Clause (B) of Section 2.01(a) of the Merger Agreement is hereby
amended by deleting the existing clause in its entirety and inserting in its
place the following clause:

(B) a contingent obligation of Merger Sub to pay $800,000 in cash on July 31,
2003 (the "Contingent Merger Payment") (of which Contingent Merger Payment 62.5%
of the payment is guaranteed severally but not jointly by USE and 37.5% of the
payment is guaranteed severally but not jointly by CSHC pursuant to guarantees
of USE and CSHC, respectively (the "Guarantees"), substantially in the form of
Exhibits 2.01(a-2) and 2.01(a-3)),

            2. Parent and Surviving Corporation hereby waive until July 31, 2003
compliance with section 2.05(a)(ii) of the Merger Agreement to the extent of the
submission of the response and objection to the Draft Effective Date Balance
Sheet.

            3. Except as amended hereby, the Merger Agreement is as hereby
ratified

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and confirmed and, as so amended, remains in full force and effect on the date
hereof.

            IN WITNESS WHEREOF, Parent and the Surviving Corporation have caused
this Agreement to be executed as of the date first written above.

                                        U.S. ENERGY SYSTEMS, INC.

                                        By: ____________________________________
                                            Name:
                                            Title:


                                        US ENERGY BIOGAS CORP

                                        By: ____________________________________
                                            Name:
                                            Title:


CONSENTED TO BY:

MAJOR SHAREHOLDER AGENT:
AJG FINANCIAL SERVICES, INC.

By: _________________________
Name:
Title: