<PAGE>   1
                                                                    EXHIBIT 10.6


                                 AMENDMENT NO. 3
                                       TO
                              EMPLOYMENT AGREEMENT
                              --------------------


         THIS AMENDMENT NO. 3 TO EMPLOYMENT AGREEMENT (this "Amendment") is made
and entered into as of January 1, 2000, by and between METRETEK TECHNOLOGIES,
INC., a Delaware corporation (the "Corporation"), and W. PHILLIP MARCUM
("Officer").

                                    RECITALS
                                    --------

         WHEREAS, the Corporation and Officer have previously entered into that
certain Employment Agreement, dated as of June 11, 1991, as amended as of July
14, 1997 and December 3, 1998 (as so amended and as the same may hereafter be
amended or otherwise modified from time to time, the "Employment Agreement");
and

         WHEREAS, the Corporation and Officer now desire to amend the Employment
Agreement in order to extend the period of Officer's employment with the
Corporation;

                                    AGREEMENT
                                    ---------

         NOW, THEREFORE, in consideration of the premises and other good and
valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto, intending to be legally bound hereby, agree as
follows:

         SECTION 1. AMENDMENT TO SECTION 2.1 OF EMPLOYMENT AGREEMENT. Effective
as of the date hereof, Section 2.1 of the Employment Agreement is hereby amended
to read in its entirety as follows:

                  "2.1 Basic Term. The term of Officer's employment under this
         Agreement shall continue for a period of four years until December 31,
         2003, unless terminated earlier pursuant to this Section 2 (the
         "Employment Period"); provided, however, that unless the Corporation or
         Officer gives to the other written notice at least six months prior to
         the expiration of such four-year term or of any successive one-year
         extension term as provided hereafter, the Employment Period shall be
         automatically extended for successive one-year terms, unless and until
         terminated pursuant to this Agreement."

         SECTION 2. EFFECT OF AMENDMENT. From and after the date hereof, the
term "Employment Period" as used in the Employment Agreement shall be deemed to
refer to the Employment Period as such term is defined in this Amendment. Except
as and to the extent expressly modified by this Amendment, the Employment
Agreement shall remain in full force and effect in all respects in accordance
with its terms, and any reference to the Employment Agreement from and after the
date hereof shall be deemed to be a reference to the Employment Agreement as
modified by this Amendment.

<PAGE>   2

         SECTION 3. GOVERNING LAW. This Amendment shall be governed by and
construed in accordance with the laws of the State of Colorado.

         SECTION 4. COUNTERPARTS. This Amendment may be executed in one or more
counterparts, each of which shall be deemed to be an original, but all of which
taken together shall constitute one and the same instrument.

         IN WITNESS WHEREOF, this Amendment No. 3 to Employment Agreement has
been duly executed and delivered by or on behalf of the undersigned as of the
date first written above.


                                 CORPORATION:

                                 METRETEK TECHNOLOGIES , INC.


                                 By:/s/ A. Bradley Gabbard
                                    --------------------------------------------
                                    A. Bradley Gabbard, Executive Vice President


                                 OFFICER:


                                 By:/s/ W. Phillip Marcum
                                    --------------------------------------------
                                    W. Phillip Marcum

                                       2