Software products (such as HOMER), using for projecting and research of alternative energy systems in Nigeria are usually developed by European or American research centers or products supplying companies. There is a need for specific software product for projecting, analysis and simulation of electric power processes in technological buildings with specific electrical control. This paper discusses the development of a computer program for determining the renewable energy sources (RES) that can be used for electric power supply of GSM base station site at any given time. The program calculates the optimum configuration of the system according to the weather data as well as the period of operation of solar cells. From the simulation results, it was shown that the optimal load sizing (source and load characteristics are effectively matched) and the supplied energy is efficiently and rationally utilized. The structural analysis of the program is described in details through data flow diagrams. The program can be adapted for application in any area.