Abstract:This research intent to enhance and design an electrical power subsystem for a communications satellite in the geosynchronous orbit for a life time mission of 7-10 years. Among many types of the well known electrical power subsystems used for satellites and spacecraft, the solar array system is used to provide the satellite power requirements. The solar arrays are used to provide power to the satellite during all mission life time, while the batteries are being used to supply power to the satellite during the eclipse when there is no available power from the solar arrays. The system is first analyzed using equations and diagrams for each stage of the electrical power subsystem for the satellite, and a theoretical design is presented using these analyses. Some characteristics curves are drawn for the DC/DC converter and the charge controller, this process is done using MATLEB 7.0, these curves are used to show the characteristics and the properties of the DC/DC converter and the charge controller based on their properties. The simulation results are used to redesign the electrical power subsystem and a comparison between the suggested system with other satellite systems is given to show the advantages of the suggested system. The electrical power subsystem is designed to meet the following power requirements: 4525 W at equinox, 3995 W at summer solistics, and 3775 W during eclipse.