The Electric Power Subsystem (EPS) is one of the most critical system on any satellite because nearly every other subsystem requires power. This makes the choice of power systems the most important task facing satellite designers. The main purpose of the Satellite EPS is to provide continuous, regulated and conditioned power to all the satellite subsystems. It has to do that withstanding the radiation, thermal cycling, and vacuum of a hostile space environment as well as the subsystem degradation over the time [1]. The EPS power characteristics are determined by both the parameters of the system itself and by the satellite orbit RI After the satellite separates from the launch vehicle (LV) to its orbit, in almost situations, all the satellite subsystems (attitude determination and control, communication, and onboard computer and data handling (OBC&DH)), take their needed power from a storage battery (SB) and solar arrays (SA) beside the consumed power in EPS management device. But at this point (separation point, detumbling mode), the satellite angular motion is high and the orientation of solar arrays with respect to the sun will change in a non-uniform way, so the amount of power generated by solar arrays will be affected. The objective of this research is to select the satellite EPS components, types; estimation the solar arrays illumination parameters and determination of the efficiency of solar arrays during both detumbling and nominal modes.