Most orbital satellites today rely on solar panels to generate the necessary power for their normal operation. A spacecraft electrical power system (SEPS) generates stores, regulates, control, and distributes required electric power to whole spacecraft loads during the space mission life time. The (SEPS) consists of three main subsystems: primary power supply subsystem (PPSS), secondary power supply subsystem (SPSS), and control - regulation -distribution subsystem (CRDS). This search is focused on the designing of the PPSS. In this work the designed system is based on the conversion of PV energy into electric power via the (PV effect) using the solar cells. The main goal of the design procedure of the spacecraft (PPSS) is the verification of the optimal solution of generating the electric power required for the spacecraft loads, during the daylight of the orbit, and also able to charge the battery (SPSS) with the required power for the spacecraft loads, during the eclipse period of the orbit. the impacts of space radiation and the variation of the operating temperature of the cell are considered, And it can compare between the several categories of PV cells.