The Electric Power Subsystem (EPS) is a critical subsystem of any satellite. It provides the electric power needed for satellite subsystems. Any failure in EPS leads to satellite mission failure or reduction of the satellite capabilities. When designing an electrical power system for CubeSats, the available energy is usually estimated first, then the energy budget is calculated, taking into account the load consumption. As is known [1], to estimate the energy budget, we need to know the orbit average power, generated by SA. It depends on many factors: orbital parameters (duration of illuminated and
shadow areas), electrical characteristics of solar cells, their temperature, and state of health; this parameter can be calculated or measured fairly easily. The power of SA also depends on the illumination which is the amount of solar power that reaches the solar cell per unit area. This energy is determined by the orientation scenario (and attitude control method) of a CubeSat, since it depends on the angle between the direction of sunlight and the normal to the SA surface. This paper aims to design the Electrical Power System (EPS) for CubeSat LEO satellites and investigate it power capability to satisfy the mission requirements. In this regard, accurate solar irradiance determination for the nadir-orientation scenario, Multi-Junction (MJ) solar cells calculations, backup batteries type and number. Designing highly-efficient power supply module circuits is done. The power supply module circuits are designed based on commercial on the shelf components. The design of those circuits is satisfying the modularity concept and perform sample of the whole function of the power supply system for CubeSat.