Using the presently available technologies in the fiber composite rotors and in magnetic bearings, it is feasible to design the flywheel energy storage to replace the battery in the spacecraft power system. The potential benefits are reductions in weight, size and cost. The fundamental design considerations in designing the photovoltaic/flywheel power system are investigated in this paper. Since low earth orbit satellites, being battery-heavy, would show greater benefits using the flywheel, the NASA Goddard Space Flight Center’s EOS-AM1 is taken as a case study example. It is seen that the power system using flywheel in this satellite has a potential of saving approximately 35 percent mass, 55 percent volume and 6.6 percent solar array area.