The Electrical Power subsystem (EPS) on the satellite is used to power the satellite form launch through end of the mission life cycle. This subsystem is made up of the units on-board the satellite used to generate, store, regulate and distribute the power needed to support the satellite. This system consists of power sources including solar arrays and batteries, charging systems, voltage and current regulation, power bus and fuse assemblies and EPS control systems.

On LEO satellites the EPS system is highly or fully automated. Since the satellite can be transitioning from solar array power to battery power every orbit it would have to replace the energy used from the batteries each time. Some LEO satellites are placed in a Sun synchronous orbit so they have power from solar arrays continuously and battery systems are used for emergency conditions.

On GEO satellites that are constantly in view,  some systems are controlled from the ground while others are automated. At GEO the satellites solar arrays will have to track the Sun or be clocked to keep them pointed at the Sun for maximum efficiency. These satellites also experience daily eclipses during approximately a 45 day period centered around the equinoxes (twice a year). The duration can be from a few minutes to about 70 minutes each. During these periods the batteries must have the capacity to support the power required and have a suitable margin in the event of anomalies.  After an eclipse the batteries must be recharged, based on the satellite design, this process can be done manually or in some cases is automated. Ground control software systems have also been used to automate this process.