There are a number of sources, depending which "energy system" you are referring to: This is typical of a geostationary satellite: The inertial energy "forward motion" is provided by the 'kick motors' during insertion into final orbit. During the process of "station keeping" there are small thrusters (usually pressurized gas) used to adjust the speed -> altitude -> 'obrital slot' position needed because of various forms of 'drag' and degradation in space (e.g., solar wind pressures, VERY slight friction...) On-board power is typically generated by solar cells and is used to run the on-board components (e.g., the communication system, the imaging system, and other actions the satellite is expected to perform.) Very similar answers address the LEO (Low Earth Orbit) and MEO satellites.