An apparatus, system and method provides electrical power in a subterranean well. A radioisotope thermoelectric generator may be positioned and installed in a downhole location in a wellbore. The location of the radioisotope thermoelectric generator may be within a completion string. A radioisotope thermoelectric generator comprises a core having a radioisotope for producing heat, and a thermocouple. The thermocouple comprises at least two different metals, and is positioned adjacent to the core. The radioisotope thermoelectric generator flows heat from the core to the thermocouple to produce electricity that may be stored in an energy storage device, or used to power a component. The produced electrical power may be employed to activate downhole sensors, valves, or wireless transmitters associated with the operation and production of an oil or gas well.