Electrical power is provided by the Radio-isotope Thermoelectric Generator (RTG). The power level decreases as the mission progresses: from 280 W at the start of the mission to just under 200 W in December 2006.
The RTG, which generates 4500 W of thermal energy, has two major components: a heat source and a converter. The General-Purpose Heat Source (GPHS) consists of several elements containing the isotopic fuel plutonium-238, in the form of PuO2. The radioactive decay energy is absorbed at the heat source converter interface where heat is produced. The Si-GE converter contains thermoelectric elements which convert the heat into electrical energy. Power is delivered to the experiments and subsystems at 28V ± 2%.