The energy source for each device was a rod of plutonium-238 providing a thermal power of approximately 1250 W. https://atomicinsights.com/nuclear-batteries-tools-for-space-science/ This fuel capsule, containing 3 kilograms of plutonium-238 in oxide form (44,500 Ci or 1.65 PBq), was carried to the Moon in a separate fuel cask attached to the side of the Lunar Module.