At launch, Voyager 2's three radioisotope thermoelectric generators supplied about 470 watts of electrical power collectively.
The spacecraft could not rely on solar panels in the dim outer Solar System, so it used MHW-RTGs fueled by plutonium-238. Each generator produced approximately 157 watts of electrical power at launch; together, the three units supplied roughly 470 watts.
RTGs generate electricity from the heat released by radioactive decay. Their output gradually declines as the plutonium fuel decays and as the thermocouples lose efficiency. The generators also produce waste heat, which helped keep spacecraft systems warm in the cold of deep space.
The figure is a launch-era total, not Voyager 2's present output. Power has fallen substantially over the decades, forcing NASA to switch off heaters and scientific instruments in stages. The same three-RTG design was used on Voyager 1.