Huh? A watt is a joule per second, [energy/time]. They are describing rates. If you build a 1.5 GW heat engine on top of Yellowstone (assuming it's possible) t

Huh? A watt is a joule per second, [energy/time]. They are describing rates. If you build a 1.5 GW heat engine on top of Yellowstone (assuming it's possible) the energy that builds below the volcano is 0, because you captured it and turned some of it into electricity, and dumped the waste heat into a river or the atmosphere.