If every idle process started running simulations full bore, that'd be a lot of heat... I have maybe 100W of computers "online" at the moment. But the heavy load draw would be well over 1000W. Idle time isn't free.

The exceptions would be some previous gen consoles like the PS3, which didn't idle efficiently, but only if you assume they're being left on continuously. I used to run folding@home during the winter to help heat my apartment.