Use the same electricity twice.
In exchange for hosting a leased server, households get heat for rather less than would otherwise be the case.
And hopefully offer a free gigabit drop as well. *grin*
Anyway, this is all too obvious. Where I live in a northern clime in a not-well-winterized century old building, I need to supplement my heating with electric. So, I have a half dozen computers on all the time. This is not a luxury, but just an easy way to use the electricity twice - no extra cost.