My AMD housefire rig draws 300 W from the outlet when folding. It'll bring the whole room up two degrees over the course of an hour. Even at idle, it still draws 120 W. It probably rivals my old Pentium 4 tower for being the most inefficient computer I've ever owned.
my old compaq was the best furnace for my room, especially when the fan would be going max rpm while I had like stepmania, firefox, and skype open all at once. my room isn't nearly as warm ever since I got a new rig and I actually kind of miss having the compaq running.