i pretty much leave all of my desktop machines off until i want to use them. my MBP gets more use than any of the others combined, and i sleep it at night, so for 90% of the time no computer is on in my room.

but if i had to choose my most power-hungry system, that would probably have to be my main gaming rig. its not mega-powerful by today's standards, but it will hold its own. the PSU in it is 650 watts and it probably uses 400-500 of those watts. ish.