I just counted, I have 4 computers in my house running at all times, each probably sucking as much power as a couple of light bulbs. Two are the desktop machines used by the humans in the household at random times during the day. One is a low-power mini-ITX machine working as a file-server and the other is hooked to the TV in the livingroom where it is occasionally used to watch downloaded video.
I’d shut the desktops down at night, but they backup to one another. The fileserver has software mirrored drives, but I like having multiple copies of things on multiple machines. The TV computer could probably be replaced with some sort of networked video player with faster startup and lower power consumption, but my encryped 802.11g network may be a little too flakey to be used to reliably play back high-quality video stored on another system.
I think I need to rethink my approach to backup and storage first, and then go from there. I want to be turning at least two of these things off every night.