I know i used to run a server 24/7 as well......it was doubling my leccy bill....
I built myself a nice Mini-itx machine. Fanless (fans failed after first year so I fitted some bigger heatsinks..). Only takes 20 watts and that's with a decent hard drive. OK, OOF would be a tad sluggish if it were hosted on such a POS and there's minimal fault tolerance bit it's fine for serving mail, as an MP3 dumping ground, etc.
I have a little gadget (they are on special offer from Maplins every so often for about 12 quid) that you can plug an appliance into and it gives you the power consumption - Watts and VA, power factor, mains voltage and frequency. It will also tot up units consumed over time so you can stick it on a fridge for a week and then calculate the average consumption. A nice little toy that has caused me a few eye openers.
Did you know a Sky box takes 20 watts on standby? And pretty much the same when it's not on standby. There's absolutely F.A. point in switching it to standby
All so it can turn on immediately on demand and not spend 30 seconds syncing to the satellite, and so that it can phone home in the night and spill the beans on your viewing habits (not that mine is anywhere near connected to a phone line).
If the environmentalists found out about that it would take the pressure off the 4x4 owners a tad!
Kevin