I have an old single CPU Power4 p630 server at home. I've always wanted to know how much it costs in electricity to run this thing, so I borrowed a device from a family member that measures electricity usage. You plug the server in to this device, and then plug the device in to the wall, tell it how much electricity costs in your area, and then it will tell you how much it is costing you.
I live in Colorado and pay somewhere around 10.5 cents per killowatt hour. Here is how much it would cost me per month to run this p630 24/7:
So it costs about $23.43 a month (about $281 a year) to run this old Power4 server 24/7. Even with the system powered down in standby mode it would cost about $1.96 a month.
I think I'll keep leaving the thing powered off (and unplugged) unless I need it :)