[dba-Tech] Power Consumption

I'm trying to teach the kids a little about electricity conservation - see if we can knock down that electric bill a bit (we pay about $.10 per kwh).  I'm going to have them calculate how much it costs to leave various things on per hour.

So the question is, if a computer has a 300 watt power supply, is it drawing 300 watts all the time?  Does it draw more when it's computing than when it's idling?  What's a good way for them to estimate how much a computer uses if they leave it on overnight? Just the computer, not the monitor.

Come to think of it, I leave my laser printer on all the time although it has a sleep mode and has to do a warm up if it hasn't printed anything for a while.  Also leave the speakers on - they use one of those little transformers that plugs directly into an outlet.  I suppose they draw minute amounts of power all the time although I never considered them to be major power users.

TIA

Rocky