Power drain

Does anyone know how much power computers suck? I'm trying an experiment. When I'm not using my main computer I'm shutting it down completely.

It's a Mac which means I can leave it on for weeks on end (which I usually do), but I'm giving this a shot. It's a PowerMac G5 Quad hooked up to a 20" screen and a 30" screen so I figure it has to be using a decent amount of power.

Here's hoping it cuts down dramatically on the electric bill.