I’ve measured the power usage on my desktop computer:

It consists of:
2×17″ tft
1xusb hub.
1xdigitenne reciever.
1xcomputer. (intel dual core, 2x hdd, 2x dvd drive and 8500GT silent video card).

Idle usage is 150-160W. (idle is watching tv, irc that kind of stuff)
Playing a game is 180-190W.
50W is used by the 2 monitors. so around 80-90W by the pc.

This is better then I expected, I assumed the computer alone would take 200W.