I've done this exercise, I have a digital watt meter.
My pc uses 120W idle and 420W with a benchmark that pushes it to the max. When playing a game it uses about 250W (depends on the game).
This is for an i5 2500K @ 4.5ghz
P67 Motherboard
GTX 980
2 x 24" led screens
4 hdd's and 1 ssd
All the fans, peripherals and headphones
I don't think it is all that bad





