As many others have said, power consumption is not fixed.

To do some calcs on total power draw in a couple of server rooms, we use one of these fairly cheap Kill-a-Watts. I've tested ours side-by-side against a Fluke DMM with clamp meter extension and it was "pretty close (TM)". Before we put a new box in production, we take readings at rest and with one cpuburn program per core. I haven't found a good program to exercise hard drives, power-wise, so would be interested if anybody knows of a "cpu-plus-diskburn"-type program.

For dual power supply boxes we put the Kill-a-Watt on one leg and unplug the 2nd power supply so we can what UPS loads we would get if a particular branch circuit or UPS failed.

Anyhow, for $25 it is pretty handy.