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)".
...
Anyhow, for $25 it is pretty handy.
That's a cool toy!
A little googling got me info about the successor, which includes the ability to hold info during power-off.
NewEgg.com has them both. The original for just $16 and the 'EZ' for $40.
Oh, the EZ will calculate the usage cost directly if you give it your rate from the power company.
Here's a
review of the Kill-A-Watt EZ.Then there's the
Watt's Up Pro(Amazon) for those who want to fiddle with the data on your computer. It's $115-$150, though, so the original P3 Kill-A-Watt is a definite must have at $20.