Kill A Watt – Take apart @ SparkFun-
Plug the Kill-a-watt into a standard 110V US outlet, then plug the device (like a TV, air-conditioner, computer, etc) you want to measure into the Kill-a-watt. The KAW will measure voltage, amps, watts, Hz, kWhr, and elapsed time (since plugged in). The unit requires no batteries (it’s plugged into the wall after all) and does a pretty good job. I bought two so I could measure intermediary loads, and of course so I could take one apart and not have to worry about destroying it.