Buy a cheap, simple watt-hour meter
A watt-hour meter is a little device that tells you how much electricity something uses, either at a given moment or over an extended period of time. Just plug the device into the meter, plug the meter into the wall, and read the display. I like the $25 Kill-A-Watt (pictured at right) sold through SmartHome. Just plug it in and see how many watts a device is drawing at any given moment, or how many kWh you’ve used since you turned it on. This is especially useful for finding the amount of kWh used in a month for devices that don’t run constantly, like refrigerators and window unit air conditioners.