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Kill-A-Watt meter in use
Porkrind, Wikimedia Commons

The Kill-A-Watt is a device that can be used to measure the power consumption of 120V appliances and electronics. Plug the Kill-A-Watt into the wall outlet and plug the appliance into the Kill-A-Watt. The display can show instantaneous power (how many watts your appliance is drawing at any moment) or cumulative energy (how many watt-hours the appliance has consumed since installing the Kill-A-Watt). It also displays voltage, current, and apparent power. These devices are inexpensive (~$20) and simple to use, but do not have data storage or transfer capabilities, so they are mostly useful for spot checks, rather than long-term data-acquisition. The memory is volatile (i.e. erased each time you unplug it), so if you are interested in the cumulative energy use data, you must be sure the read and record the display before unplugging the device from the outlet.