Watt meters can be helpful in measuring the energy used in standby mode by "phantom electronics"—TV sets, cable boxes, cell-phone chargers, and the like. To assess total phantom energy use, leave those appliances in standby mode, plug them all into one power strip, then plug the strip into a watt meter and the watt meter into a wall outlet.
"Learn which appliances are costing you the most!" says the maker of Kill A Watt P4400. Watts Up, offers "a convenient way to monitor and project energy costs."
The devices work in much the same way: You plug any 120-volt AC appliance into the watt meter and plug the meter into a wall socket, and the meter starts counting wattage use. You can leave meter and appliance connected for as long as you typically use the appliance: all day for a fridge, or an hour while a space heater warms a room, for example. The meter's readout tells you how much electricity the appliance has sucked up during that time. Watts Up also figures out the cost of energy use for you, after you've entered your cost per kilowatt-hour into the meter. With Kill A Watt, you do the math, multiplying the kWh reading by your cost per kWh.
Both devices are sold only online. Kill A Watt costs $25; Watts Up, about $96.
We used both watt meters to measure the electricity consumed daily by a refrigerator and a computer and compared their readouts with those from a calibrated watt meter in our labs.
Guess watt? Kill A Watt and Watts Up were accurate, and both can teach you how much an appliance contributes to utility costs. If that makes you use the appliance less or at lower-cost times of day (ask your utility when those are), you could save money. The devices can also help you compare energy costs of an appliance you own now with projected costs listed on any new one you might buy.