Electricity usage monitors are easy to use and can measure the electricity usage of any device that runs on 120 volts. (But it can’t be used with large appliances that use 220 volts, such as electric clothes dryers, central air conditioners, or water heaters.) You can buy electricity usage monitors at most hardware stores for around $25-$50. Before using a monitor, read the user manual.

To find out how many watts of electricity a device is using, just plug the monitor into the electrical outlet the device uses, and then plug the device into the monitor. It will display how many watts the device uses. If you want to know how many kilowatt-hours (kWh) of electricity the devices uses in an hour, or a day, or longer, just leave everything set up and read the display later.

Monitors are especially useful for finding the amount of kWh used over any period of time for devices that don’t run constantly, like refrigerators. Some monitors will let you enter the amount your utility charges per kilowatt-hour and provide an estimate how much it cost to run the device since it was plugged into the monitor.

Many appliances continue to draw a small amount of stand-by power when they are switched "off." These "phantom loads" occur in most appliances that use electricity, such as televisions, stereos, computers, and kitchen appliances. Most phantom loads will increase the appliance's energy consumption a few watt-hours, and you can use a monitor to estimate those too. These loads can be avoided by unplugging the appliance or using a power strip and using the switch on the power strip to cut all power to the appliance.