device that measures and records over time the electric power flowing through a circuit. Although there are several different types of watt-hour meters, each consists essentially of a small electric motor and a counter. A precise fraction of the current flowing in the circuit is diverted to operate the motor. The speed at which the motor turns is proportional to the current in the circuit, and, therefore, each revolution of the motor's rotor corresponds to a given amount of current flowing through the circuit. The counter is connected to the rotor and adds and displays the amount of power the circuit has carried based on the number of revolutions of the rotor. The counter is usually marked in kilowatt-hours (1,000 watt-hours).

Mercury-type and commutator-type watt-hour meters measure power in direct-current circuits. Induction-type meters measure power in alternating-current circuits and are the type commonly seen on the outside of houses. Specialized watt-hour meters include totalizing meters, which record the power used in more than one circuit, and highly accurate portable meters, which are used for testing installed watt-hour meters.