The device that measures and records electricity usage is called a watt-hour meter or just simply a meter. Think of the meter as the "cash register" of the utility system. Meters are the last device in the distribution system from the distribution substation to the customer. After the meter, the wires enter the customer's own electrical service panel. Meters measure the total amount of energy consumed by a given customer.
A meter is one of the least expensive pieces of hardware a utility can buy, but also one of the most important. It is only through the meters that a utility knows how much of its electricity is being used and thus how much to bill its customers. Today's meters are extremely accurate and very seldom need replacing due to damage or inaccuracy.