What Electric Meters Do
Electrical meters measure the amount of power a building uses in a unit called Kilowatt-hours (kWh.) Electrical power is measured in watts. Electrical appliances are rated by how fast they use electricity in a unit called watts. For example, a typical incandescent light bulb might use 75 watts, while a microwave oven would use 700 watts. A kilowatt hour is the amount of energy used by a 1000-watt appliance in a single hour.
How Electric Meters Run
Most electric meters are powered by a spinning metal disk. Electricity coming into the building is passed by the disk in coils, which generate a weak magnetic field. That magnetic field pulls on the disk, causing it to spin. The more energy used in the building, the stronger the field and the faster the disk spins.
The disk itself measures the amount of energy used. It is hooked up to a series of dials that record this measurement. The disk is first hooked to a dial which spins around once every time the house uses a kilowatt hour. That dial is hooked up by gears to another dial which spins ten times slower than the first, going around every time the house uses 10 kilowatt hours. That dial is hooked up to a 100 kilowatt hour dial, and so on all the way up to 10,000 kilowatt hours.
Although the electro-mechanical meters work pretty well, they have one big drawback: a meter reader has to come out and read each and every meter individually. Modern meters get around this by sending the information directly back to the power company. Instead of using a whole series of dials to show how many kilowatt hours the meter has run for, a computer digitally records the number and sends it back to the power station. Technicians only have to come out when they need to repair the meter.