Electric power drives many trains and subway systems. On farms, electric machinery performs such tasks as pumping water, milking cows, and drying hay. Huge electric generators in power plants produce almost all the world's electric power. The majority of these plants burn coal, oil, or natural gas to run the generators. Most other plants drive the generators by means of nuclear energy or the force of falling water.
Wires carry the electric current from power plants to the cities or other areas where it is needed.
The electric power is then distributed to individual consumers. Electric power is measured in units called watts. For example, it takes 100 watts of electric power to operate a 100-watt light bulb. Ten 100-watt bulbs require 1,000 watts, or 1 kilowatt. The amount of energy used is expressed in kilowatt-hours.