A Watt Hour is a metric used to measure energy usage. For example, if a 100 Watt (W) light bulb was turned on for one hour, its total energy usage would be 100 Watt Hours. This same quantity of energy would power a 50 W light bulb for 2 hours. Electric Utilities companies usually charge its customers per Kilo-Watt Hour (kWh) or 1,000 W per hour. Prices vary greatly, but range from 5 cents to 20 cents per kWh.