Power (Watts) is the rate at which energy is consumed at any given moment. Energy (Watt-hours) is the total amount of power used over a period of time.

For example, a 100-watt light bulb left on for 10 hours consumes 1,000 watt-hours (or 1 kilowatt-hour) of energy. Utility companies bill you based on the total energy (kWh) you consume, not the peak power you use.