Energy Metrics: Btus, Watts, and Kilowatt-Hours

We begin with the definition of energy: the capacity to do work, such as creating heat, light, or motion. Energy is typically measured in British thermal units (Btus) or, in the metric system, joules (J).

Power is a measure of the rate at which energy flows, and in electrical systems it is measured in watts (W). A watt is energy flowing at a rate of one joule per second, which is approximately 3.4 Btus per hour.