The BTU (British Thermal Unit) equals approximately 1,055.06 joules, defined as the heat needed to raise one pound of water by one degree Fahrenheit. It is the primary unit for rating heating and cooling equipment in the United States, appearing on air conditioner specifications, furnace ratings, and energy efficiency labels. One BTU equals about 0.2931 watt-hours or roughly 0.252 kilocalories.

The watt-hour (Wh) equals 3,600 joules, representing the energy consumed by a one-watt device operating for one hour. It is commonly used for small battery capacities, solar panel output ratings, and electric vehicle charging metrics where kilowatt-hours would be too large a unit. One watt-hour equals exactly 0.001 kilowatt-hours or about 3.412 BTU.