A British thermal unit (Btu) is a standard unit of energy that is used in the United States and sometimes in the U.K. It represents the amount of thermal energy necessary to raise the temperature of one pound of pure liquid water by one degree Fahrenheit at the temperature at which water has its greatest density (39 degrees Fahrenheit). The Btu is a measure in the so-called English system of units (the foot-pound-second system). Other countries use the joule, the unit of energy in the International System of Units (SI). A Btu is equivalent to approximately 1055 joules (or 1055 watt-seconds).
The Btu is often used as a quantitative specification for the energy-producing or energy-transferring capability of heating and cooling systems such as furnaces, ovens, refrigerators, and air conditioners. The heat output of computer equipment is often specified so that it can be considered when planning the size of climate control systems in buildings. Computer device heat output is expressed in Btus per hour. 3.7 Btus per hour is equivalent to 1 watt of heat dissipation.