British Thermal Unit (BTU)

The measure used to gauge the heating quality of various fuels. It is the amount of heat needed to increase the temperature of one pound of water one degree Fahrenheit from 58.5 to 59.5 degrees under standard pressure of 30 inches of mercury at or near its point of maximum density. General conversion factors are: 1 Btu = 252 calories,1,055 joules, or 0.293 watt hours.