The standard measure of heat energy. It takes one Btu to raise the temperature of one pound of water by one degree Fahrenheit at sea level. For example, it takes about 2,000 Btu to make a pot of coffee. One Btu is equivalent to 252 calories, 778 footpounds, 1055 joules, and 0.293 watthours. Note: In the abbreviation, only the B is capitalized.