The British thermal unit is a measurement of energy. One Btu is the heat needed to raise 1 pound of water 1 degree Fahrenheit. The term is useful for comparing energy sources (rough-and-ready cost/benefit calculations, for instance). A furnace and boiler’s output (or input) is measured in Btu. The relation-ship between the appliance’s Btu input and its usable Btu output is its efficiency. How do various fuels compare? A gallon of fuel oil contains about 139,000 Btu; a cubic foot of propane, about 2570 Btu; and a cubic foot of natural gas, 1028 Btu. A cubic foot of seasoned hardwood contains 180,000 to 220,000 Btu.

Expert Tip

Use this energy calculator from the Energy Information Administration to convert energy from Kilowatt Hours to BTU (and back), or find out the number of BTUs you use in your monthly natural gas consumption.