Energy is used in homes in a variety of forms: heat, light, mechanical, and electrical; by far the most of it in the form of heat. Some 40% to 60% of the energy used in American homes is consumed in maintaining the temperature within the home through space heating and space cooling. Naturally, these uses vary widely from region-to-region.

Heat is a form of energy. It is actually the motion or agitation of molecules and atoms in a substance. The amount or quantity of heat is commonly measured in British Thermal Units (Btus). One Btu is equal to the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. It is roughly the amount of heat given off by burning a wooden match. To measure larger quantities of heat, a unit equal to one thousand Btus called the MBtu, is used.