A BTU (British Thermal Unit) is a unit of measurement that shows how much energy is required to raise the temperature of one pound of liquid water by one degree Fahrenheit. It is a measure of the heat content of fuels or energy sources. There are several different definitions of the BTU that differ slightly, reflecting the fact that the temperature change of a mass of water due to the addition of a specific amount of heat depends slightly upon the waters initial temperature. The BTU is often used to express the conversion-efficiency of heat into electrical energy in power plants. It is also used to compare energy sources or fuels on an equal basis, as fuels can be converted from physical units of measure (such as weight or volume) to a common unit of measurement of the energy or heat content of each fuel.

BTUs are used in the world of HVAC systems to measure the heating or cooling power of appliances such as air conditioners, furnaces, heat pumps, and cooking equipment. For example, with air conditioners, BTUs denote how much heat a unit can remove per hour. A higher BTU rating means more power. Understanding BTUs is important when choosing the right HVAC system to keep you comfortable while making sure your energy bills stay affordable.