British thermal units (BTUs) are a measurement of energy used primarily in heating and cooling systems. One BTU is defined as the amount of energy needed to raise the temperature of one pound of water by one degree Fahrenheit. This unit helps quantify the energy output of appliances like furnaces, air conditioners, and water heaters. In the context of heating, a higher BTU rating indicates a more powerful system capable of heating or cooling a larger space. For example, a furnace with a higher BTU rating can effectively warm a bigger room compared to one with a lower rating, making it essential for selecting the right equipment for specific needs.