The BTU measure of power for furnaces and air conditioners
The British Thermal Unit, or BTU for short, is a traditional measure of energy. In North America, it is commonly used to describe the power of heating and cooling systems. When used to describe power, BTU per hour (BTU/hr) is actual the technically correct unit, but it usually gets abbreviated to simply BTU.
The BTU also tends to be more commonly used with furnaces rather than air conditioners, which tend to get described in tons instead. Nevertheless, it is an easy conversion since 1 ton is equal to approximately 12,000 BTU. The higher the BTU rating of your system, the larger its heating or cooling capacity (meaning it can handle bigger areas) and the more energy it uses. This is why proper furnace and air conditioner sizing is so important.