You’ll find numbers for British thermal units on heaters and air conditioners. Without getting too technical, 1 British thermal unit (Btu) is roughly the amount of heat produced by burning a wooden kitchen match. With a heater, the number tells you the amount of heat that can be produced in an hour: the higher the number, the more powerful the heater. In an air conditioner, the number tells you how much heat can be removed from a room in an hour: the higher the number, the more powerful the air conditioner.