BTU or British Thermal Unit, is a unit of heating power; Watts, the SI unit of power, can also be used to refer to heating power, but here’s the confusing part: when rating these large space heaters, the watts typically refers to how much electrical power is required to operate the heater, not specifically to how much heat it outputs. Unlike machines that do some other kind of work, however, heaters are typically very “efficient” since thermodynamics is on their side. There’s very little energy wasted when all the energy is “wasted” as heat.