In more detail
One BTU is about 1,055 joules, roughly the energy needed to warm one pound of water by 1 °F. Equipment is rated in BTU per hour, which is power: 1 W is about 3.412 BTU/h, so a 12,000 BTU/h ("one ton") air conditioner moves about 3.52 kW of heat. Gas is often billed in therms of 100,000 BTU. A rough guide for room air conditioners is about 20 BTU/h per square foot, adjusted for sun, ceiling height, occupants and kitchens.
Why it matters
An undersized air conditioner runs constantly and never catches up; an oversized one cycles on and off quickly and removes less humidity, leaving the room cold but clammy. The BTU/h rating is heat moved, not electricity used: the electrical input is much smaller, which is what efficiency ratings such as EER and SEER describe.
Example
Room 12 × 15 ft = 180 ft² × 20 BTU/h = 3,600 BTU/h → a 5,000 BTU/h unit is the usual smallest size
12,000 BTU/h × 1,055.06 J / 3,600 s = 3,517 W of cooling
Furnace rated 80,000 BTU/h input = 23.4 kW
Common confusion
BTU is energy and BTU/h is power, the same distinction as kWh and kW (see Kilowatt-hour (kWh)). Product labels often drop the "/h".