BTU SizerBTU, Tons and Kilowatts Explained
Three units, one quantity. A contractor can quote you all three in a single conversation without noticing, and a listing can print two of them for the same machine.
What each one means
BTU per hour is a rate of heat flow: how much heat the machine moves in an hour. One BTU is the energy that raises a pound of water by one degree Fahrenheit — about the heat of a kitchen match. It is the US convention for equipment ratings, and the “per hour” is almost always dropped in conversation, which causes confusion of its own: a “12,000 BTU air conditioner” means 12,000 BTU per hour.
Tons of refrigeration come from the ice trade. Before mechanical cooling, buildings were cooled with delivered ice, and equipment was rated by how much ice it replaced. One ton of refrigeration is the cooling produced by a ton of ice melting over 24 hours: exactly 12,000 BTU per hour. It has nothing to do with equipment weight.
Kilowatts are the SI unit of power and what almost every catalogue outside the United States prints. One kilowatt is 3,412.14 BTU per hour.
The two conversions worth knowing
- BTU/h ÷ 12,000 = tons (exact)
- BTU/h ÷ 3,412 ≈ kW (near enough for reading a quote)
So a 12,000 BTU mini split is a one-ton, 3.5 kW machine. A three-ton central system is 36,000 BTU/h or 10.5 kW. A 1,500 W space heater is 5,118 BTU/h.
The trap: output kilowatts versus input kilowatts
This one costs money. For a heat pump, the kilowatts in the specification are heat delivered, not electricity consumed. A 3.5 kW heat pump might draw 1 kW of electricity to deliver that 3.5 kW of heat, because it moves heat rather than making it. The ratio is the COP.
For an electric resistance heater, delivered and consumed are the same number — which is exactly why resistance heating is the expensive way to stay warm and a heat pump is not.
The other trap: furnace input versus output
Gas furnaces are advertised by input rating. An “100,000 BTU furnace” at 80% AFUE delivers 80,000 BTU/h into the house and sends the rest up the flue. Sizing calculations always mean output. Mixing the two by 20% is easy and common: take the output you need, divide by the AFUE, and shop for that input number.
And portables
Portable air conditioners have two BTU figures in circulation — the current SACC rating and the older ASHRAE one — which differ by 30 to 40% for the same machine. If a listing shows a large number with no mention of SACC, assume the old scale.
The BTU converter does all of these in one place, and shows the common equipment sizes in all three units so you can recognise a machine whichever way it has been labelled.