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Method and sources checked on by the Brikalo editorial team · Our method
Air conditioner output is rated in kW or in BTU/h. NIST gives 1 BTU/h = 0.2930711 W, so a 9,000 BTU/h unit delivers 2.64 kW of cooling and 12,000 BTU/h, one ton of refrigeration, is 3.52 kW.
BTU/h and kW: two units for the same thing
Portable and split air conditioners sold in the UK are often described by their BTU rating: 7,000, 9,000 or 12,000 BTU. Strictly this is BTU per hour, a rate of energy just like the watt. The NIST guide to units, published by the US national measurement institute, gives the factor: 1 BTU/h = 0.2930711 W.
Example: 9,000 BTU/h × 0.2930711 = 2,637.6 W, or 2.64 kW. The other way round, a 3.5 kW unit is 3,500 ÷ 0.2930711 = 11,942 BTU/h.
BTU/h to watts and kW conversion table
Converted with the NIST factor, 1 BTU/h = 0.2930711 W. Example: a 9,000 BTU/h air conditioner has a cooling capacity of 2.64 kW, and 12,000 BTU/h is exactly one ton of refrigeration.
| BTU/h | Watts | Kilowatts | Tons of refrigeration |
|---|---|---|---|
| 5,000 BTU/h | 1,465 W | 1.47 kW | 0.42 |
| 7,000 BTU/h | 2,051 W | 2.05 kW | 0.58 |
| 9,000 BTU/h | 2,638 W | 2.64 kW | 0.75 |
| 12,000 BTU/h | 3,517 W | 3.52 kW | 1.00 |
| 18,000 BTU/h | 5,275 W | 5.28 kW | 1.50 |
| 24,000 BTU/h | 7,034 W | 7.03 kW | 2.00 |
| 30,000 BTU/h | 8,792 W | 8.79 kW | 2.50 |
| 36,000 BTU/h | 10,551 W | 10.55 kW | 3.00 |
Reading the data sheet
The BTU/h and kW on the label are cooling capacity: the heat the unit removes from the room. They are not the electricity it uses, which appears separately on the data sheet as power input. To estimate running costs, start from that electrical figure and the hours of use.
American data sheets sometimes use the ton of refrigeration, which is 12,000 BTU/h, or 3,516.853 W according to NIST. A 2-ton unit therefore delivers about 7.03 kW of cooling.
Common conversions
With the same factor, 5,000 BTU/h is 1.47 kW, 7,000 BTU/h is 2.05 kW, 12,000 BTU/h is 3.52 kW, 18,000 BTU/h is 5.28 kW and 24,000 BTU/h is 7.03 kW. One kilowatt is 3,412 BTU/h.
If a radiator or heater is listed in BTU/h, the same factor applies: the conversion does not care whether the heat goes in or out. On a reversible air conditioner, though, heating and cooling capacity are two different figures on the data sheet.
Estimating what a room needs
How much cooling a room needs depends on which way it faces, the size of the windows, the insulation, the number of people and the equipment giving off heat. The watts-per-square-metre rules found online come from retailers and do not agree with each other; no official source we checked sets one. The calculator therefore offers no default figure.
If your installer or a heat-gain survey gives you a figure, enter it with the floor area or volume: 20 m² at 100 W/m² is 2,000 W, so 2 kW or 6,824 BTU/h.
Special cases
- Multi-split systems: add up the need for each room, then compare it with the outdoor unit and with each indoor unit.
- Portable units: the BTU rating converts in exactly the same way.
- Server or plant rooms: the load comes mostly from the heat of the equipment, added in watts, because every watt of electricity used ends up as a watt of heat to remove.
Common mistakes
- Reading 9,000 BTU as an amount of energy: it is a rate, 9,000 BTU every hour.
- Converting the wrong way: multiply BTU/h by 0.2930711 to get watts, or divide BTU/h by 3,412 to get kW.
- Mixing up cooling capacity with electrical power input.
A well-insulated room needs less cooling and less heating: the insulation thickness has its own calculator. Running costs come from the electricity cost calculator, and ventilation rates are worked out separately.
Method and sources
References used:
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