Watts to BTU per hour
Convert watts to BTU per hour. Multiply by 3.4121, with air conditioner sizing tables and what a "ton" of cooling means.
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The formula
BTU/h = watts × 3.412141633A BTU is 1055.05585262 joules, and an hour is 3600 seconds, so one BTU per hour is 0.29307107 watts. The reciprocal is the factor above.
Both are units of power. BTU/h is energy per unit time, exactly as a watt is; the “per hour” is what makes it comparable, and it is the part advertising leaves out.
Air conditioner sizing
This is the conversion’s job. Air conditioners are rated in BTU/h in the US, Hong Kong and much of Asia, and in kilowatts in Europe, Japan and Taiwan:
| BTU/h | kW | Suits |
|---|---|---|
| 5000 | 1.47 | small bedroom, ~10 m² |
| 7000 | 2.05 | bedroom, ~14 m² |
| 9000 | 2.64 | living room, ~18 m² |
| 12,000 | 3.52 | large room, ~25 m² |
| 18,000 | 5.28 | open-plan, ~35 m² |
| 24,000 | 7.03 | whole floor, ~45 m² |
| 36,000 | 10.55 | small commercial |
Area guidance is rough — insulation, glazing, ceiling height, floor level and how much sun a room gets all move it, sometimes by half.
The “ton” of cooling
12,000 BTU/h = 1 ton of refrigeration = 3.517 kW.
The unit is literal: it is the rate of cooling produced by a ton of ice melting over 24 hours. It survives as a sizing convention in the US and in commercial HVAC:
| Tons | BTU/h | kW |
|---|---|---|
| 0.5 | 6000 | 1.76 |
| 1 | 12,000 | 3.52 |
| 1.5 | 18,000 | 5.28 |
| 2 | 24,000 | 7.03 |
| 3 | 36,000 | 10.55 |
| 5 | 60,000 | 17.58 |
Cooling output is not electricity consumed
The rating is how much heat the unit moves, not how much power it draws. A heat pump moves several times more heat than the energy it consumes, which is the entire point of one:
| Cooling output | Typical draw at COP 3.2 |
|---|---|
| 2.64 kW (9000 BTU/h) | 0.83 kW |
| 3.52 kW (12,000 BTU/h) | 1.10 kW |
| 5.28 kW (18,000 BTU/h) | 1.65 kW |
So a “12,000 BTU” unit does not add 3.5 kW to the electricity bill. Efficiency is published as COP or EER — in Taiwan as CSPF, in Japan as APF — and the higher it is, the smaller the draw for the same cooling.
Quick reference
| Watts | BTU/h |
|---|---|
| 100 W | 341 BTU/h |
| 500 W | 1706 BTU/h |
| 1000 W | 3412 BTU/h |
| 1500 W | 5118 BTU/h |
| 2000 W | 6824 BTU/h |
| 3000 W | 10,236 BTU/h |
Estimating in your head
Multiply by 3.4, or triple it and add 40%. 1000 W × 3.4 is 3400 against a true 3412. Going the other way, divide BTU/h by 3.4 — 12,000 ÷ 3.4 is 3529 W against a true 3517.