Power Converter (W, kW, hp, BTU/h)
Convert power between watts, kilowatts, megawatts, mechanical and metric horsepower, BTU per hour, and kilocalories per hour.
Power conversion reference
| 1 unit | W | kW | hp | BTU/h |
|---|---|---|---|---|
| 1 mW | 0.001 | 0.000001 | 0.00000134102 | 0.00341214 |
| 1 W | 1 | 0.001 | 0.00134102 | 3.41214 |
| 1 kW | 1 000 | 1 | 1.34102 | 3 412.14 |
| 1 MW | 1 000 000 | 1 000 | 1 341.02 | 3 412 140 |
| 1 hp | 745.7 | 0.7457 | 1 | 2 544.43 |
| 1 PS | 735.499 | 0.735499 | 0.98632 | 2 509.63 |
| 1 BTU/h | 0.293071 | 0.000293071 | 0.000393015 | 1 |
| 1 kcal/h | 1.163 | 0.001163 | 0.00155961 | 3.96832 |
Converting power units
Power is the rate of energy transfer — energy per unit time. The SI unit is the watt (one joule per second), but engines are rated in horsepower and heating and air-conditioning equipment in BTU per hour, so converting between them comes up constantly.
The two horsepowers
There are two in common use and they differ slightly. Mechanical horsepower (the one usually meant in the US and UK) is 745.7 W, while metric horsepower (PS, used across much of Europe) is 735.5 W — about 1.4% smaller. A car quoted at 150 PS is roughly 148 hp, which is why the same engine can carry two slightly different numbers.
Heating and cooling
Air-conditioners and boilers are often rated in BTU per hour, where 1 BTU/h ≈ 0.293 W. A 12 000 BTU/h air-conditioner is therefore about 3.5 kW of cooling. Confusingly, that same 12 000 BTU/h is also called "one ton" of cooling in the US trade.
Worked example
A 2 kW kettle is 2000 / 745.7 ≈ 2.68 hp, or about 6824 BTU/h. A 100 hp engine is 100 × 745.7 = 74.6 kW. To turn power into the energy actually used over time, multiply by the hours — theenergy converter handles the units.
Common questions
- Is horsepower the same everywhere?
- No, and this trips people up on car specifications. Mechanical horsepower (the British and American one) is 745.7 W, defined as 550 ft·lbf per second. Metric horsepower — written PS, CV or pk — is 735.5 W, about 1.4% smaller. A car quoted at 150 PS is about 148 hp, so the same engine gets two slightly different numbers depending on the market.
- What is the difference between watts and watt-hours?
- A watt is a rate; a watt-hour is an amount. A 100 W bulb burning for 10 hours uses 1,000 Wh, or 1 kWh — which is what an electricity meter counts. Confusing the two is the commonest error in energy arithmetic: a battery is rated in watt-hours because it stores a quantity, while a charger is rated in watts because it delivers a rate.
- What is BTU per hour, and why do air conditioners use it?
- It is a rate of heat flow — one BTU is the energy to raise a pound of water by one degree Fahrenheit, and BTU/h is that per hour. It persists in North American heating and cooling because the industry standardised on it long ago. For conversion, 1 W is about 3.412 BTU/h, so a 12,000 BTU/h air conditioner is roughly 3.5 kW of cooling.
- Why is an air conditioner's power rating not what it draws?
- Because the two numbers measure different things. The BTU/h figure is the heat moved; the electrical rating is the power consumed. A heat pump moves several times more heat than it consumes in electricity — that is the point of it — so a unit rated 12,000 BTU/h of cooling might draw only 1.2 kW. The ratio between them is the efficiency, quoted as SEER, EER or COP.