Power is the rate at which energy is used or delivered: how fast a motor does work, a heater produces heat or an air conditioner removes it. Cars are rated in horsepower (or in kilowatts and PS in Europe), appliances in watts, furnaces and air conditioners in BTU per hour or tons, and power plants in megawatts. This converter relates all of them with exact definitions, and it also shows how much energy a device at that power uses in one hour and in a full day.
How to use the power converter
- Enter the power in Value.
- Choose its unit in From and the unit you need in To.
- The tape shows the result, the factors in both directions and the energy used in 1 hour and 24 hours, in kilowatt-hours.
- The table lists the power in all 14 units.
Power conversion formula
The watt, one joule per second, is the base unit:
The horsepower family is where most confusion lies:
- Mechanical horsepower (hp): 550 ft·lbf/s = 745.69987 W. This is James Watt’s unit and the one used for US engine ratings.
- Metric horsepower (PS, CV, ch): 75 kgf·m/s = 735.49875 W, used for European and Japanese cars.
- Electrical horsepower: exactly 746 W, used on electric motor nameplates.
- Boiler horsepower: about 9,809.5 W, an old rating for steam boilers.
Energy follows from power by multiplying by time: E = P × t. A 1 kW device running for one hour uses 1 kWh.
Worked example: a 300 hp engine
A pickup truck’s engine is rated at 300 hp. In kilowatts and metric horsepower:
300 hp × 0.74569987 = 223.71 kW
223.71 kW ÷ 0.73549875 = 304.16 PS
Because metric horsepower is slightly smaller, the same engine sounds about 1.4% more powerful in PS. If it somehow ran at full output for an hour, it would deliver 223.7 kWh of work.
Common power conversions
| From | To | Multiply by |
|---|---|---|
| horsepower (mechanical) | kilowatts | 0.7456999 |
| kilowatts | horsepower | 1.3410221 |
| metric horsepower (PS) | kilowatts | 0.73549875 (exact) |
| horsepower | PS | 1.0138697 |
| kilowatts | BTU per hour | 3,412.1416 |
| watts | BTU per hour | 3.4121416 |
| tons of refrigeration | kilowatts | 3.5168528 |
| tons of refrigeration | BTU per hour | 12,000 (exact) |
| horsepower | ft·lbf/s | 550 (exact) |
Power ratings in daily life
| Device | Typical power |
|---|---|
| LED bulb | 8–12 W |
| Laptop charger | 45–100 W |
| Microwave oven | 1,000–1,200 W |
| Space heater | 1,500 W (5,118 BTU/h) |
| Window air conditioner | 5,000–12,000 BTU/h (0.4–1 ton) |
| Central air conditioner | 2–5 tons (24,000–60,000 BTU/h) |
| Family car engine | 100–200 kW (135–270 hp) |
| Large wind turbine | 2–5 MW |
A 1,500 W heater produces 1,500 × 3.412 = 5,118 BTU/h, which is why portable heaters are often advertised as “5,100 BTU.” Electric resistance heat converts every watt into heat, so the two figures describe the same output.
Tips for reading power labels
- Input versus output. A motor’s nameplate horsepower is mechanical output; the electrical input is higher because of losses. A 1 hp motor at 85% efficiency draws about 877 W.
- Peak versus continuous. Audio amplifiers and generators often advertise peak power, which can be double the continuous rating.
- BTU means BTU/h. On heating and cooling equipment, “BTU” is shorthand for BTU per hour.
To find power from work and time, use the power calculator; for watts from volts and amps, the Ohm’s law calculator. To convert an amount of energy rather than a rate, switch to the energy converter.
Frequently asked questions
How many watts are in one horsepower?
Mechanical horsepower, the usual US rating, is about 745.7 W: it is defined as 550 foot-pounds per second. Metric horsepower (PS or CV) is 735.5 W, and electrical horsepower is exactly 746 W.
How do I convert kW to hp?
Multiply kilowatts by 1.341022 to get mechanical horsepower, or by 1.359622 for metric horsepower (PS). A 150 kW electric motor is about 201 hp or 204 PS.
What does a ton of refrigeration mean?
It is a cooling rate of 12,000 BTU per hour, about 3.517 kW. The name comes from the heat needed to melt one short ton of ice in 24 hours. A 3-ton home air conditioner removes about 36,000 BTU/h.
Are BTU and BTU/h the same?
No. BTU is an amount of energy; BTU per hour is a rate, or power. Air-conditioner and heater ratings labeled simply "BTU" almost always mean BTU per hour.