Power is energy per second
Power tells how fast energy is used or delivered. One watt is one joule per second, so a device rated 100 W uses 100 joules every second it runs. Different trades kept their own units: engines are rated in horsepower, heaters and air conditioners in BTU per hour, cooling systems in tons, and electrical equipment in watts and kilowatts. All of them measure the same thing and convert with a fixed factor.
How to use this converter
Type a value in any field and every other unit updates as you type. Start from the number you have: an engine rating from a brochure, the BTU figure on an air conditioner box, or the wattage on an appliance plate. Each common pair below has its own page with the formula, a worked example and a chart.
Key equivalents
- 1 kW = 1.341 hp = 1.360 PS
- 1 hp = 745.7 W, and 1 PS = 735.5 W
- 1 W = 3.412 BTU/h, and 1 kW = 3,412 BTU/h
- 1 ton of refrigeration = 12,000 BTU/h ≈ 3.517 kW
- 1 kcal/h ≈ 1.163 W
Two horsepowers
The horsepower was invented in the eighteenth century to sell steam engines by comparing them with the horses they replaced. Britain and the US kept the mechanical horsepower of 550 foot-pounds per second, while continental Europe defined its own metric version from 75 kilogram-meters per second. The gap is small, about 1.4%, but it explains why the same car shows slightly different figures in American and European brochures. Today the official rating in the EU is in kilowatts, with PS or ch shown alongside.
Heating and cooling capacity
A heater or air conditioner moves heat, and its capacity is a power. In the US it is written in BTU per hour, often shortened to BTU on the box. Larger systems use the ton of refrigeration, worth 12,000 BTU/h. Remember that cooling capacity is not electric consumption: a heat pump or air conditioner moves two to four times more heat than the electricity it draws.
Power units compared
| Unit | Symbol | In watts |
|---|---|---|
| Watt | W | 1 |
| Kilowatt | kW | 1,000 |
| Megawatt | MW | 1,000,000 |
| Horsepower | hp | 745.7 |
| Metric horsepower | PS | 735.499 |
| BTU per hour | BTU/h | 0.293071 |
| Ton of refrigeration | TR | 3,516.85 |
| Kilocalorie per hour | kcal/h | 1.16222 |
The units in detail
What is a watt?
The watt is the SI unit of power: one joule of energy per second. Light bulbs, chargers and small appliances are rated in watts; an LED bulb draws about 9 W and a hair dryer up to 1,800 W.
What is a kilowatt?
The kilowatt equals 1,000 watts. It rates electric heaters, car engines outside the US, solar installations and home electrical service. Running 1 kW for one hour uses 1 kilowatt-hour of energy.
What is a megawatt?
The megawatt equals one million watts. It measures power plants and large wind turbines: a modern onshore turbine produces 3 to 6 MW at full output, and a nuclear reactor around 1,000 MW of electricity.
What is one horsepower?
Mechanical horsepower, defined as 550 foot-pounds per second, equals about 745.7 watts. It is the unit of US and UK car engines, lawn mowers and boat motors. The word stays the same in the plural.
What is one metric horsepower?
Metric horsepower, 735.5 watts, is the power that lifts 75 kilograms by one meter in one second. German brochures write it PS and French ones ch. It is about 1.4% smaller than mechanical horsepower.
What is one BTU per hour?
BTU per hour is the US unit for heating and cooling capacity, about 0.293 watt. Window air conditioners range from 5,000 to 15,000 BTU/h, and the label often drops the per hour and simply says BTU.
What is a ton of refrigeration?
A ton of refrigeration is 12,000 BTU per hour, about 3.517 kW of cooling. It comes from the heat needed to melt one short ton of ice in a day. Central air systems for homes are sold in sizes of 1.5 to 5 tons.
What is a kilocalorie per hour?
The kilocalorie per hour, about 1.163 watts, appears in older European boiler specifications and in exercise physiology. A resting adult gives off roughly 80 to 100 kcal/h, close to a 100 W light bulb.