Power units and their equivalents
| Unit | Symbol | In watts |
|---|---|---|
| Watt | W | 1 Wbase unit |
| Kilowatt | kW | 1,000 W |
| Megawatt | MW | 1,000,000 W |
| Mechanical horsepower | hp | 745.7 W |
| Metric horsepower | PS | 735.499 W |
| BTU per hour | BTU/h | 0.293071 W |
| Kilocalorie per hour | kcal/h | 1.163 W |
| Milliwatt | mW | 0.001 W |
| Gigawatt | GW | 1,000,000,000 W |
| Foot-pound per second | ft·lbf/s | 1.35582 W |
Frequently asked questions
What is the difference between hp and PS?
The metric horsepower, written ch in France, CV in Spain and Italy, PS in Germany, is 735.49875 W, the power needed to lift 75 kilograms by one metre in one second. The mechanical horsepower is 745.6998716 W, defined on 550 foot-pounds per second. A car rated 150 hp is 152 PS, and one rated 150 PS is 148 hp.
How do I go from horsepower to kilowatts?
Divide by 1.36 for metric horsepower, by 1.34 for mechanical horsepower. 100 PS is 73.5 kW, 100 hp is 74.6 kW. European registration documents state power in kilowatts precisely to avoid the ambiguity, and the tax rating printed beside it is not a power unit at all but a taxation band.
What is BTU per hour used for?
Sizing air conditioners and heat pumps in North American and Asian markets. A 12,000 BTU/h unit delivers 3.5 kW, a figure often called one ton of cooling, inherited from the power needed to melt a short ton of ice in 24 hours.
What is the difference between a kilowatt and a kilowatt-hour?
The kilowatt is a rate, the kilowatt-hour a quantity. A 2,000 W kettle running for half an hour uses 1 kWh, and the kilowatt-hour is what the supplier bills; the kilowatt only states what the installation draws at a given instant. A solar array rated 3 kWp quotes a peak power, not an output: what it produces over a year is counted in kilowatt-hours.
Why is a generator rated in kVA rather than kW?
Because the volt-ampere measures apparent power, what the cable and the breaker have to carry, while the watt measures the power actually turned into heat or motion. The ratio between them is the power factor: a 5 kVA generator delivers only 4 kW at a factor of 0.8, a common figure as soon as motors are involved. On a heater the factor is 1 and the two numbers coincide, which is why French domestic supply contracts are sold in kVA, 6, 9 or 12.
Two horsepowers on one spec sheet
The horsepower is not one unit but two, and the gap shows on every car spec sheet. Metric horsepower, written ch in France, PS in Germany, CV in Italy and Spain, is 735.49875 watts: the power that lifts seventy-five kilograms by one metre in one second. British horsepower is 745.6999 watts, that is five hundred and fifty foot-pounds per second. One point four per cent separates them, enough to turn a 300 ch car into a 296 hp one without a single bolt moving.
James Watt set the first of those figures himself, by measuring what a brewery horse lifted while turning a mill: he wanted to sell his engines by promising how many animals they replaced. Then comes the costliest confusion in the field, between the watt and the watt-hour. The first is a power, a rate of energy; the second a quantity of energy. A 2000 watt heater left on for three hours uses six kilowatt-hours, and it is that second quantity, never the first, that the electricity supplier bills.