What this calculator does
Power is energy per unit time, and the watt is one joule per second. Everything else on this list is a multiple of it or an older unit that predates it. Kilowatts and megawatts do most of the metric work, and horsepower survives almost entirely in engine ratings.
There are two horsepowers and they are not the same. Mechanical horsepower, the one used in Britain and the United States, is about 745.7 watts. Metric horsepower, written PS or CV and used across much of Europe, is about 735.5 watts, roughly 1.4% smaller. The same engine therefore has a higher number in PS than in hp, which is worth knowing when comparing two brochures.
The formula
Every unit has a fixed factor in watts and the conversion multiplies then divides through that base. The metric prefixes are exact powers of ten. The two horsepower figures come from their historical definitions, mechanical horsepower from 550 foot-pounds per second and metric horsepower from 75 kilogram-force metres per second, which is why neither lands on a round number of watts.
| Term | Meaning |
|---|---|
| Watt (W) | One joule per second, the SI unit of power. |
| Horsepower (hp) | Mechanical or imperial horsepower, about 745.7 W, from 550 foot-pounds per second. |
| PS | Metric horsepower, about 735.5 W, also written CV or pk. About 1.4% smaller than mechanical horsepower. |
| BTU/h | British thermal units per hour, about 0.293 W, the unit air conditioning and heating equipment is usually rated in. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The power value to convert. |
| From | The unit it is currently in. Check whether a horsepower figure is hp or PS before choosing. |
| To | The unit you want the answer in. |
When to use it
Comparing engine or motor ratings
Brochures quote kW, hp or PS depending on the market, and the two horsepower units differ enough that a direct comparison of the numbers is misleading.
Sizing heating or cooling equipment
Air conditioners are rated in BTU per hour in some markets and kilowatts in others. A 12,000 BTU/h unit is about 3.5 kW, and knowing that makes the two catalogues comparable.
Reading an electrical specification
Appliance and generator ratings are in watts or kilowatts, and converting a horsepower figure for a pump or compressor is the usual first step in sizing the supply.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What is one of each power unit in kilowatts?
A single unit of each kind, converted to kilowatts to compare their scale.
| From | In kilowatts |
|---|---|
| Watt | 0.001 kW |
| Megawatt | 1,000 kW |
| Horsepower (mechanical) | 0.7457 kW |
| Metric horsepower (PS) | 0.7355 kW |
| BTU per hour | 0.00029307 kW |
Converting 150 kW to the other power units
A single power figure of 150 kilowatts expressed in each of the other units.
| To | Result |
|---|---|
| Watts | 150,000 W |
| Horsepower (mechanical) | 201.1532788 hp |
| Metric horsepower (PS) | 203.942896 PS |
| BTU per hour | 511,821.3675 BTU/h |
Questions
How many watts in a horsepower?
About 745.7 for mechanical horsepower and about 735.5 for metric horsepower (PS). Neither is round, because both were defined from older force and distance units rather than from the watt.
What is the difference between hp and PS?
PS is metric horsepower and is about 1.4% smaller than mechanical horsepower, so a given engine has a slightly higher figure in PS. The two are frequently treated as interchangeable in casual use, which introduces a small but systematic error when comparing cars from different markets.
How do I convert BTU/h to kilowatts?
Divide by 3,412. A 12,000 BTU/h air conditioner is about 3.5 kW, and a 24,000 BTU/h unit about 7.0 kW. Note this is cooling capacity rather than electrical consumption, which is several times lower.
What is the difference between power and energy?
Power is the rate, energy is the amount. A kilowatt is a rate of use; a kilowatt hour is what you get by sustaining that rate for an hour, and it is energy that appears on a bill. Confusing the two is the most common error in reading an electricity statement.
For energy rather than the rate it is delivered at, see the energy converter. For pressure, another quantity where imperial and metric units sit side by side, see the pressure converter.