How to Convert Horsepower to Kilowatts (and Other Power Units)
Where horsepower came from, why hp and PS are not quite the same, how BTU per hour fits in, and the difference between a kilowatt and a kilowatt-hour.
Power shows up on almost everything you buy that moves, heats or cools. A car brochure lists 150 hp or 110 kW, a space heater says 1,500 W, and an air conditioner promises 12,000 BTU. These are all the same physical quantity in different clothes. This guide explains where each unit comes from, how to move between them and how to avoid the classic trap of mixing up power and energy.
What power actually measures
Power is the rate of using or delivering energy: how much work gets done per second. The SI unit is the watt (W), one joule per second. A 60 W bulb turns 60 joules of electrical energy into light and heat every second. A 1,000 W microwave uses energy about 17 times faster.
Because power is a rate, every power unit is "energy per time" in disguise. Horsepower is foot-pounds per second, BTU per hour is heat per hour, and kilocalories per hour is food energy per hour. Converting between them always means going through the watt.
Where horsepower came from
In the late 1700s, James Watt wanted to sell steam engines to mine owners who used horses to pump water. He needed a way to say how many horses one engine could replace. He settled on a horse lifting 550 pounds by one foot every second, and that became the mechanical horsepower: 550 ft·lbf/s, or about 745.7 W.
Continental Europe later defined its own version in metric units: lifting 75 kilograms by one meter every second. That metric horsepower is exactly 735.49875 W and is written PS in German, CV in French, Italian and Spanish, and ch in some French documents. It is about 1.4% smaller than the mechanical horsepower, so the same engine has a slightly higher number in PS than in hp.
Today, car makers in Europe usually state power in kW with PS in brackets, while US and UK sources mostly use hp.
The conversion step by step
To convert any power unit, multiply by its size in watts, then divide by the size of the target unit in watts.
- 1 hp = 745.69987 W
- 1 PS = 735.49875 W
- 1 kW = 1,000 W
- 1 BTU/h ≈ 0.293071 W
- 1 ton of refrigeration = 12,000 BTU/h ≈ 3,516.85 W
- 1 kcal/h ≈ 1.162222 W
Worked example: comparing two car specs
You are comparing an American review that lists 150 hp with a European spec sheet that says 110 kW (150 PS). Are these the same engine?
- 150 hp × 745.69987 W = 111,855 W = 111.85 kW.
- 110 kW ÷ 745.69987 W = 147.51 hp.
- 110 kW ÷ 735.49875 W = 149.56 PS, which the brochure rounds to 150 PS.
So "150 hp" and "150 PS" are not the same: 150 PS is about 147.5 hp. The difference is small, but it explains why the same car can be listed with slightly different numbers in different markets.
A handy rule of thumb: kW × 1.34 ≈ hp, and hp × 0.75 ≈ kW. A 200 kW electric motor is about 268 hp; a 300 hp engine is about 224 kW.
Heating and cooling: BTU per hour
The British thermal unit is the heat needed to warm one pound of water by one degree Fahrenheit. Heaters and air conditioners in North America are rated in BTU per hour, often shortened to just "BTU".
- A 1,500 W space heater gives off about 5,118 BTU/h. Nearly all electricity in a resistive heater ends up as heat, so input and output are the same.
- A 12,000 BTU/h air conditioner moves about 3,517 W of heat. That is also called one ton of refrigeration, from the cooling power of melting a short ton of ice over 24 hours.
Note the word "moves". An air conditioner rated at 3.5 kW of cooling typically draws only 1 kW or so of electricity, because it pumps heat from inside to outside rather than creating cold. The ratio is the efficiency rating, and it varies by model, so check the label for the actual electrical input.
How to use the converter
The Power Converter handles eleven units: watts, kilowatts, megawatts, milliwatts, mechanical horsepower, metric horsepower, BTU per hour, tons of refrigeration, foot-pounds force per second, calories per second and kilocalories per hour. Type a value, pick the unit you have and the unit you want, and press Convert. The swap button flips the direction, and a table below shows the same power in every unit at once, which is handy when a spec sheet uses one unit and a forum uses another.
Power versus energy: the most common mix-up
A kilowatt (kW) is power, a rate. A kilowatt-hour (kWh) is energy, an amount. Running a 1 kW appliance for one hour uses 1 kWh; running it for 30 minutes uses 0.5 kWh. Your electricity bill charges for kWh, not kW.
The same logic links food and exercise. A person doing steady work at 100 W, for example on an exercise bike, is turning energy over at about 86 kcal per hour. Food labels state energy (kcal), and your body's rate of use is power (kcal per hour).
Good to know
- Engine power is measured at a specific engine speed. A "150 hp" rating is the peak, usually near the top of the rev range. Everyday driving uses much less.
- Other horsepowers exist. Electrical horsepower is 746 W and boiler horsepower is about 9,810 W. Engine specs almost always mean mechanical hp or metric PS.
- Wheel power is lower than engine power. Measurements on a dynamometer at the wheels come out lower than the factory figure because the drivetrain loses some energy.
- Rounding hides differences. Brochures round to the nearest whole number, so converting a rounded figure back may not give the exact original.
FAQ
Is bhp the same as hp?
Brake horsepower (bhp) is mechanical horsepower measured at the engine's output shaft, before losses in the gearbox and wheels. In the UK, "bhp" on a spec sheet is usually the same figure that US sources call hp.
How many watts does a human produce?
A fit adult can sustain roughly 100 to 200 W of mechanical power for an hour on a bike, and trained cyclists can hold more. Short sprints can reach well over 1,000 W for a few seconds, which is more than one horsepower.
Why do electric cars use kW instead of hp?
Electric motors are electrical machines, and their power is naturally measured in watts like the battery and charger. Many makers still give hp or PS as well, because buyers are used to comparing cars that way.
How many kilowatts is a 5,000 BTU air conditioner?
5,000 BTU/h of cooling is about 1.47 kW. The electricity it uses is lower, typically a few hundred watts, depending on its efficiency rating.
