Horsepower to Kilowatts: Converting Power Units
A kilowatt is 1.34 horsepower, or near enough. The complication is that there is more than one horsepower. The Power Converter handles watts, kilowatts and horsepower.
Which horsepower
Mechanical horsepower, the British and American one, is 745.7 watts. It came from James Watt's estimate of what a working horse could sustain, which he arrived at by watching pit ponies lift coal.
Metric horsepower, used across continental Europe and written PS in German or CV in French and Italian, is 735.5 watts - about 1.4 per cent less.
So a car advertised at 150 PS in Germany is 148 hp in Britain. A small gap, and enough to make two spec sheets for the same car disagree. There is also an electrical horsepower, defined as exactly 746 watts, used in motor ratings.
Quick arithmetic: kilowatts to horsepower, multiply by 1.34. Horsepower to kilowatts, divide by 1.34 - or multiply by 0.75, which is close enough for a conversation.
Power is not torque
These get used interchangeably in car talk and they measure different things.
Torque is twisting force - how hard the engine turns the shaft. It is what you feel as the shove when pulling away. Power is the rate of doing work, which is torque multiplied by how fast it is turning.
So an engine can have a lot of torque and modest power if it only revs slowly - a diesel, or a tractor. Or modest torque and high power if it revs hard, which is a motorcycle engine. Neither number alone describes a car, which is why both are quoted, and the Torque Converter handles that side of it.
Where kilowatts have taken over
Electric cars are quoted in kilowatts almost everywhere, partly because the battery and charger are already measured that way and quoting one in horsepower would be an odd translation.
Which is also where the power-versus-energy distinction bites. A car's motor might be 150 kW - a rate. Its battery might be 60 kWh - an amount. The first says how fast it goes, the second how far, and they are completely different quantities measured in confusingly similar units. The Energy Converter covers the second.
Charger ratings are power too: a 7 kW home charger adds about 7 kWh an hour, so a 60 kWh battery takes something over eight hours from empty.
Common questions
How much is one horsepower really?
745.7 watts, mechanical. Watt's original estimate was generous - a real horse can sustain rather less for a full working day.
Is PS the same as hp?
Almost. PS is metric horsepower, 1.4 per cent smaller. Specifications quoting both will show slightly different figures.
What does a kilowatt feel like?
A domestic kettle is about 3 kW. A family car's engine is 75 to 150 kW, so roughly 25 to 50 kettles.
Why do electric cars feel faster than their power suggests?
Because an electric motor delivers its full torque from a standstill, while a petrol engine has to rev to reach its peak.
Got a power figure to convert?