Three horsepower, one word
Like the ton, horsepower is several different units sharing a name — and unlike the ton, most people meet the difference without ever being told it exists.
Mechanical horsepower(hp) is 550 foot-pounds per second, which works out at 745.699872 watts. This is what “horsepower” means in the United States and the United Kingdom.
Metric horsepower — written PS in Germany, CV in France and Spain, pk in the Netherlands — is 75 kilogram-force metres per second, or 735.49875 watts. It is about 1.4% smaller, and it is the figure on European car specifications.
So a car sold as 150 PS in Germany is 148 hp in Britain, and both figures describe the same engine correctly. The gap is small enough to look like rounding and large enough to matter when comparing two cars quoted in different markets — or when a figure is copied between them without conversion.
A third, electrical horsepower, is defined as exactly 746 W for motor ratings, and boiler horsepower is a different quantity again at about 9,810 W. Neither is offered here, because they belong to specific industries that state which they mean.
The unit that avoids all of this is the kilowatt, which is why EU type approval states engine power in kW and lets PS appear as a supplementary figure. A specification quoting kW is unambiguous in every market.
What a rating actually promises
A power figure is a measurement made under a test standard, and the standard matters as much as the unit. Engine outputs have been quoted under SAE gross, SAE net, DIN, ECE and JIS at various times, and they are not interchangeable.
The starkest case is American figures from before about 1972, quoted under SAE gross: measured on a test stand without the alternator, water pump, exhaust system or air cleaner fitted. The same engine measured as installed — SAE net, the modern standard — often reads 20% lower. A muscle-car horsepower figure and a modern one are not comparable, and the engine did not change.
Electrical equipment carries its own version of the problem. A device rated in volt-amperes is quoting apparent power, which equals real power in watts only when the power factor is 1. A UPS rated 1,500 VA at a 0.6 power factor delivers 900 W, so sizing one against a watts figure without checking is a 40% error.
And most ratings are maxima. Continuous output is lower, sometimes considerably, for anything that heats up — which is why generators quote both and why a “peak” figure on consumer equipment should be read as marketing rather than specification.
Air conditioning and BTU per hour
Heating and cooling capacity is rated in BTU per hour in the United States and in kilowatts almost everywhere else, and the two appear on the same product in different markets. 12,000 BTU/h is 3.517 kW — and it is also, by a historical accident, one “ton” of refrigeration, being the rate at which a short ton of ice melts over 24 hours.
That gives a third unrelated meaning to the word ton, in a field that also uses BTU per hour and kilowatts. A 2-ton air conditioner is 24,000 BTU/h or about 7 kW, and has nothing to do with the weight of anything.
One distinction worth carrying into any comparison: the BTU/h or kW figure is output capacity, not the electricity the unit consumes. A modern air conditioner delivers several times more cooling than it draws in power — the ratio is its SEER or COP — so a 3.5 kW cooling unit might consume 1 kW. Reading the capacity as consumption overstates running cost by a factor of three or more.
Power against energy on a bill
Power is a rate; energy is an amount; a bill charges for the amount. A watt is one joule per second, and multiplying a power by a time gives an energy — which is what a kilowatt-hour is, and why the unit has a time in its name.
So an appliance’s wattage tells you how fast it uses energy, not how much it will cost. A 2,000 W kettle boiling for three minutes uses 0.1 kWh; a 40 W fridge running continuously uses about 0.96 kWh a day. The fridge has a twentieth of the power rating and uses nearly ten times the energy, because it never stops.
The arithmetic for a running cost is watts ÷ 1,000 × hours × your unit rate. Keeping those as three separate steps makes the tariff visible, which matters more than the conversion when the tariff changes.
Doing it in a spreadsheet
Write the factors out, because there is no unit code that distinguishes the two horsepower: =A1*0.745699872 for mechanical hp to kW, =A1*0.73549875 for metric PS to kW, =A1/0.745699872 and =A1/0.73549875 to come back, and =A1*0.29307107 for BTU/h to watts.
Excel’s CONVERT has "HP" for mechanical horsepower and "PS" for metric — worth knowing, and worth not relying on, since the two codes look interchangeable and are not.
If a sheet mixes markets, put the horsepower convention in the column header rather than trusting the reader to infer it. A column headed simply “hp” that contains both British and German figures is out by 1.4% on an unknown subset of its rows, which is the kind of error that never announces itself.
Sources and methodology
Nothing here is fetched and there is no data feed. Both horsepower are exact derived figures rather than measurements — mechanical from the 1959 pound and standard gravity, metric from 75 kilogram-force metres per second — so the difference between them is a definition. The sources settle those definitions and the EU rule that states engine power in kilowatts, which is why a European specification carries two numbers.