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Power Converter
Convert watts, kilowatts, horsepower and BTU/h for motors, engines and appliances.
A car brochure boasts 150 hp. A washing machine label says 2,200 W. Neither number means anything next to the other until you convert them into the same unit, and the conversion factor is not the round number most people assume.
A horse was a real measurement once
James Watt needed a way to sell his steam engines to mine owners who were used to paying for horses. In the 1780s he timed a horse turning a mill wheel and calculated it could lift 33,000 pounds one foot in one minute. He called that rate one horsepower, and the unit stuck long after the horses left the mines.
That figure survives today as the imperial mechanical horsepower: 745.7 watts. A separate metric horsepower (used across continental Europe, denoted PS) is defined slightly differently and works out to 735.5 watts. The two are close enough to confuse people and different enough to matter in a precise spec sheet.
Common power units side by side
| Unit | In watts | Typical use |
|---|---|---|
| 1 watt (W) | 1 | Base SI unit — light bulbs, small electronics |
| 1 kilowatt (kW) | 1,000 | Appliances, electric motors, solar panels |
| 1 horsepower (hp, mechanical) | 745.7 | Engines, pumps, US-market vehicles |
| 1 metric horsepower (PS) | 735.5 | European car and machinery specs |
| 1 BTU per hour | 0.293 | Air conditioner and heater ratings, mostly in the US |
Where the mismatch actually bites
- Comparing a European car spec in PS against a US spec in hp — they differ by about 1.4%, small but not zero.
- Sizing an air conditioner: US units are rated in BTU/h, everywhere else in kW, and the two numbers look nothing alike for the same appliance.
- Reading an old industrial motor plate stamped in horsepower against a modern kW-rated replacement.
- Solar inverter shopping, where output is always in kW but consumption habits are often described in appliance-watts.
A number worth memorising
1 kW is roughly 1.34 hp. So a 100 kW electric motor is about 134 hp — useful for sanity-checking a conversion without reaching for a calculator, and close enough for comparing two rough specs before you need the exact figure.
Power conversion questions
How many watts are in one horsepower?
One mechanical horsepower equals 745.7 watts. The metric horsepower (PS), common in European specifications, equals 735.5 watts instead — close, but not identical, so check which one a spec sheet is using.
How do I convert kilowatts to horsepower?
Multiply the kilowatt figure by 1.341 to get mechanical horsepower. A 75 kW motor works out to roughly 100.6 hp.
Why do air conditioners use BTU per hour instead of watts?
It is largely a US convention carried over from older heating and cooling engineering, where BTU (British Thermal Unit) was the standard measure of heat energy. One BTU/h equals about 0.293 watts, so a 12,000 BTU/h unit is roughly 3.5 kW.
Is 1 hp exactly 746 watts?
Close but not exact. The precise mechanical horsepower is 745.699872 watts, commonly rounded to 746 or 745.7. For everyday conversions the rounding makes no practical difference.
Why do car adverts sometimes show two different hp figures?
Because manufacturers sometimes quote metric horsepower (PS) and imperial horsepower (hp) side by side, or convert between measurement standards like SAE and DIN, which test engine output slightly differently. Always check which standard is being quoted before comparing two cars.
