How many watts is a horsepower? Converting power units
A car's spec sheet says "300 hp," a microwave says "1,000 W," and an air conditioner says "12,000 BTU/hour." All three describe power — how fast energy is being delivered or converted — but in units that don't share a common root, so none of those numbers can be compared at a glance.
The method
Convert everything to watts, the SI base unit for power. The fixed relationships: 1 mechanical horsepower = 745.699872 watts, 1 kilowatt = 1,000 watts, and 1 BTU/hour = 0.29307107 watts (a BTU/hour is a rate — one BTU of energy delivered every hour — not the same thing as a plain BTU, which is an amount of energy). To convert A to B: multiply A by its watts-per-unit, then divide by B's watts-per-unit.
A real example: a 300 hp engine in kilowatts
Multiply by the watts-per-horsepower constant: 300 × 745.699872 = 223,709.9616 watts. Divide by 1,000 to get kilowatts: 223,709.9616 ÷ 1,000 = 223.71 kW. A 300 hp engine is putting out about 223.7 kW — the number European car listings usually quote instead of horsepower.
A real example: a 12,000 BTU/hour window AC unit in watts
Multiply by the watts-per-BTU/hour constant: 12,000 × 0.29307107 = 3,516.85 watts, or 3.52 kW. "12,000 BTU" window units (often marketed as "1-ton" in HVAC shorthand) draw roughly 3.5 kW of cooling capacity — note this is cooling output, not the electrical power the unit draws from the wall, which is lower because an AC moves heat rather than generating it.
A real example: converting a 2.2 kW motor rating to horsepower
Convert kW to watts first: 2.2 × 1,000 = 2,200 watts. Divide by the watts-per-horsepower constant: 2,200 ÷ 745.699872 = 2.95 hp. A "2.2 kW" motor nameplate — common on European power tools and pumps — corresponds to just under 3 hp, which is why a tool rated 3 hp in the US and one rated 2.2 kW in the EU are, for practical purposes, the same machine.
Why there's more than one "horsepower"
745.7 W is mechanical horsepower, the figure most engines and tools use. Metric horsepower (PS, used on some European spec sheets) is defined slightly differently at about 735.5 W, and electrical horsepower and boiler horsepower are different again. The gap between them is under 2% — small enough to ignore for a quick estimate, large enough that a precise engineering comparison should confirm which definition a spec sheet is using before treating two "horsepower" figures as equal.
Run your own numbers
The free Power Converter converts between watts, kilowatts, horsepower, megawatts and BTU/hour instantly, entirely on your device.