Watts, Horsepower and BTU/hr: A Plain-English Guide to Power Conversion

By the Super Simple Digital Tools Team · Updated June 2026 · Converters

Power describes how quickly energy is delivered, and almost every industry invented its own unit to measure it. Electricians and physicists use watts and kilowatts, the auto world clings to horsepower, and the heating and cooling trade speaks in BTU per hour. None of these is more correct than the others; they are simply different rulers for the same thing. Once you accept that, conversion stops being intimidating, because every unit ties back to the watt through a fixed, well-defined factor.

The cleanest way to think about conversion is to picture the watt as a central hub. To go from any unit to any other, mentally pass through watts first. Kilowatts are just 1,000 watts. Mechanical horsepower is 745.7 watts, metric horsepower is 735.5 watts, and BTU per hour is 0.293 watts each. Multiply or divide to reach watts, then convert outward to your target. This tool does both hops in one step, but knowing the hub model helps you sanity-check whether a result looks reasonable.

The single biggest pitfall is the word horsepower. American and British engineers mean mechanical horsepower (745.7 W), while German, French, Italian and Japanese specs usually mean metric horsepower, written PS, CV, or pk (735.5 W). The two differ by about 1.4 percent, which is why a European car advertised as 150 PS shows up as 148 hp on a US spec sheet. If you are comparing engines across markets, always confirm which horsepower the source used before trusting the number.

Heating and cooling adds its own twist with the BTU per hour and the refrigeration ton. A ton of cooling is 12,000 BTU/hr, which is about 3.517 kW. So when you size a mini-split rated at 9,000 BTU/hr, you are looking at roughly 2.64 kW of cooling. Translating that into kilowatts is handy when you want to estimate the electrical draw or compare it against a heat pump quoted in kW, though remember that cooling capacity and electrical input are not the same number.

Finally, keep power and energy in separate mental boxes. A 1,500 W heater has a fixed power rating, but the energy it consumes depends on how long it runs: leave it on for two hours and it uses 3 kWh. This converter deals only with the rate side of that equation. When a bill, battery, or fuel figure is involved, you are dealing with energy, and you multiply power by time to bridge the two.

Quick tips

• Before converting any horsepower figure, check whether the source uses mechanical HP (745.7 W) or metric PS/CV (735.5 W) to avoid a 1.4 percent error.
• To size an air conditioner against a kW-rated heat pump, divide its BTU/hr rating by 3,412 to get kilowatts (12,000 BTU/hr is about 3.52 kW).
• Remember a refrigeration ton equals 12,000 BTU/hr, so a 2-ton unit is about 7.03 kW of cooling capacity.
• For energy or running-cost estimates, do not stop at power; multiply the watt or kW result by the number of hours of operation to get kilowatt-hours.

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