Horsepower to Kilowatts: The Math Behind Any Power Rating
A car's spec sheet lists horsepower, a European appliance's label gives watts, an air conditioner is rated in BTU per hour, and a generator or motor might be rated in kilowatts. These are all the same kind of quantity — power, energy delivered per unit time — just scaled by different fixed multiples, so converting between them is one multiplication, done through a common base unit.
The method: convert through a fixed base unit
This tool uses watts as the base. Every other power unit is a fixed number of watts — one mechanical horsepower is always 745.699872 watts, one BTU per hour is always 0.29307107 watts, by definition. To convert a value from one unit to another: multiply by the source unit's watts-per-unit, then divide by the target unit's watts-per-unit.
answer = value × (source watts-per-unit) ÷ (target watts-per-unit)
That's the whole method. Horsepower to kilowatts, BTU/hour to watts — all the same formula, just a different pair of numbers.
The numbers
- 1 watt = 1 watt (the base)
- 1 kilowatt = 1,000 watts
- 1 horsepower = 745.699872 watts (mechanical horsepower)
- 1 megawatt = 1,000,000 watts
- 1 BTU/hour = 0.29307107 watts
A worked example: a 300 hp engine in kilowatts
Multiply by horsepower's 745.699872 watts: 300 × 745.699872 = 223,709.9616 watts. Divide by a kilowatt's 1,000 watts: 223,709.9616 ÷ 1,000 = 223.71 kW — the figure a European spec sheet would print for the same engine.
A second example: a 1,500-watt microwave in horsepower
Divide instead of multiply, since watts is already the base: 1,500 ÷ 745.699872 = 2.01 hp (2.0115… exactly). It's a useful gut check on how small "one horsepower" actually is in household terms — a countertop microwave already draws about two of them.
A third example: sizing an air conditioner, BTU/hour to kilowatts
A "5-ton" central air conditioner is rated at 60,000 BTU per hour. Multiply by 0.29307107 watts per BTU/hour: 60,000 × 0.29307107 = 17,584.2642 watts. Divide by a kilowatt's 1,000 watts: 17.58 kW — consistent with the HVAC rule of thumb that one ton of cooling runs about 3.5 kW (17.58 ÷ 5 ≈ 3.52 kW per ton). The same rating in horsepower: 17,584.2642 ÷ 745.699872 = 23.58 hp, the electrical draw a whole-house AC unit's compressor motor is working against.
A fourth example: a 100 kW motor, back to horsepower
100 kW is 100,000 watts. Divide by horsepower's 745.699872 watts: 100,000 ÷ 745.699872 = 134.10 hp — the figure a US spec sheet would print for a motor a European one lists in kilowatts.
A sanity check you can do without a calculator
One horsepower should convert to about 745.7 watts — a figure worth memorizing, since it's the single constant every other horsepower conversion in this tool runs through. If a conversion tool doesn't land close to that for 1 hp, something is wrong with the tool, not the fact.
What the tool adds
The numbers above are all you need, on paper, for any pair of power units. The tool's job is just speed and fewer mistyped digits: pick the two units, type the value, and it runs the same multiplication instantly across any of the five units it knows, entirely on your device.