How to use the Power Converter
- Enter the power rating you have.
- Choose its unit and the unit you want.
- Check whether a horsepower figure is mechanical (hp) or metric (PS) — they differ by about 1.4%.
- Use BTU per hour or tons of refrigeration for heating and cooling equipment.
- Multiply power by running time to get energy, then use the energy converter.
How the calculation works
Power is energy per unit time, and the SI unit is the watt: one joule per second. All conversions here run through the watt. The distinction from energy matters commercially — a 2 kW heater is a power rating, while the 12 kWh it consumes over six hours is energy, and only the latter appears on a bill.
Two horsepower definitions are in active use and they are not the same. Mechanical or imperial horsepower is 550 foot-pounds per second, or 745.70 W, and dominates in the US and UK. Metric horsepower, written PS or CV, is 75 kilogram-force metres per second, or 735.50 W, and is standard across continental Europe and Japan. A car advertised at 300 PS is about 296 hp — small, but enough to explain why the same model appears with different figures in different markets.
HVAC keeps its own units. A ton of refrigeration, 3,516.85 W, is the cooling rate that would freeze one short ton of water in 24 hours, a definition inherited from the ice trade. It equals 12,000 BTU per hour, which is why domestic air conditioners cluster around round BTU numbers that correspond to whole and half tons.
power_target = (P_source × W_per_source) ÷ W_per_target; 1 hp = 745.6999 W; 1 PS = 735.49875 W; 1 BTU/h = 0.29307 W; 1 ton refrigeration = 3,516.85 WSource: BIPM SI Brochure 9th edition; ISO 80000-4; ASHRAE Handbook — Fundamentals.
Worked example
A hot-hatch is listed at 261 PS in a German brochure and 4.5 tons of air conditioning is specified for an office.
- Enter 261 and select metric horsepower: 261 × 735.49875 = 191,965 W.
- Convert to mechanical horsepower: 191,965 ÷ 745.70 = 257.4 hp.
- For the office, enter 4.5 and select tons of refrigeration: 4.5 × 3,516.85 = 15,826 W.
- That is 15.8 kW, or 54,000 BTU per hour.
261 PS is 257 hp, and 4.5 tons of cooling is 15.8 kW — the same machines, described in two different traditions.
Frequently asked questions
How many watts in one horsepower?+
745.7 W for mechanical horsepower, 735.5 W for metric horsepower (PS). Roughly three-quarters of a kilowatt either way.
What is the difference between kW and kWh?+
kW is the rate of energy use; kWh is the amount consumed. A 2 kW appliance running one hour uses 2 kWh.
How many BTU per hour is one kilowatt?+
3,412 BTU/h. A 3.5 kW air conditioner is about 12,000 BTU/h, or one ton of cooling.
Why do European and US horsepower figures differ for the same car?+
Because metric horsepower is defined slightly smaller than mechanical horsepower, so the same engine produces a marginally higher PS number.
Last reviewed September 1, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.