Convert power (watt, kilowatt, horsepower)

The value is converted into watts, then into the target unit. The split between metric and mechanical horsepower is why the same car is quoted at 150 ch in Europe and about 148 hp in an American brochure. French fiscal horsepower is not a physical power and therefore does not appear here.

P(W) = x × kfrom ; y = P(W) / kto

1 metric hp = 735.49875 W

1 mechanical hp = 745.6999 W

The watt is the SI unit of power, one joule per second. Metric horsepower comes from a historical definition: raising 75 kilograms-force by one metre in one second. Mechanical horsepower is based on the foot-pound. Both survive, which is why they are listed separately.

Value

The power alone, without the unit.

From

Starting unit. The default is metric horsepower, used on European spec sheets.

To

Target unit. The default is the kilowatt, the legal unit.

Assumptions

• “ch” means metric horsepower (735.49875 W).
• “hp” means mechanical horsepower (≈ 745.7 W).
• Mechanical and electrical power are treated alike: the watt is common to both.

Limits

• French fiscal horsepower is not a physical power.
• No distinction between rated, peak and continuous power.
• Motor efficiency is not modelled: input power exceeds useful output.

A 150 hp car produces about 110 kW. A registration document lists that power in kilowatts alongside a fiscal rating computed by an administrative formula with no direct relation. For a household appliance, watts and kilowatts are enough: horsepower has no place there.

• Mixing metric and mechanical horsepower.

  The gap is about 1.4%: 150 metric hp is roughly 148 mechanical hp.

• Confusing fiscal horsepower with real power.

  Fiscal horsepower serves a tax calculation, not a description of the engine.

• Treating power as consumption.

  A power in watts becomes energy only once multiplied by a duration.

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Written by Rédaction HexaCalc (editorial team). Content last updated: August 25, 2026. No third-party medical or financial review is claimed.

• NIST Guide to the SI, Appendix B: Conversion Factors — National Institute of Standards and Technology
• The International System of Units (SI) — Bureau international des poids et mesures (BIPM)