Power units: kW, hp, and PS

Power measures how quickly energy is transferred. Car brochures, motor nameplates, and generator specs still juggle kilowatts (kW), mechanical horsepower (hp), and metric horsepower (PS / cv). They describe the same idea with different historical definitions — close, but not identical.

Definitions

• Watt / kilowatt — SI: 1 kW = 1,000 joules per second.
• Mechanical horsepower (hp) — about 745.7 W; common in US/UK marketing.
• Metric horsepower (PS) — exactly 735.49875 W in the usual metric definition; widespread on continental European engine plates.

Because PS and hp differ by roughly 1.4%, converting with the wrong horsepower unit adds a small but real error when you compare datasheets across regions.

Use the converter

Our Power converter keeps kW, hp, and PS synchronised through watts. For energy totals (kWh, joules, BTU) use the Energy converter instead.

Worked examples

A traction motor rated 150 kW is about 201 hp or 204 PS. An engine advertised at 100 PS is roughly 98.6 hp or 73.5 kW. Peak versus continuous ratings can differ — always compare like with like (same test standard, same measurement point).

Where ratings mislead

• Crankshaft vs wheel power — drivetrain losses sit between them.
• Brake horsepower (bhp) — still horsepower, measured under a particular setup.
• Boiler horsepower — a different historical steam unit; not the mechanical hp here.

Common mistakes

• Assuming PS = hp numerically.
• Buying “more horsepower” without checking torque and gearing for the job.
• Confusing a kW power rating with kWh energy cost.

FAQs

Which figure should EU car buyers trust?

Many EU materials quote PS or kW; convert explicitly when reading US hp claims.

Is electric motor kW “the same” as petrol hp?

Same physical dimension (power), but delivery curves and gearing differ — apples-to-apples comparisons need full charts.

Where does fuel economy fit?

Economy is energy per distance — see Travel and Trip Fuel.

Nameplates and marketing

Electric motors often list rated kW alongside efficiency class and RPM. Peak power for a few seconds can exceed continuous rated power; EV launch figures sometimes highlight the peak. Petrol and diesel brochures historically preferred hp or PS; the same engine can be legal-quoted in kW in one market and hp in another. Convert with our Power tool when comparing, then check whether the figure is DIN, SAE, ECU-limited, or wheel-measured.

Human-scale intuition

A healthy adult’s sustained mechanical output is only a fraction of a kilowatt. A compact kettle is about 2 kW of electrical power while boiling. Those contrasts explain why small efficiency gains matter on continuous industrial motors but “huge horsepower” on a car is still swamped by aerodynamic drag at motorway speed.

Sources

SI watt definition; conventional metric PS value used in automotive literature; mechanical horsepower engineering constant ≈ 745.7 W.