Power and Weight
PS, hp and kW: Three Numbers for One Engine
500 PS is 493 hp and 368 kW, and the difference is a unit rather than a rounding error. The measurement standard behind the figure shifts it by another 1.4 per cent.
The same engine carries three different numbers because PS, horsepower and kilowatts are three different units, and a fourth source of difference sits underneath them: the standard the measurement was corrected to. 500 PS is 493 hp and 368 kW. Measure that same engine to SAE J1349 instead of DIN 70020 and the figure drops another 1.4 per cent, because the two reference atmospheres are not the same.
The three units, and where each comes from
PS, from the German Pferdestärke, is metric horsepower, defined as the power to lift 75 kg by one metre in one second, which is 735.5 W. Horsepower in the mechanical or imperial sense is defined as 550 foot-pounds per second, which is 745.7 W. The kilowatt is the SI unit and needs no definition beyond itself. Metric horsepower is therefore the smallest of the three, which is why a PS figure is always the largest number.
| In PS | In hp | In kW | Where you see the PS figure |
|---|---|---|---|
| 300 PS | 296 hp | 221 kW | Germany, Japan, much of Europe |
| 500 PS | 493 hp | 368 kW | Same car, UK and US press |
| 700 PS | 690 hp | 515 kW | Widened gap, same engine |
| 1,000 PS | 986 hp | 736 kW | The hypercar rounding trap |
| 1,600 PS | 1,578 hp | 1,177 kW | A real published pair |
The bottom row is worth noting because it appears in the wild. A car quoted at 1,578 hp is 1,600 PS, a satisfying round metric figure that becomes an awkward imperial one. Whenever an imperial power figure looks oddly specific, converting it to PS usually reveals the number the engineers actually targeted.
The fourth variable: which standard
Net power is measured to one of three standards, and they do not share reference conditions. SAE J1349 corrects results to 99 kPa of dry air at 25 °C. DIN 70020 and UNECE Regulation 85 correct to 101.3 kPa at 20 °C. The second reference is both higher pressure and lower temperature, so it is denser air, and denser air produces a larger number.
The working conversion is to divide a DIN figure by 1.0139 to reach SAE net, a difference of about 1.4 per cent. That is small enough to be invisible in casual reading and large enough to move a 500 PS engine by 7 PS, which is more than the difference between many model variants.
Net power, and what gets subtracted
All three standards measure net power, meaning the engine is running with the ancillaries it needs in the car: its own cooling system, its alternator, its exhaust system. Older gross figures, measured on an engine stripped of everything it could be stripped of, are substantially higher and are not comparable to anything current.
Net power is still a crankshaft figure. It is not what reaches the road, and the gap between the two is the drivetrain. That is a separate subtraction with its own conventions, and it is the reason a chassis dynamometer reading will always sit below the number on the specification sheet.
A short procedure
- Identify the unit first. A large round number is usually PS. 500, 700 and 1,000 are metric targets, and their imperial equivalents are 493, 690 and 986.
- Convert everything to kW. It is the only unit with no ambiguity, and it makes power to weight ratios directly comparable.
- Check the standard if precision matters. A DIN or ECE figure is about 1.4 per cent above the SAE net equivalent for the same engine.
- Do not mix crank and wheel figures. A dynamometer reading and a brochure figure are measuring different points in the drivetrain.
Questions readers ask
What is the difference between PS, hp and kW?
PS is metric horsepower at 735.5 W, hp is mechanical horsepower at 745.7 W and kW is the SI unit. Metric horsepower is the smallest, so a PS figure is always the largest number. 500 PS is 493 hp and 368 kW.
Why do I see 1,578 hp rather than a round number?
Because the round number is in the other unit. 1,578 hp is 1,600 PS. Whenever an imperial power figure looks oddly specific, converting it to PS usually reveals the target the engineers actually set.
Does the measurement standard change the figure?
Yes, by about 1.4 per cent. SAE J1349 corrects to 99 kPa and 25 °C, while DIN 70020 and UNECE Regulation 85 correct to 101.3 kPa and 20 °C. The denser reference produces the larger number, and dividing a DIN figure by 1.0139 gives the SAE net equivalent.
Is net power what reaches the wheels?
No. Net power is measured at the crankshaft with the engine's own ancillaries fitted. What reaches the road is lower by the drivetrain loss, which is why a chassis dynamometer always reads below the specification sheet.
What was gross power?
An older convention that measured an engine stripped of ancillaries such as the alternator, full exhaust and cooling system. Gross figures are substantially higher than net ones and are not comparable with any modern standard.
Sources
- UNECE Regulation No. 85, on net power measurement for internal combustion engines and electric drivetrains.
- Engine power correction standards, on the reference conditions of SAE J1349, DIN 70020 and ECE 85 and the 1.0139 conversion factor.
- Bugatti Chiron Super Sport 300+, the source of the 1,578 hp figure used as the worked example.
Conversions use 1 PS = 735.5 W and 1 hp = 745.7 W. Figures in the table are rounded to the nearest whole unit.