Horsepower (International) to Horsepower (Metric)

By the mid-20th century, at least five different horsepower definitions existed: British mechanical, metric (PS), electric, boiler, and water. International trade required a single reference. The 1956 agreement standardized the mechanical/British value (745.699872 W) as the international benchmark. This didn't eliminate the others — metric PS persists in Europe, electric hp in US motors — but it gave engineers a common reference when precision matters or when "hp" appears without qualification.

SAE J1349 specifies measuring net horsepower with all production accessories (alternator, water pump, AC compressor) attached, at standard atmospheric conditions. Before 1972, US manufacturers used gross hp (engine on a test stand with minimal accessories), which inflated numbers by 15–25%. The switch to SAE net ratings famously caused "overnight" power drops: a Corvette went from "350 hp" (gross) to "255 hp" (net) in 1972 — same engine, honest measurement.

Japan officially uses metric PS (called 馬力, "horse power," abbreviated PS after the German). Japanese car specs list PS, and JIS standards define power in PS. However, for international export, Japanese manufacturers convert to international hp or kW depending on the destination market. A Nissan GT-R produces 570 PS for the Japanese market and 565 hp for the US market — the same engine, different unit systems, and the ~1% gap occasionally causes forum arguments.

The Wärtsilä-Sulzer RTA96-C, a marine diesel engine used in the largest container ships, produces about 109,000 hp (international) — 80,080 kW from 14 cylinders each the size of a small apartment. It's 13.5 meters tall and weighs 2,300 tonnes. At 102 RPM, it turns propellers the size of houses. For comparison, a Saturn V rocket's five F-1 engines produced about 217 million hp combined, but only for 2.5 minutes.

Probably, but slowly. The EU already legally requires kW; China uses kW; scientific and engineering communities prefer kW. But cultural inertia is powerful — Americans have been buying cars by horsepower for over a century, and "how many horses under the hood" is deeply embedded in car culture. The transition to EVs may accelerate the shift, since electric motors are naturally rated in kW. Give it 20–30 years, and hp may join the furlong and the gill in the museum of obsolete units.

EU regulations require engine power in kilowatts, but consumers prefer a familiar number. Continental Europe adopted metric horsepower (PS) in the 19th century, and car culture cemented it. Germans say "PS," French say "CV," Italians say "CV" too. The UK uses "bhp" (British horsepower). A 200 PS car is 197 hp — close enough that most people don't notice the 1.4% difference. Japanese manufacturers use PS as well (sometimes written 馬力).

In a light car (1,000 kg), 100 PS gives a power-to-weight ratio of 100 PS/tonne — adequate for city driving with 0–100 km/h in about 10–11 seconds. In a heavy SUV (2,000 kg), 100 PS feels sluggish, struggling on hills and taking 15+ seconds to reach highway speed. The magic number for "fun" is roughly 150–200 PS per tonne — which is why a 90 PS Mazda MX-5 (1,000 kg) feels livelier than a 200 PS family SUV (1,800 kg).

As of 2025, the Rimac Nevera holds the production EV record at 1,914 PS (1,408 kW). For combustion engines, the Bugatti Chiron Super Sport delivers 1,600 PS. Koenigsegg's Jesko Absolut produces 1,600 PS. But the real mind-bender is that a Formula 1 car's power unit produces about 1,050 PS from just 1.6 liters — over 650 PS per liter, achieved through turbocharging and energy recovery systems at 15,000 RPM.

Because they're defined differently. British hp = 550 ft·lbf/s = 745.7 W. Metric hp = 75 kgf·m/s = 735.5 W. The metric definition uses round metric numbers (75 kg, 1 m, 1 s) rather than being an exact conversion of the British unit. The ~1.4% gap is small enough that it rarely matters practically, but it means a car rated at 200 PS is technically 197 hp. Marketing departments sometimes quietly use whichever number is larger.

Both, depending on market. Tesla lists kW in tech specs but PS/hp in consumer marketing because buyers understand horsepower intuitively. A Tesla Model 3 Performance produces about 460 PS (340 kW). The shift toward kW is accelerating because EVs make the kW connection obvious — if you charge at 11 kW and your motor outputs 150 kW, the relationship is clear. Eventually kW may replace PS entirely, but decades of "how many horses?" thinking won't die easily.