Mechanical horsepower is a fixed SI-derived unit—745.7 watts exactly—not the same as metric PS on some import labels. This guide shows how to convert kilowatts to mechanical horsepower for US industrial equipment, VFD matching, and nameplate cross-checks.
Benefits
- Mechanical hp = kW ÷ 0.7457 (exact definition: 550 ft·lbf/s).
- Separates US mechanical hp from metric PS (≈ 0.7355 kW).
- Aligns inverter kW ratings with NEMA hp class motors.
How it works
- Take real shaft or electrical power in kilowatts (kW).
- Divide by 0.7457—the kW equivalent of one mechanical horsepower.
- Read mechanical hp for catalogs, motor stamps, and load comparisons.
FAQ
How do I convert kilowatts to mechanical horsepower?
Mechanical HP = kW ÷ 0.7457. Example: 3.73 kW → 3.73 ÷ 0.7457 = 5.0 hp mechanical. The constant comes from 745.7 W per hp. Do not use 0.7355—that is metric horsepower (PS), not mechanical.
Why is mechanical horsepower 0.7457 kW?
One mechanical horsepower is defined as 550 foot-pounds per second, which equals 745.699872 W—rounded to 745.7 W or 0.7457 kW in practice. NEMA, IEEE, and US pump curves use this definition. European DIN horsepower (PS) is slightly smaller.
Does efficiency change the kW-to-hp conversion?
The conversion factor is unit math only—hp and kW describe the same power magnitude. Motor efficiency affects how many electrical kW you draw to deliver a given shaft hp, but converting 7.5 kW shaft power still yields ~10.06 mechanical hp regardless of efficiency.
Technical specifications
- Mechanical HP = kW ÷ 0.7457.
- 1 mechanical hp = 745.7 W = 0.7457 kW.
- Metric PS: kW ÷ 0.7355 (different unit—not mechanical hp).
- Related: kw-to-hp-converter, kva-to-kw, inverter-sizing.
Mechanical horsepower has a precise definition
Unlike colloquial hp in ads, mechanical horsepower in engineering is 550 ft·lbf/s—745.7 watts. Dividing kilowatts by 0.7457 converts electrical or shaft power into that standard unit. A 15 kW drive output is 20.1 mechanical hp, not 15 hp. Spec sheets that skip the conversion mis-rank motor frame sizes and overload settings.
Do not confuse mechanical hp with metric PS
Imported compressors and compact engines sometimes list PS (Pferdestärke). One metric horsepower ≈ 0.7355 kW—about 1.4% less than mechanical. Converting 7.5 kW with the wrong constant gives 10.2 PS but 10.06 mechanical hp. For US NEC load calcs and NEMA replacements, mechanical hp is the correct target. Note the unit on the source label before converting.
Where mechanical hp still drives procurement
Pump affinity laws, belt drives, and farm implement PTO ratings reference hp classes. VFDs and battery inverters quote kW. Convert kilowatts to mechanical horsepower at the interface—when approving a 10 hp replacement for a 7.5 kW rated line, or when a utility interconnection cap in kW must be compared to hp-limited machinery on the same panel.