When you need this conversion
Motors, water pumps, and compressors are often rated in horsepower (hp), while solar inverters, generators, and electrical circuits are rated in kilowatts (kW). Converting correctly is essential when sizing an inverter or generator to start and run a motor-driven load, such as a solar water pump, well pump, or workshop tool.
The formula in plain terms
Kilowatts equal horsepower multiplied by a conversion factor that depends on the horsepower standard used. Mechanical (imperial) horsepower, the most common type on US and UK equipment, uses kW = hp times 0.7457. Metric horsepower (PS or CV), common on European equipment, uses kW = hp times 0.7355. Electrical horsepower, used on some motor nameplates, uses kW = hp times 0.746. The differences between the three are small, generally under 2 percent, but matter for precise sizing.
Common hp to kW values (mechanical hp)
| hp | kW |
|---|---|
| 1 hp | 0.75 kW |
| 2 hp | 1.49 kW |
| 5 hp | 3.73 kW |
| 10 hp | 7.46 kW |
| 20 hp | 14.91 kW |
Practical tips
Check the nameplate for the horsepower type
Before converting, check whether the equipment lists mechanical, metric, or electrical horsepower. When unsure, mechanical (0.7457) is the safest default for US-market equipment.
Size for starting, not just running, power
Induction motors and pumps draw a surge of three to seven times their running kW for one to a few seconds at startup. An inverter or generator sized only for the converted running kW may fail to start the motor at all.
Add margin for inverter and wiring losses
Inverters lose a small percentage of power to heat, and voltage drop over long cable runs reduces power available at the motor. Add 10-20 percent margin beyond the converted kW figure when sizing a solar or generator system around a motor load.