Here is the correction that separates a correct horsepower-to-kVA conversion from a sloppy one: a motor's nameplate horsepower is not its electrical demand. Horsepower measures the motor's mechanical output at the shaft — the generator supplies the electrical input, and the motor loses a portion to friction, heat, and magnetic losses.
The turning force the motor delivers to the pump, compressor, or fan. A 100 hp motor is 74.6 kW of shaft power — the number on the nameplate, not the electrical draw.
The real electrical power drawn by the motor. At 84% efficiency, the same 100 hp motor draws 88.8 kW of electrical power — higher than the horsepower value alone suggests.
At 0.8 power factor, 88.8 kW becomes 111 kVA. The naive formula (hp × 0.746 ÷ 0.8) gives only 93 kVA — a 19% under-size.
The motor's efficiency means the electrical demand is higher
than the horsepower suggests. A 100 hp motor is 74.6 kW of shaft power.
At 84% efficiency it draws 88.8 kW of electrical power,
which at 0.8 power factor becomes 111 kVA.
This is why the calculator asks for efficiency, and why the default is a realistic
0.85 rather than the 100% the thin tools assume. If you do not know
the efficiency, use 0.85 for motors and treat the result as a floor.
The motor converts a portion of electrical input into mechanical output. The rest is lost as heat and friction. Use 0.85 as a realistic default — never assume 100%.
Motors draw apparent power (kVA) that exceeds real power (kW) because they magnetize their windings. Use 0.8 as the standard for diesel generator sets and motor loads.
100 HP × 0.746 = 74.6 kW shaft power.
74.6 kW ÷ 0.84 = 88.8 kW electrical input.
88.8 kW ÷ 0.8 = 111 kVA generator required.
If you do not know the efficiency, use 0.85 for motors and treat the result as a floor. The actual generator size may need to be larger depending on the motor's real efficiency and starting method.
The conversion runs in two steps, because horsepower is mechanical power and kVA is electrical apparent power — two different quantities with two different corrections between them. Use Forward Mode to convert HP to kVA, or Reverse Mode to find the horsepower a generator can run.
Enter the motor's horsepower and efficiency. The tool converts to kilowatts (mechanical to electrical) then to kVA at your selected power factor. Default efficiency is 0.85 — realistic for most industrial motors.
Enter the generator's kVA rating, the motor's efficiency, and the power factor. The tool returns the maximum horsepower a motor can deliver when powered by that generator.
At the standard 0.8 power factor, with efficiency set aside, the conversion collapses to kVA = hp × 0.93. The table below shows that rule of thumb across the range, with the electrical kW beside it.
| Horsepower (hp) | Electrical kW (0.8 PF) | Generator kVA (0.8 PF, no efficiency) |
|---|---|---|
| 1 hp | 0.75 kW | 0.93 kVA |
| 5 hp | 3.73 kW | 4.66 kVA |
| 10 hp | 7.46 kW | 9.33 kVA |
| 25 hp | 18.65 kW | 23.31 kVA |
| 50 hp | 37.3 kW | 46.6 kVA |
| 100 hp | 74.6 kW | 93.3 kVA |
| 150 hp | 111.9 kW | 139.9 kVA |
| 200 hp | 149.2 kW | 186.5 kVA |
| 500 hp | 373 kW | 466 kVA |
Three examples, calculated with the formulas above, to show how the corrections stack up. Each example walks through the conversion from horsepower to kVA, with real efficiency and power factor values.
kW: 100 × 0.746 ÷ 0.84 = 88.8 kW
kVA: 88.8 ÷ 0.8 = 111 kVA
Specify a 125 kVA generator (or larger once the starting surge is counted).
kW: 150 × 0.746 ÷ 0.91 = 123.0 kW
kVA: 123.0 ÷ 0.91 = 135.1 kVA
Specify a 150 kVA generator as the running-load floor.
kW: 25 × 0.746 ÷ 0.84 = 22.2 kW
kVA: 22.2 ÷ 0.8 = 27.75 kVA
Specify a 30 kVA generator for this small workshop load.
Running kVA is only half the story. When a motor starts, it draws several times its running current. The starting surge — not the running load — is often what dictates the generator size.
A 150 hp motor can pull 795 kVA at startup (based on NEMA Code F, 5.3 kVA/hp). This is what usually dictates the generator size rather than the running load. Convert the horsepower here, then check the surge on our companion motor starting generator sizing calculator.
Convert horsepower here, then check the starting surge on our companion motor starting generator sizing calculator — and you have both halves of the sizing decision.
We build diesel generator sets from 8 kVA to 4000 kVA — on Cummins, Perkins, Weichai, and Yuchaiengines with Stamford alternators — and our engineers specify the running kVA and the starting kVA against your actual equipment.
A: Multiply the horsepower by 0.746, then divide by the motor efficiency and the power factor: kVA = hp × 0.746 ÷ (efficiency × PF). At 0.8 PF and ignoring efficiency, that simplifies to kVA = hp × 0.93.
A: At 0.8 power factor and 84% motor efficiency, a 100 hp motor draws about 111 kVA. Without the efficiency correction, the naive answer is only 93 kVA, which under-sizes the generator by roughly 19%.
A: Multiply the horsepower by 0.746: kW = hp × 0.746. For a motor’s electrical input, divide by the efficiency as well: kW = hp × 0.746 ÷ efficiency. One horsepower equals 0.746 kilowatts, and one kilowatt equals 1.341 horsepower.
A: Yes. Horsepower first becomes kilowatts (real power), which must then be divided by the power factor to reach kVA (apparent power). At the standard 0.8 power factor, kVA is always 25% higher than the kilowatt figure.
A: One mechanical horsepower equals 0.746 kW, and one kilowatt equals 1.341 horsepower. A metric horsepower (PS or CV) is slightly smaller at 0.7355 kW, so be clear which unit the equipment nameplate uses.
A: Multiply the kVA by the power factor and the motor efficiency, then divide by 0.746: hp = kVA × PF × efficiency ÷ 0.746. For example, a 111 kVA generator at 0.8 PF can run about 100 hp of motor at 84% efficiency.
A: Horsepower measures the motor’s mechanical output at the shaft, while kilowatts measure the electrical input to the windings. The electrical input is higher than the shaft output by the motor’s efficiency, which is why a 100 hp motor draws about 88.8 kW of electrical power rather than 74.6 kW.
A: At 0.8 power factor and 84% efficiency, a 100 hp motor needs about 111 kVA running — but the starting surge usually dictates more. A 100 hp motor can draw several hundred kVA at startup, so size for the surge, not just the running load.
