How this instrument works
A generator's nameplate rates it in kVA, apparent power, because the alternator's physical limit is current and voltage rather than how much of that current actually does useful work. Copper windings heat according to RMS current alone, and the insulation is rated for a fixed voltage — neither constraint cares whether current and voltage are in step. Real power, kW, is the smaller number that survives once the connected load's power factor is applied, and it is the figure that decides whether whatever you are plugging in will actually run.
The formula is just the real side of the power triangle: apparent power S is the hypotenuse, real power P is the side adjacent to the phase angle φ, and PF = cos φ links them, so P = S × PF. A purely resistive load — simple electric heat, incandescent lighting — draws current perfectly in step with voltage, PF sits at 1, and the generator delivers its full nameplate kVA as kW. A motor, ballast, or switch-mode supply pulls part of its current out of step instead, PF drops below 1, and the same kVA rating yields fewer real kilowatts.
Diesel and gas gensets typically quote a kW rating right next to the kVA figure, and that kW number is almost always this same arithmetic evaluated at 0.8 lagging power factor, the assumption long used across the engine-driven generating-set industry for a mixed load of motors and electronics. What this formula does not capture is the instant a motor starts: inrush current spikes several times higher at a lower momentary power factor than the running figure, so a unit sized correctly for steady-state kW can still stall on startup — sizing for that surge needs a separate allowance beyond this calculation.
- Enter the generator's apparent-power capacity into Generator rating, kVA — the figure stamped on the alternator's data plate, not any kW number printed beside it.
- Set Power factor to a decimal between 0 and 1 that matches the load — 1.0 for pure resistive heat, 0.8 for a typical mixed load of motors and electronics, lower for a job site running several induction motors.
- Read Real power output in kW — switch the unit to W for a small load — this is the true capacity available once the load's power factor is accounted for.
- Add up the running watts of everything you intend to connect and compare that total against Real power output, not against the kVA nameplate figure, before plugging anything in.
Worked example — a 20 kVA genset on a 0.8 PF load
A facilities engineer is checking whether a 20 kVA standby generator can carry a building's mixed load of lighting, electronics, and a couple of small motors, measured at a typical 0.8 power factor. Enter 20 into Generator rating, kVA and 0.8 into Power factor: Real power output returns 16, in kW. The arithmetic is exactly 20 × 0.8 = 16.0 kW — the generator's full apparent-power rating is 20 kVA, but only 16 kW of that is real, usable power once this load's power factor is applied.
That 4 kW gap between the 20 kVA nameplate and the 16 kW real output is not wasted energy; it is reactive power the alternator still has to supply and the windings still have to carry, even though it never converts to heat, light, or motion at the load. Swap in a purely resistive load instead — PF = 1.0 — and the same generator delivers its full 20 kW, exactly the nameplate figure; drop to a motor-heavy job site at PF = 0.7 and the usable output falls to 14 kW, which is why relying on the kVA number alone is how standby generators end up undersized.
Questions
Why is a generator rated in kVA instead of kW?
Because the alternator's limit is current and voltage, not phase angle — its windings heat according to RMS current alone, so its ceiling is apparent power in kVA regardless of how much of that power the load actually converts to work. Real power in kW depends on the connected load's power factor too, so one kVA figure covers every load type; a kW figure alone would need a stated PF to mean anything, which is exactly what this calculator supplies.
Is a 20 kVA generator the same as a 20 kW generator?
No, unless every load on it is purely resistive, PF = 1. At the 0.8 lagging power factor commonly used to quote genset kW ratings, that same 20 kVA machine delivers only 16 kW of real power — the exact figure this instrument returns for a 20 kVA rating and a 0.8 power factor. Treating the kVA number as if it were kW is the single most common generator-sizing mistake.
Why do motors push the power factor down?
A motor's windings need current to build a magnetic field before they can produce torque, and that magnetizing current lags the supply voltage without doing any real work itself. The larger that reactive share, the lower the power factor — a lightly loaded induction motor can sit near 0.6 PF, while a well-loaded one reaches 0.85 to 0.9. Compressors, pumps, and elevator motors are exactly the loads that catch people out with a generator that was never actually short on kVA.
What power factor should I use if I don't know the load?
0.8 lagging is the standard assumption for a mixed commercial or residential load carrying some motors and electronics, and it is the figure manufacturers use to quote a diesel genset's kW rating alongside its kVA rating. For a load that is entirely resistive heating or incandescent lighting, use 1.0; for a job site running several induction motors, 0.7 to 0.75 is a safer estimate.
Does this account for a motor's starting surge?
No — this formula gives the steady-state real power once a load is already running, not the brief inrush a motor draws while starting. A motor can pull five to seven times its running current for a second or two at a much lower momentary power factor, which is why generator sizing guides apply a separate motor-starting allowance on top of this steady-state figure.
What happens if I overload the real power output?
The engine has to supply more torque than it is rated for, so engine speed sags, output frequency drops below its 60 Hz or 50 Hz target, and the unit's overload protection typically trips the main breaker before real damage occurs. Running consistently above the calculated real power output — even while comfortably under the kVA rating — is what actually stalls a generator, not exceeding the apparent-power number.