In short. Kilowatts measure real power, the part that does work. Kilovolt-amperes measure apparent power, the total the alternator must supply. They are linked by power factor: kVA = kW / PF and kW = kVA x PF. A diesel generator is rated in kVA at 0.8 power factor, so a 100 kVA set delivers 80 kW. Converting a load to kVA is the first arithmetic step in sizing, not the last.
Real power, apparent power and the alternator
Real power in kilowatts is what heats an element, turns a shaft or lights a hall. The engine of a generating set supplies real power, and its fuel consumption follows kW almost exactly. Apparent power in kilovolt-amperes is the product of voltage and current regardless of their timing. The alternator, cables, breakers and transformer carry current, so they are rated in kVA. Reactive power in kilovolt-amperes reactive is the difference between the two: current that magnetises motors and transformers and returns to the source every half cycle without doing work.
A generating set is therefore limited twice. The engine limits kW, and the alternator limits kVA. The nameplate quotes both, almost always at 0.8 power factor lagging, because that is a representative industrial mix of motors and resistive load. At 0.8 PF the two limits are reached together. At a higher power factor the engine limit is reached first and the alternator has spare current capacity. At a lower power factor the alternator reaches its thermal limit before the engine is fully loaded.
The conversion formulas
kVA = kW / PF, kW = kVA x PF, PF = kW / kVA
Here kW is real power, kVA is apparent power and PF is the power factor of the load under the operating condition in question, a number between 0 and 1. For a three-phase load the same relationship applies to the total; the line quantities are kVA = 1.732 x V x I / 1000 with V the line-to-line voltage in volts and I the line current in amperes. The generator current guide works through the current side.
| Load | Power factor | Apparent power | Comment |
|---|---|---|---|
| 100 kW | 1.00 | 100 kVA | Resistive heaters, incandescent lighting |
| 100 kW | 0.90 | 111 kVA | Well-loaded motors, corrected plant |
| 100 kW | 0.80 | 125 kVA | Standard generator rating point |
| 100 kW | 0.70 | 143 kVA | Lightly loaded motors, old fluorescent lighting |
| 100 kW | 0.60 | 167 kVA | Motors at low load, welding sets |
The table shows why power factor is not a detail. The same 100 kW of useful work demands two thirds more alternator capacity at 0.6 than at 1.0. The figures are arithmetic and exact; what must be verified on any real site is the power factor itself.
Choosing the right power factor
The most common error is to take the generator’s rated 0.8 and apply it to the load. The load has its own power factor, and the generator rating has nothing to do with it. A hospital with LED lighting, UPS systems and corrected chillers may run above 0.95. A stone crusher with large motors running well below their rating may be at 0.7. Use a measured value from a power analyser or the DISCOM meter wherever it exists, use manufacturer data for individual machines, and only use 0.8 as a stated screening assumption when nothing better is available.
Displacement and true power factor
Displacement power factor is the cosine of the phase angle between fundamental voltage and current. True power factor includes the effect of harmonic distortion, and is always lower where non-linear loads such as drives, rectifiers and LED drivers are present. A UPS may have a displacement power factor near unity while its true power factor is 0.9 or lower. For generator work the true value matters, because the alternator carries the harmonic current whether or not it does useful work. The power factor guide covers measurement.
Leading power factor
Capacitor banks left in service on generator, long lightly loaded cables, and some UPS input filters make the load capacitive, so current leads voltage. A diesel alternator tolerates only a small leading power factor before its automatic voltage regulator loses control and the voltage rises. The load’s kVA may be correct and the set still unstable, so the inventory should flag any capacitor bank and arrange for it to be switched out when on generator.
Worked examples (illustrative)
A workshop has 80 kW of mixed load at a measured 0.8 PF. The apparent power is 80 / 0.8 = 100 kVA, which matches a 100 kVA set on paper. Before accepting that, the largest motor start, derating and growth must be checked.
A data room has 150 kW of IT load through a modern UPS with a true input power factor of 0.95. Apparent power is 150 / 0.95 = 158 kVA. The set must also supply battery recharge current after an outage, often 10 to 25 percent above the output load until the battery recovers, and must tolerate the UPS’s harmonic current, so the alternator selection will be larger than 158 kVA even though the arithmetic is correct.
A 200 kW motor-heavy plant at 0.9 measured PF is about 222 kVA. The same plant assumed at 0.8 would have been quoted at 250 kVA, a full frame size larger with a correspondingly lower running load percentage. The difference between measuring and assuming is often a frame size.
Reading a generator nameplate
A nameplate reading 125 kVA, 100 kW, 0.8 PF, 415 V, 174 A, 50 Hz tells you the alternator may carry 174 A per phase continuously at rated temperature rise, and the engine may deliver 100 kW of electrical output at the stated rating category and site conditions. If your load is 100 kW at 0.95 PF, the current is only about 146 A and the alternator is comfortable, but the engine is at its limit. If your load is 95 kW at 0.7 PF, the current is 189 A and the alternator is overloaded even though the engine is not. Always check both limits, which is why the generator sizing guide asks for kW and PF separately rather than a single kVA.
Common mistakes
- Using the generator’s rated 0.8 as the load power factor, which oversizes resistive or corrected plants and undersizes motor-heavy ones.
- Treating the kVA result as the generator rating, so the largest motor start, harmonic loading and site derating are never applied.
- Dividing motor shaft kW by power factor without allowing for efficiency, which understates electrical demand by 8 to 15 percent.
- Leaving power factor correction capacitors connected on generator, which can push the set into a leading power factor and cause voltage instability.
What to send us
Send the kW or kVA of your load with the power factor and how it was obtained, whether measured, taken from a bill or assumed, together with a note of any UPS, drives or capacitor banks on the system. If you have a nameplate photograph of an existing generator and a recent electricity bill, include both so the conversion can be checked against real demand. Send your application engineering enquiry.
Questions engineers ask
How many kW does a 100 kVA generator give?
At the standard rating point of 0.8 power factor, 80 kW. If the load has a higher power factor the engine still limits output to 80 kW; if it has a lower power factor the alternator current limit is reached before 80 kW is delivered.
Why are generators rated in kVA and not kW?
The alternator is a current-carrying machine whose heating depends on amperes, not on useful work, so its capacity is stated in kVA. The engine’s capacity is stated in kW. Both appear on the nameplate, linked by the 0.8 reference power factor.
Does a higher power factor let me use a smaller generator?
It reduces the kVA for a given kW, so the alternator can be smaller, but the engine must still produce the full kW. In practice a set is selected on both limits, and at high power factor the engine, not the alternator, decides the size.
Is 0.8 a safe assumption when I do not know the power factor?
It is a reasonable screening value for mixed industrial load and it should be labelled as an assumption. For motor-heavy plants running at part load it may be optimistic, and for modern commercial buildings it is usually pessimistic. Measure before ordering.
How do I convert single-phase kVA to current?
For single-phase, current in amperes equals kVA x 1000 divided by the voltage. A 10 kVA single-phase load at 230 V draws about 43 A, which is why larger loads are almost always three-phase.
Looking for a specific rating? See the generator full-load current chart for every common size from 7.5 to 3,000 kVA at 415 V and 11 kV.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.