kVA vs kW: Why a Generator Is Rated in Both and What Each Means
kVA is apparent power, kW is real power, and power factor bridges them. Understand both ratings so the set you buy actually covers the load you measured.
A generator is rated in both kVA and kW because they are not the same thing. kW is the real power that does the work; kVA is the total power the alternator must carry, including the reactive part. The power factor, typically about 0.8, is the bridge between them: kW equals kVA times power factor. Read both numbers from the sizing method before you choose a set.
What kW and kVA actually measure
kW is the power your equipment turns into useful work: a pump moving water, a motor turning a conveyor, a lamp giving light. kVA is the total current the alternator and cables must handle, which is larger than kW whenever the load has inductance or capacitance, such as motors and transformers. Most genset ratings are stated in kVA because that is what stresses the machine; the kW figure depends on your load's power factor.
| Term | Measures | Set by |
|---|---|---|
| kW (real power) | Useful work delivered to the load | The load's actual demand |
| kVA (apparent power) | Total current the alternator carries | The alternator and cables |
| Power factor | Ratio of kW to kVA (0 to 1) | The type of load, usually ~0.8 |
The power factor bridge
The relationship is simple arithmetic. At a power factor of 0.8, a 100 kVA alternator delivers 80 kW of real power. Lower the power factor and the same alternator delivers fewer kW for the same kVA, which is why a quote always states the power factor it is rated at. Induction motors, the most common industrial load, are exactly the loads that pull the power factor down and the kVA up.
If your load is heavily motor-dominated, the starting surge also drops the power factor sharply for the moments a motor accelerates. Size the alternator for that transient, not just the steady running power factor, or the voltage will sag on start.
Why your quote shows both ratings
A complete quote shows prime and standby in both kVA and kW, plus the power factor and the rating standard. A set quoted only in kW leaves you guessing at the alternator's kVA limit, and a set quoted only in kVA leaves you guessing at the usable kW. Both belong on the datasheet, and both feed the ISO 8528 rating you are actually buying.
- kVA tells you the alternator and cable limit, the part that fails first on motor start.
- kW tells you the usable work, the part your electricity bill and load list care about.
- Power factor (usually 0.8) is the published bridge between them on the nameplate.
Questions buyers ask about kVA and kW
Frequently asked questions
Which rating should I size to?
Size the load in kW first, then confirm the alternator's kVA covers it at your power factor, including motor starting. If you only check kW you can still trip the set on the kVA limit during a motor start.
What does 0.8 power factor mean on a quote?
It means the set is rated to deliver 80 percent of its kVA as real kW. A 100 kVA set at 0.8 pf delivers 80 kW. A different pf changes the kW without changing the kVA.
Can I run a 0.8 pf set at 1.0 pf?
Yes, at unity power factor the set delivers its full kVA as kW, which is the best case for the alternator. Real sites rarely sit at 1.0 because motors and transformers are inductive.
Why does my motor load need more kVA than kW?
Induction motors draw reactive current to build their magnetic field, which adds to kVA without adding to kW. The larger and harder-starting the motor, the wider the gap between the two.
Do silent and open sets differ in kVA?
The enclosure does not change the electrical rating. The same alternator is rated the same kVA inside a silent canopy or an open skid; the enclosure changes noise, weather and service access, not power.