kVA to kW: what the difference actually means
A generator rated 100 kVA doesn't give you 100 kW. The gap between the two numbers is the power factor, and getting it wrong is the single most common reason people undersize or oversize a generator, UPS or transformer.
6 min read · Updated: 24 August 2026
Key points
- Real power P (kW) = apparent power S (kVA) × power factor (cos φ). kVA and kW are only equal when cos φ = 1.
- Generators, UPS units and transformers are rated in kVA because their windings and semiconductors are limited by current, not by how useful that current is.
- If you don't know your load's power factor, 0.8 is the standard assumption for a mixed load of motors, electronics and lighting.
- Reactive power Q (var) is the part of the current that does no useful work – it's what makes S bigger than P in the first place.
- Undersizing a generator by ignoring power factor is a common and expensive mistake: a 100 kVA generator only reliably delivers 80 kW at cos φ = 0.8, not 100 kW.
Why kVA and kW aren't the same thing
Apparent power (S, measured in kVA) is simply voltage × current. Real power (P, measured in kW) is the portion of that which actually does useful work – turning a motor, lighting a bulb, heating an element. The two are only equal when the load's power factor is exactly 1, which almost never happens outside a purely resistive load like a heating element.
The relationship is P = S × cos φ, where cos φ is the power factor. A load with cos φ = 0.8 pulls 100 kVA of apparent power from the supply but only converts 80 kW of it into useful output. The remaining capacity isn't wasted as heat – it's reactive power, oscillating back and forth between source and load without ever being consumed.
Why generators and UPS units are rated in kVA, not kW
A generator's alternator and a UPS unit's inverter are limited by current – their windings and semiconductors overheat past a certain amperage regardless of what that current is doing. Since apparent power (S) is what determines current draw at a given voltage, that's the number the equipment is rated in.
How many kW you actually get out of a kVA-rated machine depends entirely on your load's power factor, which the manufacturer doesn't control and can't put on the nameplate. This is why two installations with identical 100 kVA generators can have very different usable capacity: one running mostly resistive loads at cos φ = 0.95 gets 95 kW, another running older induction motors at cos φ = 0.7 gets only 70 kW.
The sizing mistake this causes
If you size a generator against your equipment's kW rating without checking power factor, you can end up short. A site with 90 kW of real load at cos φ = 0.75 needs 120 kVA of generator capacity, not 90 kVA – undersizing by 25%.
What power factor to use
Typical power factor by load type
| Load type | Typical cos φ |
|---|---|
| Resistive heating, incandescent lighting | 1.0 |
| Modern electronics with active PFC (PCs, LED drivers) | 0.95 – 0.99 |
| Mixed commercial load (offices, general equipment) | 0.8 – 0.9 |
| Older induction motors, uncorrected | 0.6 – 0.75 |
If you genuinely don't know your load's power factor, 0.8 is the standard planning assumption for a mixed load – it's what most generator sizing guides and UPS spec sheets default to. For a known, mostly-electronic load, checking the equipment's own PFC rating usually gives a more accurate number than the 0.8 default.
Worked examples
Converting between kVA and kW
| Given | Power factor | Result |
|---|---|---|
| 100 kVA generator | cos φ = 0.8 | 80 kW usable |
| 50 kVA UPS | cos φ = 0.9 | 45 kW usable |
| 10 kW load | cos φ = 0.8 | 12.5 kVA required |
| 230 kW load | cos φ = 0.95 | ≈ 242.1 kVA required |
Convert your own figures
Enter the apparent power and power factor to get real power (or the other way round) – the calculator also shows the reactive power Q.
Go to the kVA ↔ kW converterReading a nameplate correctly
A transformer or generator nameplate usually states kVA capacity and, separately, a power factor the manufacturer assumed for a secondary kW figure – often 0.8 for generators, sometimes 1.0 for UPS units running IT loads. Always check which power factor that secondary figure assumes; a UPS quoted at "100 kVA / 100 kW" is telling you it assumes cos φ = 1 for its kW figure, not that kVA and kW are interchangeable for your actual load.
Frequently asked questions
Why are generators and UPS units rated in kVA instead of kW?
Their windings and semiconductors are limited by current – that is, by apparent power – regardless of the connected load's power factor. How many kW you actually get depends on your load's cos φ, which the manufacturer doesn't know in advance.
What power factor should I use if I don't know my load's?
0.8 is the standard assumption for a mixed load of motors, electronics and lighting. Purely resistive loads are 1.0, modern electronics with active power factor correction run 0.95–0.99, and older uncorrected induction motors can be as low as 0.6–0.75.
Can the kW value ever be higher than the kVA value?
No. Since cos φ can be at most 1, real power can never exceed apparent power – in the ideal case (a purely resistive load) they're equal.
How do I convert 100 kVA to kW?
Multiply by the power factor: 100 kVA × 0.8 = 80 kW. Without knowing the power factor, you can't get an exact kW figure from a kVA rating alone.
What is reactive power?
The component of power (Q, in var) that oscillates between source and load – for example in motor magnetic fields or capacitors – without doing useful work. It's calculated as Q = √(S² − P²).
Convert your own figures
Enter the apparent power in kVA and the power factor to get the real power in kW, or the other way round.
Go to the kVA ↔ kW converter