kW is the work actually done; kVA is what the supply must deliver to do it. Whenever current and voltage fall out of step, the kVA is larger than the kW — and the gap is the power factor. That is why transformers, generators and supplies are all rated in kVA, not kW.
kVA vs kW
kW is real power. kVA is apparent power — the volts × amps the windings actually carry. They are linked by the power factor:
kW = kVA × PF, and going the other way, kVA = kW ÷ PF.
A 100 kVA supply at 0.8 PF delivers up to 80 kW of real power. Turn it around and an 80 kW load at 0.8 PF needs the full 100 kVA of supply capacity — the reactive current still has to flow through the transformer even though it does no useful work.
kVA to amps
| Supply | Amps = |
|---|---|
| Single-phase | kVA × 1,000 ÷ volts |
| Three-phase | kVA × 1,000 ÷ (√3 × volts) |
So a 100 kVA three-phase transformer at 480 V supplies about 120 A per phase (144 A at 400 V), and a 5 kVA single-phase generator at 240 V gives about 20.8 A (21.7 A at 230 V).
kVA Calculator
Enter kVA, kW or amps with the supply type and power factor to get the other two — plus reactive power.
What is reactive power (kVAr)?
The part of the apparent power that shuttles back and forth without doing work — it magnetises motors and transformers. It follows from the power triangle: kVAr = √(kVA² − kW²). Utilities charge large customers for it, which is why power factor correction pays: raising the power factor shrinks the kVAr and frees up supply capacity you have already paid for.
What power factor to assume
Use the measured or nameplate figure when you have it. Otherwise: 0.8 is the standard assumption for mixed and motor loads (and how generators are rated), 0.9–0.95 suits modern commercial buildings with corrected loads, and 1.0 applies only to purely resistive loads like heaters. When sizing a generator, the 0.8 rating is why a 100 kVA set is often quoted as “80 kW.”