kVA vs kW: The Difference, and How to Size a Generator or UPS

Generators and UPS systems are sold in kVA, but appliances list their power in kW or watts. They aren't the same thing, and buying a 10 kVA generator for a 10 kW load is a common, expensive mistake. Here's the difference and how to size a generator, UPS or inverter correctly.

kW is real power, kVA is apparent power

kW (kilowatts) is the power that actually does work: turning a motor, heating, lighting. kVA (kilovolt-amperes) is voltage × current, the total the supply has to deliver. With motors, compressors and electronics, current and voltage drift out of step, so some current flows without doing useful work. The ratio of the two is the power factor (PF).

kW = kVA × power factor  ·  kVA = kW ÷ power factor

Generators are rated at a power factor of 0.8. So a 10 kVA generator delivers 8 kW, not 10 kW.

Generator ratings in kW

Sizing a generator: worked example

1. Add up the running load in kW. Say a shop with lights, fans, two ACs and a fridge totals 9 kW.
2. Convert to kVA: 9 ÷ 0.8 = 11.25 kVA.
3. Add 25% for motor starting and future additions: 11.25 × 1.25 = 14.06 kVA.
4. Choose the next standard size: a 15 kVA generator.

Motors draw 3 to 6 times their running current for a moment when they start. If you have a large pump, lift or compressor, ask the supplier to check the starting load as well.

From kW to amps (for cables and breakers)

Current decides the wire size and the MCB rating. At Indian supply voltages:

• Single-phase (230 V): amps = watts ÷ (230 × PF). A 2 kW load at PF 0.9 draws 9.7 A.
• Three-phase (415 V): amps = watts ÷ (√3 × 415 × PF). A 9 kW load at PF 0.8 draws 15.7 A per phase.

The watts to amps calculator and wire size calculator take it from there.