kVA to kW Calculator

What's the difference between kVA and kW?

kVA (kilovolt-amperes) is apparent power: the total power a generator or supply delivers, calculated simply as voltage × current. kW (kilowatts) is real power: the part of that total that does useful work, such as turning a motor, heating an element or lighting a building.

The gap between the two comes from the power factor (PF) of the load. Inductive equipment such as motors, pumps, compressors and transformers draws current that isn't all turned into useful work, so the kW figure is lower than the kVA figure. For a purely resistive load, like an electric heater, kVA and kW are the same.

That's why converting kilovolt-amperes to kilowatts always needs a power factor. Without one there's no single answer.

How to convert kVA to kW

1. Find the kVA rating. Take it from the generator's data plate or specification, a transformer, or the load you've calculated.
2. Find the power factor. For diesel generators, use 0.8 unless you know otherwise.
3. Multiply: kW = kVA × PF.

So a 100 kVA generator delivers 80 kW of real power at 0.8 PF, and a 500 kVA generator delivers 400 kW. If you'd rather search by real power, browse 80 kW generators or 400 kW generators instead.

Why are diesel generators rated at 0.8 power factor?

Generator manufacturers rate their sets at 0.8 power factor (lagging) as an industry standard. The engine produces the kW, the real mechanical power. The alternator is sized to carry the full kVA, including the extra current drawn by motors and other inductive loads. Rating at 0.8 PF means the alternator can supply typical mixed site, commercial and industrial loads without overheating.

In short, kVA tells you the alternator's capacity and kW tells you the engine's capacity. Both matter when you're choosing a diesel generator.

Using a kVA to kW conversion to size a generator

• Check standby and prime ratings. A standby rating is the maximum output for emergency back-up during a mains failure, usually for limited hours per year. A prime rating is the output available for unlimited hours with a varying load, and is typically around 10% lower. If the generator will be your main source of power, size against the prime rating. For back-up only, see our standby generators.
• Allow headroom. Add 20-25% above your calculated load to cover motor start-up surges and future expansion. The calculator above does this for you.
• Check the starting current. Motors can draw several times their running current when they start. Use our kW to Amps calculator to work out running currents.
• Don't oversize too far. A diesel generator that runs very lightly loaded for long periods won't perform at its best, so aim for a set that runs comfortably loaded most of the time.