Why a 10 kVA generator gives you 8 kW

A spec sheet offers a generator at 10 kVA. Elsewhere on the same sheet it says 8 kW. Neither figure is marketing, and they are not alternatives.

They are describing two different limits, and the distance between them is the one piece of electrical theory worth carrying around.

Apparent power and real power

Real power, in kilowatts, is energy actually converted into work. Heat, light, a turning shaft.

Apparent power, in kilovolt-amperes, is voltage times current, which is what the conductors and windings physically have to carry. In a circuit with motors or transformers, some of the current is busy building and collapsing magnetic fields rather than doing work, and that current still flows, still heats the wire and still occupies the alternator.

Power factor is the ratio between them. Multiply kVA by power factor and you get kW, so at 0.8 a 10 kVA machine delivers 8 kW of useful output.

Why the machine is rated in kVA

Because the alternator’s constraint is thermal, and heat comes from current. Current follows apparent power. A generator does not know or care whether the current it is supplying is doing useful work, only how much of it there is.

The engine behind that alternator is the opposite. It produces mechanical power, which maps onto kW, so the engine is sized in kW while the electrical end is rated in kVA. One machine, two honest numbers, each describing the part of it that limits first.

Why 0.8 became the convention

The industry settled on 0.8 lagging because it matches the behaviour of typical industrial load, which is mostly motors and transformers, and because it lets a given alternator claim its highest sensible rating against the smallest engine.

That makes it a sensible default rather than a measurement of your site. A building full of electronics, LED lighting and resistive heating can run well above 0.8. One with many lightly loaded motors can run below it, and an underloaded motor has a worse power factor than a loaded one, which is one of the quieter arguments against oversizing machinery.

Where the confusion costs money

The practical damage happens when two machines are compared in different units.

A portable generator advertised at 5,000 watts is implicitly claiming a power factor of 1.0, which is true for a heater and false for anything with a motor. A 5 kVA industrial set rated at 0.8 offers 4 kW. The first number looks bigger and the second machine is usually the one that will actually start a compressor.

The kVA calculator converts between kVA, kW and amps at whatever power factor you give it, and it reports the current as well, which is the figure that decides cable and breaker sizing.

The current is the part that bites

Take 10 kVA on a 240 V single phase supply. That is 10,000 divided by 240, which is 41.7 A, and it is 41.7 A regardless of the power factor, because apparent power is what sets current.

So a poor power factor does not just lose you useful output. It means more amps for the same work, which means more heat in the cable, a larger conductor and a bigger breaker for no benefit. That is why industrial users are billed for it and why correction capacitors exist.

What to do with the numbers

For sizing a generator against a load, work in kW for the load and then convert, rather than mixing the units along the way.

Add up the real power your equipment consumes, apply a realistic power factor for the type of load, and that gives the kVA the machine has to carry. The generator size calculator handles the running and starting side of that in watts, which is the right unit for a household, and the kVA conversion matters once you are into three-phase equipment or a hire company’s spec sheet.

One last caution. Where a spec sheet gives only kVA and no power factor, the safe assumption is 0.8, and the safe follow-up question is what the actual figure is. A supplier quoting a bare kVA number against a kW requirement is either being careless or hoping you will not ask.