kVA vs kW: Understanding Generator Set Ratings

In Short

kVA is the apparent power a generator set is rated at, while kW is the real power it delivers to the load. The two differ by the power factor, so a kVA rating alone does not say what the set can run. Rating type, motor starting and the nature of the load also matter.

Generator sets are sold in kVA and loads are listed in kW, and the gap between the two is where many mistakes start. This guide explains what each unit means, why a generator's kVA rating is not the power it can deliver, and which points - rating type, motor starting, step loads, non-linear loads - matter when choosing a generator.

kVA, kW and Power Factor

kW is real power: the power that does work and that the engine has to produce. kVA is apparent power: what the alternator and the cables have to carry. The ratio between them is the power factor. Generator sets are rated at a standard power factor, so the kVA figure on the nameplate is always higher than the kW the engine can deliver. Some loads - many motors, transformers at light load - use up the alternator before the engine; others use up the engine before the alternator.

Standby, Prime and Continuous

The rating type matters as much as the number. A standby rating is the maximum the set delivers during an outage, for limited hours a year. A prime rating allows unlimited hours at a variable load and is lower than the standby figure for the same set. A continuous rating is for a constant load without time limit and is lower again. A site that runs its generator for long periods - weak grids, remote sites, data centers expecting long outages - must choose prime or continuous, not standby.

Motors, Step Loads and Non-Linear Loads

A large motor started direct-on-line draws many times its running current for a few seconds, and a generator has far less short-circuit capacity than the grid: the voltage dips and contactors may drop out. Soft starters and frequency converters reduce the inrush. Load steps - the whole building transferred at once by the ATS - are limited by how fast the engine can respond, so loads are often connected in stages.

UPS rectifiers, frequency converters and LED drivers draw distorted currents that heat the alternator and disturb the voltage regulator. For sites with large non-linear loads the generator and the UPS have to be matched, and the makers' figures are checked together.

What a Supplier Needs to Know

The generator maker needs the running loads, the largest motor starts or load steps, the share of non-linear loads, the rating type for the hours the set will run and the site conditions - altitude and ambient temperature reduce engine output on hot and high sites. AXON builds the package around the set the customer chooses - enclosure or container, fuel, exhaust, switchgear and load bank - and checks the package against the same site conditions.

Running load: kW and kVA, with power factors
Starting: largest motor start or largest load step
Non-linear share: UPS, drives, LED
Rating type: standby, prime or continuous
Site conditions: altitude and ambient
Room for growth

In short

kW = real power for the engine; kVA = apparent power for the alternator
The kVA on the nameplate is always higher than the kW the engine delivers
Standby for outages, prime for long variable running, continuous for constant load
Motor starting and load steps matter as much as the running load
UPS and drives need the generator and the equipment matched
Altitude and ambient temperature reduce engine output

What to send with an enquiry

Load list with kW, kVA or power factor, and the largest motors
Starting methods of the motors
UPS and drives: ratings and types
Running hours and the rating type required
Site altitude and ambient temperature
Generator set make and model, if already chosen

Send an enquiry Send us files

Questions we are often asked

Why are generators rated in kVA?

Because the alternator, cables and switchgear are limited by current, which kVA describes. The engine is limited by kW; at the standard power factor the two ratings correspond.

How many kW is a generator rated in kVA?

Less than the kVA figure: the kW rating is the kVA rating multiplied by the standard power factor, and the generator data sheet states both.

Can a generator run at its full standby rating?

Yes, during an outage for the hours its rating allows. For long running at high load it should be chosen on its prime or continuous rating.

How much bigger must a generator be for a UPS?

Larger than the UPS rating, by an amount that depends on the UPS type and its input filter. The UPS and generator makers give the figure.

Does altitude change the generator I need?

Yes. Engines lose power with altitude and ambient temperature; the engine maker gives the reduction for the site, so state the site conditions in the enquiry.