02/11/2025
Generators are rated in kVA (kilovolt-amperes) because it represents the total apparent power, which accounts for both real power (kW) and reactive power (kVAR). Since manufacturers don't know the power factor of the load that will be connected, they rate the generator's capacity by its maximum voltage and current handling (kVA) to ensure the conductors and insulation can handle the total electrical load, regardless of whether it's a resistive or inductive load. The real power (kW) is what actually performs work, and it is always less than or equal to the apparent power, as it depends on the load's power factor.
Why kVA is used for generators
It's a measure of total capacity: Generators produce both useful (real) power and non-useful (reactive) power. The kVA rating accounts for both components, providing a more complete picture of the generator's total power-handling capability.
It's based on physical limits: The generator's physical components, like the conductors and insulation, are designed to handle a specific voltage and current. The kVA rating is a direct measure of this capacity, as kVA is the product of voltage and current, and is independent of the load's power factor.
It doesn't depend on the load's power factor: The real power (kW) output is calculated by multiplying the apparent power (kVA) by the power factor (kW = kVA x pf). Since the power factor can vary, and the generator must be able to handle any load connected to it, a single kW rating is not sufficient. A 100 kVA generator could supply anywhere from 100 kW (at a power factor of 1) to a lower value for an inductive load with a power factor of 0.8, for example.
Why kW is not sufficient for generators
kW only measures real power: The kW rating only accounts for the power that does useful work (like heating or light). It does not account for the reactive power needed to establish magnetic fields in inductive loads like motors, which are very common.
It's a dependent value: Because the kW output depends on the power factor of the load, it is not a reliable indicator of the generator's overall capacity. Manufacturers need a rating that represents the generator's physical limits, which is why kVA is used.
In short, kVA is a rating of a generator's capacity to supply power, while kW is a rating of how much of that power is being used for useful work at any given moment.
