Kilowatts vs. kilovolt-amps

Real and apparent power

Real power is the net rate of energy transfer to the load, averaged across the AC cycle. Apparent power describes the product of RMS voltage and RMS current. When current is out of phase with voltage or is distorted by a nonlinear load, the apparent-power value can exceed real power. [2]

Power factor is real power divided by apparent power. At unity power factor, the numerical values of kW and kVA are equal. At a lower positive power factor, delivering the same real power requires more apparent power and, at the same voltage, more current. This can affect alternator capacity even when the engine’s real-power limit has not been reached. [1]

Conversion and an illustrative example

If the load is 12 kW at a power factor of 0.8, its apparent power is 12 ÷ 0.8 = 15 kVA. A 15 kVA rating must not automatically be read as 15 kW. This example illustrates the arithmetic; the equipment’s own rating conditions still determine usable output. [1]

The rated power factor printed on a generator specification is a design or rating condition. It does not force every connected appliance to operate at that power factor. The combined load characteristics and the generator’s capabilities need to be evaluated together. [1]

Single-phase and three-phase relationships

For single-phase AC, kVA = Vᵣₘₛ × Iᵣₘₛ ÷ 1,000. For a balanced three-phase system, kVA = √3 × VLL × Iline ÷ 1,000, where VLL is RMS line-to-line voltage and Iline is RMS line current. Multiply either apparent-power result by the applicable power factor to obtain kW. [3]

Why both ratings matter

The engine must supply the real power plus losses, while the alternator must also carry the electrical current and provide the required excitation. Low power factor, motor starts and nonlinear loads can influence the alternator selection. A kW-only load list may therefore miss important electrical constraints. [3]

For an unbalanced three-phase system, the balanced formula alone is insufficient. Loading and current must be checked by phase. Neither a kW conversion nor a kVA conversion establishes acceptable motor-start performance or voltage quality.