kW vs. kVA: Real Power, Apparent Power, and Why the Difference Bites in Practice

July 2026

A glowing right triangle of light, one leg brighter representing real power, the other dimmer representing reactive power, the hypotenuse a blended glow representing apparent power.

Sparked by a post making the rounds explaining the difference between kW and kVA — worth a fuller, practical treatment here since this trips up generator, UPS, and inverter sizing constantly, including sizing math already touched elsewhere on this site (the DC to AC Across the Spectrum guide).

The three numbers, and how they relate

AC power comes apart into three related quantities, and conflating them is the single most common real-world electrical sizing mistake:

Real power (kW) — the power actually doing useful work: turning a motor shaft, producing heat, lighting a bulb. This is what your utility bill charges for.
Apparent power (kVA) — the total power the source has to supply, combining real power with reactive power. This is what determines the actual current flowing through wires, breakers, transformers, and generator windings — and therefore what those components have to be sized for.
Reactive power (kVAR) — power that sloshes back and forth between source and load without doing useful work, created by the inductance (motors, transformers, ballasts) or capacitance in a circuit. It doesn’t show up on a utility meter, but it still demands real current-carrying capacity from every conductor and switching device upstream.

These three combine as a right triangle: kVA is the hypotenuse, kW is the adjacent side, kVAR is the opposite side. Power factor (PF) is the ratio between them — PF = kW / kVA — and it’s the single number that tells you how close apparent power is to real power for a given load. A purely resistive load (an incandescent bulb, a heating element) has PF ≈ 1.0, meaning kW and kVA are essentially the same number. A motor or anything with a transformer or ballast has PF meaningfully below 1.0 (commonly 0.7–0.9), meaning the kVA the source must supply is noticeably larger than the kW actually being used.

Why this matters in practice, not just on paper

Generators, UPS units, and inverters are rated in kVA, not kW — because that’s what actually determines the current their internal wiring, breakers, and windings have to handle, regardless of how much of that current does useful work. Buying or sizing one using only a kW figure for the load is a common and genuinely costly mistake:

A generator rated 10 kVA at a typical 0.8 power factor can only deliver 8 kW of real power, not 10 kW — the other 2 kVA of capacity is consumed by reactive current the moment you connect motor-driven or transformer-based loads.
Running a heavily inductive load (well pump motors, air compressors, HVAC) close to a generator’s kVA limit — even though the kW figure looks like it has headroom — is exactly the scenario that trips overload protection or causes voltage sag under starting current, which is a real, low-power-factor inrush event, not a real-power problem.
UPS sizing for anything with a switch-mode power supply (most modern electronics) needs the same care: many UPS spec sheets publish both a VA rating and a lower W rating for exactly this reason, and the W number is the one that actually constrains what you can run.

The practical rule of thumb

When a spec sheet gives you only one of the two numbers, treat it as this could be worse than it looks: a kW rating alone hides how much actual current-carrying capacity you need upstream, and a kVA rating alone hides how much real work you’ll actually get out of it. Ask for the power factor (or the other number) before sizing anything real — a generator, a UPS, an inverter, or a breaker — against it.

See Practical Applications for the section this belongs to.