Real, apparent & reactive power (kW · kVA · kVAr)
Three different 'powers' on an AC system — what each one is, and why they don't simply add up.
Three powers, one load
On AC, an inductive load deals with two things at once: real power (kW) that does the actual work — turning the shaft, making heat, giving light — and reactive power (kVAr) that only shuttles energy in and out of magnetic fields without doing work.
The supply has to deliver both together. Their combination is the apparent power (kVA) — the product of the voltage and the total current actually flowing. It's what sizes cables, transformers and switchgear, because they carry the whole current, working or not.
The power triangle
Because the reactive part is 90° out of phase with the real part, they don't add arithmetically — they add as a right-angled triangle. Real power is the base, reactive power the upright, and apparent power the hypotenuse. Power factor is the ratio of the base to the hypotenuse.
That geometry is why correcting power factor (shortening the reactive upright) brings the apparent power down close to the real power — the supply ends up delivering nearer to just the working load.
Reading a nameplate
A motor rated in kW tells you its mechanical output; the kVA it draws from the supply is larger, set by its efficiency and power factor. Sizing the supply, protection and cable from kW alone undersizes them — the current comes from the kVA.
Related faults
Three-phase distribution board badly unbalanced
One phase of a three-phase board runs much hotter / higher current than the others — nuisance tripping on that phase, a hot neutral, or a warm phase conductor at the board.
Large motor start trips the distribution board
Starting a large three-phase motor (lift, pump, compressor, aircon) trips the board feeding it or dips the supply — an inrush/coordination issue rather than a running fault.
Motor running hot / overheating
The motor runs but gets excessively hot — too hot to touch, smell of hot insulation, or thermal protection cutting in after a while.
Related definitions
Power factor
How much of the current actually does useful work — and why a poor figure costs you capacity and money.
Ohm's law & power
The relationship between voltage, current and resistance — and how it gives you power.
Diversity & load balancing
Not everything runs at once, and loads should be spread across phases — the basis of sizing and balance.