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Reactive Power (kVAR) Calculator

Work out the reactive power (kVAR), apparent power (kVA), real power (kW) and power factor of an AC load from any two of those quantities. It uses the power triangle, so it doubles as a power-factor-correction sizing check.

100% FreeRuns in your browserIntermediate
How to use this calculator

Enter Values

Minimum 0

Minimum 0

Minimum 0

Range 0–1

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter exactly two of: real power P (kW), reactive power Q (kVAR), apparent power S (kVA) or power factor (0–1).
  2. Leave the rest blank; the calculator solves them and the phase angle φ.
  3. Read Q to size a capacitor bank, or compare S against your supply rating.

How it works

In an AC circuit the apparent power S drawn from the supply splits into real power P (which does work) and reactive power Q (which oscillates between the source and the load's magnetic or electric fields). They form a right triangle: S² = P² + Q². The power factor is the cosine of the angle between S and P, pf = cosφ = P/S, and Q = S·sinφ. Given P and pf, S = P/pf and Q = √(S²−P²). Given P and Q, S = √(P²+Q²) and pf = P/S. The tool picks the matching formulas for whichever two boxes you fill.

Worked example

Worked example. A motor draws 8 kW at a power factor of 0.8. Apparent power S = P/pf = 8/0.8 = 10 kVA, reactive power Q = √(10² − 8²) = 6 kVAR, and the phase angle φ = acos(0.8) = 36.87°.

Common mistakes

  • Entering a power factor above 1 — cosφ can never exceed 1.
  • Confusing kW (real, billed as energy) with kVA (apparent, sizes cables and transformers).
  • Giving an impossible pair such as P greater than S, which no real load can produce.

Frequently asked questions

What is reactive power actually doing?

Reactive power flows back and forth between the supply and the load's inductance or capacitance without being consumed. It does no net work but still adds to the current, so it heats cables and loads the transformer.

How do I use this for power-factor correction?

Solve for the present reactive power Q at your existing power factor, then repeat at your target power factor. The difference in Q (kVAR) is roughly the capacitor bank size you need.

Why does a poor power factor cost more?

For the same real load, a lower power factor means a larger apparent power and therefore more current. Utilities charge for that extra kVA (or apply a penalty) because it burdens their network.

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Cite this tool

Zerdly. (2026). Reactive Power (kVAR) Calculator. Retrieved from https://www.zerdly.com/tools/reactive-power-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.