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.
Enter Values
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How to use this calculator
- Enter exactly two of: real power P (kW), reactive power Q (kVAR), apparent power S (kVA) or power factor (0–1).
- Leave the rest blank; the calculator solves them and the phase angle φ.
- 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.
Related tools
- Power Factor Calculator
- Ohms Law Calculator
- Three Phase Power Calculator
- Voltage Drop Calculator
- kVA to kW Calculator
- Single Phase Power Calculator
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Tip: Enter any known values to calculate the remaining results.
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