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Reactive Power Calculator

Find reactive, real and apparent power.

How it is calculated

The tool treats the values as a single-phase circuit. Apparent power is S = V × I in VA, with voltage in volts and current in amperes. Real power is P = S × pf in watts, and reactive power is Q = S × √(1 − pf²) in VAR, which is the same as S × sin φ when pf = cos φ. The three values satisfy S² = P² + Q². The power factor must be a decimal between 0 and 1, so 0.8 is correct and 80 or a negative value gives no result. Voltage and current may not be negative. The tool does not say whether the load is lagging or leading, and there is no √3 factor, so for a balanced three-phase load you must enter phase quantities or multiply the results yourself.

Worked example

A load on a 230 V supply drawing 10 A at a power factor of 0.8. Apparent power is 230 × 10 = 2,300.00 VA. Real power is 2,300 × 0.8 = 1,840.00 W. The sine part is √(1 − 0.64) = √0.36 = 0.6, so reactive power is 2,300 × 0.6 = 1,380.00 VAR. As a check, 1,840² + 1,380² = 5,290,000, which equals 2,300². The tool reports 1,380.00 VAR reactive, 1,840.00 W real and 2,300.00 VA apparent.

Try this example →

Common mistakes

  • Entering the power factor as a percentage (80). The field takes a value from 0 to 1, so 0.8 is right and 80 is rejected.
  • Using a power factor from the nameplate at full load for a motor running lightly loaded. Power factor drops at partial load, so the result will not match what the meter sees.
  • Applying the numbers directly to a three-phase machine using line voltage and line current. This calculation has no √3 factor, so the powers come out too small.
  • Expecting a sign for inductive or capacitive loads. Q is always shown positive, so you must keep track of whether the reactive power is lagging or leading yourself.
  • Adding apparent powers of different loads together arithmetically. Real and reactive powers add; VA values add only when the loads share the same power factor.

About this calculator

In an AC circuit, total power splits into three related quantities that form the power triangle: real power P (measured in watts) does actual work, reactive power Q (measured in VAR) sloshes back and forth into inductive or capacitive loads like motors and transformers without doing net work, and apparent power S (in VA) is their vector sum. This calculator finds Q = V·I·√(1−pf²) — equivalently V·I·sin φ — from voltage, current and power factor (pf = cos φ, the phase angle between voltage and current).

Electrical engineers and plant electricians use reactive power calculations to size power-factor-correction capacitors, since utilities often penalize industrial customers with low power factor through surcharges on their bill. Facility managers, HVAC and motor technicians, and electrical engineering students use it to understand why a circuit's apparent power (what the utility bills for capacity) can exceed the real power actually consumed as useful work.

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Comments (1)

  • Editorial team

    To see what power-factor correction would do, run the tool twice at the same real power. The 230 V, 10 A, 0.8 example carries about 1,840 W and 1,380 VAR. Correct to 0.95 and the same watts need only about 8.42 A, leaving roughly 605 VAR, so a capacitor bank near 775 VAR does the job. The tool is single-phase and does not show lagging or leading. What kind of load are you looking at?

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