POWER · CALCULATOR

Reactive power calculator

Free reactive power calculator: get reactive power from apparent power and power factor. The formula and a worked example are shown; verify against the NEC.

Written by Chameera Sampath · Not yet reviewed by a licensed electrician. See the methodology.

US (NEC) only · NFPA 70 / NEC — locally adopted editionDefaults are US values. Verify the locally adopted NEC edition and project data.Official reference ↗

Project inputs

Measurements

Apparent power

kVA

Power factor

Updates as inputs change. Unit changes preserve the same physical values.

THE MATH BEHIND THE ANSWER

Method & assumptions

Q = S × √(1 − PF²)

Assumes sinusoidal waveforms and lagging load.

Results do not check every aspect of an electrical installation. The calculators follow US (NEC) practice only; a regional preset changes defaults such as voltage and units, and no result certifies NEC compliance.

Sources and validation →

Worked example

Apparent power: 10 kVA · Power factor: 0.85

Reactive power: 5.268 kvar

This example uses the stated inputs; results for the entered values appear above.

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Estimating and invoicing tool

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