Power Factor Calculator (AC)
Find the power factor, phase angle and reactive power of an AC load from its real (watts) and apparent (VA) power.
How it works
PF = cos φ = P (W) ÷ S (VA) · Q = √(S² − P²)
Power factor is the fraction of the current that actually delivers useful power. It equals real power (watts) divided by apparent power (volt-amps), or equivalently the cosine of the phase angle between voltage and current. A power factor of 1 is ideal; inductive loads such as motors pull it below 1, meaning the utility must supply extra current (and reactive power, in VAR) that does no work but still heats the wires.
Worked example
A motor draws 1,000 VA of apparent power but produces 800 W of real power. Power factor = 800/1000 = 0.8. The phase angle is arccos(0.8) ≈ 36.87°, and the reactive power is √(1000² − 800²) = √360,000 = 600 VAR.
Sources: Power factor — Wikipedia
Frequently asked questions
Why is a low power factor a problem?
For the same useful work a low power factor means more current, which causes larger I²R losses in cables and transformers. Utilities often bill commercial customers a penalty for a poor power factor.
How do you improve power factor?
Inductive loads are usually corrected by adding capacitors that supply the reactive power locally, bringing the phase angle back toward zero and the power factor toward 1.
What is the difference between W, VA and VAR?
Watts (W) are real power that does work, volt-amps (VA) are apparent power (voltage × current), and volt-amps reactive (VAR) are the reactive power that shuttles energy back and forth without doing net work. They form a right triangle: S² = P² + Q².
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This calculator is for educational purposes. Double-check important results before acting on them.