Inductive loads such as motors, transformers and fluorescent ballasts draw current that lags the voltage. Part of that current shuttles energy back and forth each cycle instead of doing work, yet it still heats conductors and uses up transformer capacity. Power factor measures how much of the current is useful. This calculator finds the power factor from measured kW, volts and amps, and sizes the capacitor bank — in kVAR and in microfarads — needed to bring it up to a target.
How to use the power factor calculator
- Choose Size a correction capacitor or Find the power factor.
- Choose single-phase or three-phase, and enter the real power in kW and the voltage (line-to-line for three-phase).
- To find the power factor, enter the measured current.
- To size a capacitor, enter the present and target power factor, the supply frequency, and for three-phase, whether the bank is connected in delta or wye.
- Read the kVAR needed, the capacitance per phase, and how much the current and kVA fall. The power triangle compares before and after.
Power factor formulas
Real power P, reactive power Q and apparent power S form a right triangle. A capacitor supplies leading reactive power locally, shortening the Q side while leaving P unchanged, so S and the current both shrink.
Worked example
100 kW at PF 0.75, corrected to 0.95, 480 V three-phase, 60 Hz (the default)
Q1 = 100 × tan(arccos 0.75) = 100 × 0.8819 = 88.19 kVAR. Q2 = 100 × 0.3287 = 32.87 kVAR.
Capacitor bank: 88.19 − 32.87 = 55.32 kVAR. In delta, each capacitor sees 480 V and needs 55,320 ÷ (3 × 377 × 480²) = 212.3 µF.
Apparent power falls from 133.3 kVA to 105.3 kVA, and the line current from 160.4 A to 126.6 A — 21.1% less.
Finding the power factor. A three-phase load measured at 100 kW, 480 V and 160 A has S = 1.732 × 480 × 160 = 133 kVA, so PF = 100 ÷ 133 = 0.752, with 87.7 kVAR of reactive power.
Multipliers for common corrections
| From \ to | 0.90 | 0.95 | 1.00 |
|---|---|---|---|
| 0.70 | 0.536 | 0.692 | 1.020 |
| 0.75 | 0.398 | 0.553 | 0.882 |
| 0.80 | 0.266 | 0.421 | 0.750 |
| 0.85 | 0.135 | 0.291 | 0.620 |
Multiply the kW by the factor to get the kVAR required. Correcting the last step to unity takes a large amount of capacitance for little extra benefit, which is why 0.95 is a common target.
Where to install correction
Capacitors can sit at the service entrance as one switched bank, on a distribution panel, or directly at large motors. Correction at the motor reduces current in every conductor upstream, but the capacitor must be sized from the motor manufacturer’s table so it never exceeds the motor’s no-load magnetizing kVAR. Central banks with automatic steps follow a changing load and avoid over-correction at night or on weekends.
The three-phase power calculator breaks a load into kW, kVA and kVAR, and the kVA calculator shows how much transformer capacity correction frees up. For the reactance of a single capacitor at line frequency, see the reactance and resonance calculator.
Capacitor banks store energy and must include discharge resistors and proper protection. These calculations are for estimation only; follow your local electrical code and have a licensed electrician or engineer design and install correction equipment.
Frequently asked questions
What is power factor?
The ratio of real power (kW) to apparent power (kVA). A power factor of 1 means all the current does useful work; 0.75 means the circuit carries a third more current than the real power requires. For sinusoidal loads it equals the cosine of the angle between voltage and current.
How do I calculate the capacitor kVAR needed?
kVAR = kW × (tan(arccos PF1) − tan(arccos PF2)). To raise 100 kW from 0.75 to 0.95: 100 × (0.8819 − 0.3287) = 55.32 kVAR. The bracketed difference is the multiplier printed in power factor correction tables.
Why do utilities charge for low power factor?
Low power factor means more current for the same energy sold, so the utility needs bigger transformers and conductors and loses more energy as heat. Many commercial tariffs add a charge when power factor drops below about 0.90 to 0.95, or bill demand in kVA instead of kW.
Can you over-correct power factor?
Yes. Too much capacitance makes the load leading, which can raise voltage, especially at light load, and fixed capacitors switched with a motor can cause self-excitation when the motor is turned off. Correct to around 0.95 rather than 1.0, or use an automatically switched bank.
Will a capacitor fix the power factor of electronic loads?
Not fully. Computers, LED drivers and variable-speed drives often draw distorted current. That distortion lowers the true power factor but is not reactive power a capacitor can cancel, and capacitors can even resonate with harmonics. Those loads need filters or drives with built-in correction.