Power Factor Calculator
Calculate power factor and the correction capacitance needed to improve it.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Power Factor Calculator does
Power factor is the ratio of real power to apparent power. A low factor means large reactive currents flowing without doing work, which wastes cable capacity and attracts utility penalties. Capacitors supply that reactive power locally instead of drawing it from the grid.
Formula
PF = Real power ÷ Apparent powerReactive power Q = S × sin(arccos(PF))Correction kVAR = Q_current − P × tan(arccos(Target PF))Capacitance = Q ÷ (2π × f × V²)
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Known values | one of 2 options | Yes | — |
| Real power | W | Yes | Accepts more than 0. |
| Apparent power | VA | In some modes | Accepts 0 or more. Shown Known values is Real and apparent power. |
| Voltage | V | In some modes | Accepts 0 or more. Shown Known values is Voltage, current and real power. |
| Current | A | In some modes | Accepts 0 or more. Shown Known values is Voltage, current and real power. |
| Supply type | one of 2 options | Yes | — |
| Target power factor | number | Yes | Accepts 0.01 or more, up to 1. |
| Supply frequency | Hz | Yes | Accepts 1 or more. |
How to use it
- Choose Known values and Supply type.
- Enter Real power, Target power factor and Supply frequency.
- Fill in the remaining inputs the form shows for your choice.
- Select Calculate.
Worked example
8,000 W real against 10,000 VA apparent, corrected to 0.95 on a 230 V 50 Hz supply.
- Real
- 8000 W
- Apparent
- 10000 VA
- Target
- 0.95
- Frequency
- 50
PF 0.8, angle 36.87°, reactive 6,000 VAR. Target needs 2,629 VAR, so 3.371 kVAR of correction — about 202.8 µF.
Frequently asked questions
What causes a poor power factor?
Inductive loads: motors, transformers, fluorescent ballasts and welders. They draw magnetising current that lags the voltage.
Can power factor exceed 1?
No. It is a ratio of real to apparent power and one cannot exceed the other. A leading factor above unity means the calculation is wrong.
Method and sources
Method. Power factor as real power over apparent power, with the reactive component and the correction capacitance required to reach a target.
Applies to. None for the arithmetic; installation of correction equipment is governed by national wiring regulations
Assumptions
- A linear load with sinusoidal supply, so displacement power factor describes the whole picture.
Limitations
- With non-linear loads, harmonic distortion contributes to apparent power and total power factor falls below the displacement figure this calculates — capacitors do not correct that component.
- Correction capacitors can resonate with system inductance and amplify harmonics, which is why correction on a distorted supply is a design exercise rather than an arithmetic one.
Sources
- The national wiring regulations and your supply agreement — Varies by country and utility. Any power-factor obligation or penalty, and the rules governing correction equipment.