Power Factor Calculator
Power factor from voltage, current and real power, with the reactive component and the capacitance that would correct it.
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The formula
S = V × I PF = P ÷ S = cos φ Q = √(S² − P²) C = (Q₁ − Q₂) ÷ (2πf V²)
Real, reactive and apparent power form a right triangle, which is why they combine by Pythagoras rather than by adding. Power factor is the cosine of the angle between the voltage and the current — at unity they are in step and every amp does useful work.
What to watch out for
Power factor is the fraction of the current you draw that actually does something:
- Resistive loads — heaters, filament lamps, kettles — sit at 1.0. Every amp does work.
- Motors and transformers run at 0.7–0.85. They need magnetising current that returns to the supply each cycle.
- Cheap switch-mode supplies can be worse than 0.6, though for a different reason — harmonic distortion rather than phase shift, which a capacitor will not fix.
A poor power factor does not increase your energy use in kWh. It increases the current, and therefore the cable losses, the transformer loading and — for industrial customers billed on kVA — the bill.
Frequently Asked Questions
What is a good power factor?
Above 0.95 is generally considered good, and many utilities penalise industrial customers below 0.9. A purely resistive load is 1.0, which is the best possible.
Does bad power factor cost me money?
Domestically, almost never — household meters bill real energy in kWh, and reactive power does not register. Commercially it often does, because larger tariffs charge for kVA demand or apply a penalty below a threshold.
What is the difference between kW and kVA?
kW is the power doing useful work; kVA is the product of voltage and current, which is what the cables and transformer must be sized for. They are equal only at unity power factor.