Electrical Power Factor Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/9/2026
Power factor is calculated with cos(φ) = real power ÷ apparent power. An installation with 800 W of real power for 1000 VA of apparent power has a power factor of 0.8.
Explanation
In an alternating-current electrical circuit, the apparent power (the product of RMS voltage and RMS current, expressed in volt-amperes) isn't always fully converted into useful work. Part of this power can be periodically sent back to the grid without ever being consumed, a phenomenon caused by the phase shift between voltage and current on so-called reactive loads (inductive, like an electric motor or a transformer; or capacitive). Power factor quantifies this efficiency: a factor of 1 (the maximum possible) means all the apparent power is actually useful, typical of a purely resistive load like a classic electric heater or an incandescent bulb; a factor below 1 means part of the power supplied by the grid does no useful work at all, even though it keeps current flowing through cables and equipment. A low power factor has a real cost: the electrical grid and wiring must be sized for the total apparent power, not just the actually useful power, which is why large industrial consumers (with many electric motors) are often billed based on their power factor, and encouraged to correct it (generally by adding capacitors that offset the inductive phase shift) to bring it closer to 1.
Example: 800 W real power, 1000 VA apparent power
Inputs
Real power: 800 W. Apparent power: 1000 VA.
Calculation
cos(φ) = 800 ÷ 1000 = 0.8.
Result
This installation has a power factor of 0.8, meaning 80% of the apparent power supplied is actually useful.
Frequently asked questions
Why does an electric motor have a power factor below 1?
Because an electric motor contains windings that create a magnetic field needed for it to operate, an inductive behavior that shifts the current out of phase with the voltage. This phase shift means part of the electrical energy flows through the circuit without ever being converted into useful mechanical work, which lowers the power factor below 1, unlike a simple purely resistive heating appliance.
How do you correct a power factor that is too low?
The most common method is adding capacitors in parallel on the installation, whose capacitive effect offsets the inductive phase shift created by motors and other reactive loads. This correction, called power factor correction, brings the factor closer to 1, reduces the apparent power demanded from the grid for the same useful power, and can thereby lower the electricity bill for large industrial consumers billed based on this factor — an especially significant concern for a three-phase installation, whose voltage-to-current relationship our transformer turns ratio calculator touches on from another angle.
Does power factor also matter for households?
For a standard residential home, power factor generally has little direct impact on the bill, since residential billing is usually based only on the real power consumed in kWh (see our electrical power calculator for that basic calculation). It becomes a significant economic concern for large industrial and commercial consumers, however, whose electricity contract often includes a penalty if the power factor drops below a contractual threshold.