Power Factor

by Steve on November 1, 2009


Power Factor

In a purely resistive AC circuit, voltage and current waveforms are in step (or in phase), changing polarity at the same instant in each cycle.

Where reactive loads are present, such as with capacitors or inductors, energy storage in the loads result in a time difference between the current and voltage waveforms.

This stored energy returns to the source and is not available to do work at the load. Thus, a circuit with a low power factor will use higher currents to transfer a given quantity of real power than a circuit with a high power factor.

A linear load does not change the shape of the waveform of the current, but may change the relative timing (phase) between voltage and current.





In Phase

For AC Current

If the Volts and Amps are in phase (for example a resistive load) then the Power Factor is 1.0


VA x 1.0 =Watts (resistive load)

(Where V is the RMS voltage and A the RMS amperage)


However, in the real world almost all end user electrical installations use AC voltage and various loads are not only resistive, but also reactive.


Resistive loads such as heating elements (filament lamps, strip heaters, cooking stoves, etc.) have a power factor of 1.0.

Reactive loads including inductive or capacitive elements (electric motors, solenoid valves, lamp ballasts, and others ) often have a power factor of less than 1.0.

Voltage & Current in Phase

Power Factor at 1.0 (Unity)

Power Factor at 1.0 (Unity)

Out of phase

A power factor of less than unity (or 1.0) means the volts and amps are ‘out of phase’.

Meaning that the peak of the voltage curve is does not happen at the same time as the peak of the current curve.


The ratio between these two curves (i.e. rms volts time rms amps) and Watts is called the power factor Power Factor (PF).

 Voltage & Current Out of Phase

Power Factor at Zero (Out of Phase)
Power Factor at Zero (Out of Phase)

With AC, if the volts and amps are not precisely in phase you have to calculate the watts by multiplying the volts times the amps at each moment in time and take the average over time.

 VA x PF = Watts


In other words, volt-amps x power factor = watts.

Similarly, KVA xPF = KW or

kilovolt-amps times power factor equals kilowatts.


When you want to know how much the electricity is costing you, you use watts or kilo watts.

When you are specifying equipment loads, fuses, and wiring sizes you use the VA, or the rms voltage and rms amperage.


This is because VA considers the peak of both current and voltage, without taking into account if they happen at the same time or not.


How to find Power Factor

This isn’t easy. For computer power supplies and other supplies that are already power factor corrected the power factor is usually over 90%.

But for high power motors under heavy load the power factor can be as low as 35%.


The industry standard ‘rule-of-thumb’ is to allow for a power factor of 60%, which can be used as an average power factor for design purposes.

It is often desirable to adjust the power factor of a system to near 1.0. This Power Factor Correction (PFC) is achieved by switching in or out banks of inductors or capacitors.


Source information:

Many thanks to: Power Stream 


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