Watts, Volt Amps and Power Factor

Uninterruptible power supplies typically use Volt Amps (VA) to define their output capacity while the load of most electrical devices is listed in watts (W).  And when dealing with AC power, the two are not the same.  The difference is the Power Factor (PF).  The good news is that many newer PC power supplies incorporate active Power Factor Correction circuits (PFC), which electronically adjusts the PF back to 1 making VA approximately equal to W.  This can help simplify sizing a UPS.


For devices with Active PFC: Watts ~ Volt Amps


For devices without PFC: Watts = Volt Amps x .7


For electrical devices that do not incorporate power factor correction, a general rule of thumb is to assume a PF = .7 (or .5 if you want to be more conservative).


Ultra 850VA Power Protection Unit and UPS Review - Cases and Cooling 28


Power factor is defined as the ratio of true power (measured in Watts) to apparent power (measured in Volt Amps).  It measures how effectively AC power is being used by a device.  The difference between true power and apparent power is expressed as the power factor and results from the way true power and apparent power are measured.  Ideally we would like to have true power and apparent power equal to one another, which would result in a PF of 1.00 or 100% effective power utilization.


Apparent Power = AC Volts x AC Amps = VA (Volt Amp)


Purely Resistive AC Load: VA = Watts (same as DC circuits)

Inductive/Reactive AC Load: VA x PF = Watts


The basic formula for true power includes the power factor (the power factor compensates for the extra reactive power produced by inductance and capacitance in an AC circuit). 


True Power = AC Volts x AC Amps x PF = Watts


True power (also referred to as working power) defines power that produces work, heat and light.  We see true power in DC circuits and in AC circuits that are powering purely resistive loads like a resistance heater or light bulb.


Apparent power is found only in AC circuits.  Along with the power doing work, apparent power also takes into account the extra power needed to create the alternating magnetic fields inside inductors (transformers and motors) and to charge capacitors.  Devices that incorporate inductance and capacitance in an AC circuit are referred to as reactive loads.  Reactive power flows into and out of a circuit but does not produce any work.  Nearly all AC devices include some form of reactive load, which causes the PF to be less than 1 and the apparent power to always be greater than the true power.


When AC is applied to a purely resistive load, the current rises and falls in almost perfect harmony with the voltage.  Plotting a graph of the AC voltage and current will result in two classic sine waves that are in alignment with each other.  But computers are not resistive loads.  The power supply inside a PC or monitor includes inductors and capacitors among other things, which result in a complex reactive load.  A reactive load causes the alternating current to become out of phase with the alternating voltage.  A load with predominantly inductive reactance will cause the current sine wave to lag behind the voltage sine wave by a certain amount (phase angle).  When the two sine waves are in perfect alignment (purely resistive load) the Power Factor is 1.00.  The more the current lags behind the voltage, the smaller the Power Factor value becomes.  Having the current out of phase with the voltage can also induce harmonic distortions back onto the power lines.


For example, when AC is used to power a light bulb (resistive load), electricity flows thru the filament and is converted into heat and light.  Under these conditions the true power and apparent power are essentially the same so the PF ~ 1.  All of the incoming AC power is being effectively used.  If that same AC has to first go thru a transformer or power supply (reactive load) before reaching the light bulb filament, extra current is required to create the magnetic fields inside the inductors and keep the capacitors charged. 


Most newer PC power supplies contain Power Factor Correction circuits.  A power supply without any PFC will normally exhibit a PF of <0.70 and will generate significant harmonics, which can distort the AC source waveform.  Power supplies with active PFC will have power factors >0.95 with minimal harmonics. 


Ultra 850VA Power Protection Unit and UPS Review - Cases and Cooling 29


Active PFC (PF=0.99)             Passive PFC (PF=0.75)     No PFC (PF= 0.55)


What this really means to the end user is that a PC power supply with PFC will pull less current from the AC mains to generate the same amount of DC power as a similar non-PFC unit.  For commercial users who are billed based on VA usage and PF, this may save you money.  However, most residential users are billed per kilowatt-hour, which ignores the load’s reactive power component.  Because of this a PSU with active PFC won’t save the typical residential user any money on their electric bills.  However if you have a room full of computers operating on a single circuit, equipping them with PFC power supplies will draw significantly less current (therefore allowing more computers 🙂


Important note: a power supply that incorporates active PFC circuitry might seem to be more efficient than one that does not have PFC. It’s true, a power supply with active PFC will pull less current than one without PFC, however it is not more efficient than a comparable unit without PFC. This is because true power (watts) is used in the efficiency calculation, not apparent power (VA).  A power supply with active PFC is more effective at converting electrical power but is not more efficient.  In fact, because of the additional PFC circuitry, a power supply with active PFC may be slightly less efficient than the same model without active PFC.


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