Power is simply the product of voltage and current and has the units VA (for Volt-Amps). You can measure the voltage, measure the current and calculate the power. Power measured in VA is therefore called Apparent Power. The real, or True Power consumed by a system is different, and this is measured in Watts. In a DC system both the Watts value and the VA value are the same. In AC systems we introduce a complication. This is because the voltage and current waveforms are sinusoidal in nature and need not necessarily be in-phase (they both reach their peaks, troughs and cross the zero axis at the same time). To explain, let us take a look at a system where the voltage and current waveforms are in phase for an AC system:

This graph shows the power waveform (P above) for a system with unity power factor. The voltage and current waveforms are both in phase. Note that the power waveform (plotted on the RHS axis) is ALWAYS POSITIVE!

As is the case with anything other than a purely resistive load, the voltage and current waveforms do not appear in phase. Depending upon the type of load, either inductive or capacitive, the current waveform can be said to be lagging the voltage waveform (inductive) or leading it (capacitive).

Let us take a look at a lagging power factor waveform:

The voltage and current are no longer in phase, and this means that the power (volts x amps) now has a NEGATIVE COMPONENT (shown hatched). This means that the real power is actually less than the total power as some of the power is effectively redirected back to the source. (This is not a good thing as it actually causes problems to the supply network resulting in harmonic problems and waveform distortion). In this instance, if we were to measure the Voltage, V, and the Current, I and multiply them together, we get the Volt-Amp rating as described above. However, the true power consumed by the system is measured in Watts, and this is the POSITIVE POWER amount on the graph above, which is NOT the same as multiplying the voltage and current. The relationship between WATTS and VA is known as the POWER FACTOR.

Power (Watts) = Power (VA) x PF

And PF is a number between -1 and 1. The sign is to show whether the power factor is leading or lagging. [If you want to get into the nitty gritty, the Power Factor can be shown to be the cosine of the phase angle between the voltage and current waveforms.]

In modern electronic systems however, the current waveform is not sinusoidal at all, but instead looks something like this:

This is the waveforms shown for a rectifier - a device used for converting AC power into DC power. It is used in practically all traditional electronic circuits.

How can you possibly calculate a power factor for this arrangement? A genius mathematician called Fourier once proved that any repeating waveform can be shown to be comprised of an infinite number of pure sine (or cosine) waveforms of varying frequencies and amplitudes (and incidentally also discovered the Greenhouse Effect, that certain gases cause global warming). This is heavy maths and too complicated for this article, but all you need to know, is that the result of this proves that the power factor of such a system is around 0.7.

Those of you alert at this stage, may of noticed that an awful lot of UPS systems are rated for a power factor of 0.7 for exactly this reason. UPS systems are historically rated in Volt-Amps (VA), however they also have a Watts rating. Neither can be exceeded or you will be overloading the UPS.

Take a 1000VA UPS at 0.7pf. This means the Watts rating is 700W. If you have a computer system that consumes 900VA of power at a power factor of 0.7 you have:

VA =900

Watts = 900x0.7 = 630W.

Therefore it is safe to use your UPS on this application.

Let us now assume, that (due to modern regulations) the PC power supply is power factor corrected, and that it still consumes 900VA, but at a power factor of 0.9. Now we have:

VA=900

Watts=900x0.9 = 810W.

Now the UPS is overloaded, and we will need to look for a UPS system that has a Watts rating in excess of 810W.

In conclusion, what you really need to know, is that you cannot depend upon the VA rating of a UPS to determine if it is suitable for your load. If you do not know the power factor of your load, assume it to be 1 and make VA and WATTS the same value. This way, you will never undersize your UPS system.