Why companies rate UPS in VAs and PSUs in Watts

1. Introduction2. Apparent Power, Real Power and Reactive Power 3. Explaining and calculating the power factor (PF)4. Sizing PSU and UPS units5. View All Pages

As we mentioned before, the vast majority of power loads will cause a phase shift between voltage and current and will draw in more current than they will actually use. For a load which will consume a certain amount of real power, apparent power increases the larger the phase shift is. The following vectors diagram can be used to explain how increasing the phase shift angle φ will increase the apparent and reactive power while real power remains unchanged.

The angle φ of this phase shift can be used to calculate the power factor, which is usually defined as the ratio between the real power P and apparent power S and/or as the cosine of the angle φ. Being the result of a cosine number it cannot ever be lower than 0 or greater than 1, which is verified by simple reason; it is impossible for real power to surpass the complex power under any circumstances.

Power factor is commonly presented as a percentage, e.g.:

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