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Why companies rate UPS in VAs and PSUs in Watts

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.

Hypothetical ideal 160W load and real 160W loads with PF's of 94% and 90% respectively

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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5 comments

  1. Great read, I found this easier to understand when compared with the first one last week.

  2. that helps clear up some things for me, thanks!

  3. Got lost a few times, but for the most parts, thumbs up !

  4. When you say that reactive power does not move any energy, you are wrong. In reality, the reactive power is generated by any coil or capacity and is “send” back to the power plant, adding extra current to the power line. This is the correct way to explain reactive power.

  5. Definition of Energy : The capacity of a system to *perform work*. Reactive power moves no energy because, like you said yourself, it is being returned to the power plant. 99% of the equipment does not make any use of it. You’re confusing current and/or power with energy, energy has an entirely different meaning.

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