Why UPS systems are rated in VAs and PSUs in Watts; Explaining Watts, VAr and VA

As we mentioned in the previous page, 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.

 Why UPS systems are rated in VAs and PSUs in Watts; Explaining Watts, VAr and VA

 

 

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.

PF (power factor) = P (real power) / S (complex power) = cos(φ)

 

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

 PF = 0.95 * (100%) = 95%

 

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As we mentioned in the previous page, 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.

 Why UPS systems are rated in VAs and PSUs in Watts; Explaining Watts, VAr and VA

 

 

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.

PF (power factor) = P (real power) / S (complex power) = cos(φ)

 

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

 PF = 0.95 * (100%) = 95%

 

Prev3 of 4Next
Use your ← → (arrow) keys to browse