What is meant real and reactive

Why should a device be used as a power supply instead of the watt (W)?

Here is an example: I have a device with an apparent power of 10 VA cosϕ = 0.7. It uses P. = S. ∗ cosϕ = 0.7 ∗ 10 W. = 7 W.. On the other hand, it also uses Q = S. ∗ sinϕ = 0.714 ∗ 10 = 7.14 W..

We could then add these two together as they both consume 7W and 7.14W at the same time and get 14.14W which is obviously wrong as the device never consumes more than 10W even if its power factor was somehow corrected back to 1.

Basically, var is used to indicate that reactive power is out of phase with active power and that you cannot add it up. On paper, power is power, but through the use of var (sometimes mistakenly called VAr, so r is not just appended, but is part of some other unit, the name of which is often misspelled), volt-amps, and watts, let's hope ( at least) to) clearly identify the context in which this unit is used. Var means not only size, but also phase.

This is actually nothing new and is also common in other areas. For example, we have radians and steradians as the units of measure for angles, which are actually 1. The same also applies to bits. Although we can technically say that the speed of a communication link is 1 MHz, this implies a different context than 1 Mbit / s.

Severo Raz

Why should you add reactive power and active power apart from units? This is what I meant by the "reminder" purpose of the var unit.


@Wolter Hellmund Well that would I not, but in my experience, if many people wanted to calculate the total load on the system (which is needed when choosing the dimensions of conductors, transformers, etc.) it would give incorrect results. Hopefully, if we use a different unit, people will remember that they can't just add up.