I am taking a course on AC circuits and so far I have seen that for the different types of power (read real, reactive and complex) different units are used.
For the real power, which is power in fact dissipated by the system, the SI compliant watt is used. I am very much in agreement with this.
However, for reactive and complex power the equivalent SI derived unit volt-ampere is used. In the special case of reactive power, a lowercase "r" is appended.
Now, I wonder: why should any other unit be used instead of watt, if the purpose of the unit is to specify the type of magnitude of a value? Moreover, why should a symbol be appended to the unit when its use is simply that of a reminder?
Also, is there a reason why using watts for reactive power would be considered wrong?
Best Answer
Simply because a watt is a measure of work done. The real part of V*A is watts. There are exact equivalents in mechanical systems. \$W=\dfrac{J}{s}=\dfrac{N*m}{s}\$ so \${N*m}\$ here is Force that does work through a distance but it can also be a measure of Torque over a distance and that is static(non moving). One is the potential to do work, one is the work itself.
Using watts for reactive power would be wrong because reactive power is stored power and not capable of doing work.
Using VAr - r for reactive is just short hand for the purely imaginary part, instead of using i or j (for you physicists out there).