trafo power rating

Thread Starter


Joined Jun 9, 2004
Just a simple question, why do some transformer power rating are specified in Watts and others in VA power?



Joined Apr 20, 2004
Hi Zack,

Great question. I always thought that since W(watts) = E(volts) X I(amps), that it was only some peculiararity of electricians that caused them to want to use volt-amps instead of watts. Unless someone can shed some light, I think there's no difference in the measurements.


Joined Jan 22, 2004

i believe the purpose of using watt to rate a transformer is to inform the user that he can only connect loads up to the rate they say. if they say 500w then you can only connect circuits that require up to 500w only. if your device needs 1000w then surely your transfo will not be able to supply and gets overloaded.

VA = Power output / efficiency
VA = 500w / 0.9 * 0.9
VA = 617


Joined Nov 17, 2003
The terminology Volt-Amps (VA) refers to the complex power rating, which is expressed in the form:

S = P +jQ

Where P is your active power in Watts (W) and Q is your reactive power in Volt-Amps-reactive (VAr).

We also know S as the Apparent Power which has great significance with AC 3-phase electrical systems.

Obviously for DC systems S = P (because Q = 0).