Hej Bent Nej ikke helt men i dette tilfælde er det tilnærmet da virkningsgraden på en elektronisk trafo er tæt på 1. På en jernkerne er den mindre end 1 og her kan det ved store trafoer have betydning.
forklaring : (fra hjemmesiden http://www.powerstream.com/VA-Watts.htm )
Since watts is volts times amps, what is VA? VA (or volt-amps) is also volts times amps, the concept however has been extended to AC power. For DC current
VA = Watts (DC current).
In AC if the volts and amps are in phase (for example a resistive load) then the equation is also
VA=Watts (resistive load)
where V is the RMS voltage and A the RMS amperage.
In AC the volts and amps are not always in phase (meaning that the peak of the voltage curve is does not happen at the peak of the current curve). So in AC, if the volts and amps are not precisely in phase you have to calculate the watts by multiplying the volts times the amps at each moment in time and take the average over time. The ratio between the VA (i.e. rms volts time rms amps) and Watts is called the power factor PF.
VA·PF = Watts (any load, including inductive loads)
In other words, volt-amps x power factor = watts. Similarly, KVA*PF = KW,
Or kilovolt-amps times power factor equals kilowatts.
When you want to know how much the electricity is costing you, you use watts. When you are specifying equipment loads, fuses, and wiring sizes you use the VA, or the rms voltage and rms amperage. This is because VA considers the peak of both current and voltage, without taking into account if they happen at the same time or not