Herbert Graf wrote:
>
>         I have a question about W and VA. I just purchased a UPS (Uninterruptible
> Power Supply) that has a rating of 400VA and 250W, my question is why are
> they not the same? I always thought that in most circumstances P = V x I,
> and doesn't VA just mean V x I, the same thing? I have a feeling that I am
> missing something huge here. Thanks for any info.
>


You are.  You are dealing with AC power, not DC power.  Watts, for DC is
P=E*I, straight and simple.  But for AC, there is another factor, called
the power factor, or Cos of Theta (a capital O with a horizontal bar
thru the middle of it).  In an AC system, you have impedances which are
a result of the inductance and capacitance of the circuit.  This
impedance may cause the current to lead or lag the voltage waveform.
Because they are not in sync, the apparent power is not the same as the
true power.  V*A is commonly used for AC devices because a "wattmeter"
(designed for DC) would not give a true power reading.

--
Jean Bourvic

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