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Re: [Phys-l] Watts and VA



Pete Lohstreter wrote:
Found this on a web site for a UPS.
Output Power Capacity 600 Watts / 750 VA Isn't a Watt = VA ? Why the difference?

Pete Lohstreter
Suppose that was an accurate statement of the UPS capability.
Suppose that the nominal output voltage is 120 volts.
Then the in-phase output current would need to be 5 amps.

But a UPS might well be driving a transformer or a motor, which involve
inductive windings, where the current lags the applied voltage.
(Quick: recite CIVIL as the relevant memory aid; C leads with I, L lags).
For this sort of load, a UPS could NOT provide 600 watts, if it could
only provide 5 amps.

So the designers allow the current to lag by up to 37 degrees while
allowing a greater current, up to 6.25 A ,in order to produce
the rated output power of 600 W

Brian W