TMG-L ArchivesArchiver > TMG > 2003-06 > 1054880208
From: Richard Brogger <>
Subject: Re: [TMG] Comment on UPS
Date: Fri, 06 Jun 2003 01:16:48 -0500
Dennis Lee Bieber wrote:
> UPS units are rated in Volt-Amps (I've never been quite clear on
> how that differs from plain old wattage).
You may not want to know. <g>
In a DC circuit Volts * Amps (VA) does equal watts. In an AC circuit,
Volts * Amps * Power factor equals watts. With a purely resistive
circuit the power factor is one (unity) and thus V*A still = Watts.
However, most loads are not purely resistive. Capacitance and
inductance will cause the voltage and current to be out of phase and
one is, so to speak, not pulling at the same angle as the other. When
volt and current are out of phase, they will not produce as much power
(watts) as when they are in phase.
Capacitance will produce a leading power factor while any load that
involves windings, such as a motor or transformer, will produce a
lagging power factor. All loads combined tend to average out to about
a 0.8 lagging power factor. If, for an AC circuit, you were to say
Volts * Amps * 0.8 = Watts, you will probably be closer to the correct
That is a rough explanation but I hope that it will help.