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Re: [Phys-l] Watts and VA



On 05/09/2009 01:48 PM, 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?

Power factor.
http://en.wikipedia.org/wiki/Power_factor

In the simplest case, this depends on the phase between the
voltage an the current. Also higher harmonics play a role.

In particular, the VA can be split into power (in watts)
plus VAR (i.e. volts times amps REACTIVE).

In an AC circuit, with nice simple sine waves, if the current
is 90 degrees out of phase with the voltage, no average power
is flowing, if you average over a complete cycle (or over
multiple cycles).

Example: Hook a huge capacitor across the power line. No
real power is dissipated in the capacitor, but there is lots
of VAR.

This is quite significant to the power company because parasitic
losses in their lines tend to depend on the magnitude of the
current, almost independent of whether the load is drawing real
power or just VAR. In many places the power company only charges
for real power, but for large and/or highly reactive users they
will insist on charging for VAR as well.

The ideal load would be purely resistive, i.e. with no VAR.
Typical real loads have some resistive component plus an
inductive component. As a result, it is sometimes worth the
trouble to install a huge capacitor in parallel, near the load,
so that the total impedance presented to the transmission lines
is closer to purely resistive.