Ask A Scientist© Physics Archive

name         Eric P.
status       other
age          30s

Question -   What is the difference between a volt-amp (VA) and a Watt?
Equipment I have seen (such as computer backup power supplies)
have 2 different numbers for this rating.  The watt rating is
typically 65% of the VA rating.

In the strictest sense, a volt-amp and a Watt are the same.  Based on the
fact that your device has a Watt equal to 65% of a volt-amp, I expect they
are not used to mean the same thing in AC electronics.  It would seem that
the volt-amp refers to the maximum power flow, while the Watt refers to a
time-averaged power flow.  In AC circuits, Power flow varies as a sine
function.  The "root-mean-square" rate of flow is approximately 65% of the
maximum flow.

Kenneth Mellendorf
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The definitions are:
             1 joule = the energy generated when 1 ampere of electrical
current, passes through an electrical potential drop of 1 volt. Energy =
Volts x Current.

              1 watt = 1 joule/second.

So joules is a quantity of energy, and watts is the RATE of generation of
energy.  Watt = Energy / second.

Vince Calder
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VA in AC circuits is "reactive power" and has nothing to do with real power.
Loads such as induction motors, do not act as pure resistors, but like
inductors. Inductors and capacitors draw AND supply power back into their
power source. Induction motors supply current BACK into the power grid ever
half cycle. These currents tend to somewhat cancel out, but are still there.
If I supply 10 amps on a wire for 1 second, then reverse the polarity and
try again, the net current flow from end to end was zero, but 10 amps were
flowing for 2 seconds.


The 65% rating is arbitrary, VA to watt ratios can be from 100% to just a
few percent. It depends on the type of equipment drawing the load.

Ken
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