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Name:  Eric P.
Status:  other
Age:  30s
Location: N/A
Country: N/A
Date: 2000-2001


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.


Replies:
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


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


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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