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Watts Versus Volt-Amps

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Brian Graham

Jan 1, 1970
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I just purchased a Kill-A-Watt to monitor energy usage and ultimately to plan my solar. It measures both Watts, and VA, among other things. But since Watts = Volts times Amps, what is the difference in the measurements? VA is higher, so I presume that is the unit I need to plan with.

Which brings me to the KWH that it records. Does anyone know if its based on the Watts or the VA? In other words, my new washer used .19 KWH for a load. Is that safe to use, or do I need to increase it by the same ratio of the VA:Watts the unit records.

While I'm at it, just what is the Power factor?

So many questions. So much to learn..
 
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daestrom

Jan 1, 1970
0
Brian Graham said:
I just purchased a Kill-A-Watt to monitor energy usage and ultimately to
plan my solar. It measures both Watts, and VA, among other things. But
since Watts = Volts times Amps, what is the difference in the measurements?
VA is higher, so I presume that is the unit I need to plan with.

Which brings me to the KWH that it records. Does anyone know if its based
on the Watts or the VA? In other words, my new washer used .19 KWH for a
load. Is that safe to use, or do I need to increase it by the same ratio
of the VA:Watts the unit records.

While I'm at it, just what is the Power factor?

In a DC system, Watts = Volts times Amps. All the time, every day. But in
an AC system, both voltage and current are alternating. If the alternating
current doesn't line up exactly with the voltage (such as might happen with
an inductive load), then Watts <> Volts times Amps. An 'adjustment' needs
to be made based on the two waveforms being out of phase with each other.

In an AC system,

Watts = Volts times Amps times Power-Factor

In simple circuits like an inductive motor, the power factor =
cosine(displacement angle). Where 'displacement angle' is how far out of
phase the voltage and current are.

When 'sizing' a system or design, you have to work with both for different
things. An inverter has to supply the VA rating (volts times amps). But if
you're trying to calculate battery storage capacity, you need to know the
watts of the load and how long it runs (to figure out the watt-hours of
energy you need to store).

daestrom
 
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