Tweet-A-Watt: Watts vs. VA?

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pdp7
 
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Joined: Sun Apr 24, 2011 1:35 am

Tweet-A-Watt: Watts vs. VA?

Post by pdp7 »

Hi - the wattcher.py script for the Tweet-A-Watt displays the "Watt draw, in VA". Does anyone know if this is supposed to represent the value on the Kill-A-Watt for watt mode or VA mode?

I would have thought VA mode, but the numbers I'm seeing are closer to the Watt mode value on my KAW than the VA mode value. Powering a Raspberry Pi with Samsung USB adapter, I get about 6.0 on VA and 3.0 on Watts on the KAW. wattcher.py is displaying average of around 4.0 for the "Watt draw, in VA". The power factor is about 0.5.

If this is supposed to match the VA value on KAW then I have a problem (6 - 4 = low by 2). If it is supposed to match Watt value on KAW, then it seems ok (3 - 4 = high by 1 but this is expected given KAW has some power usage of its own - the TAW instructions say ~2W is ok).

So my understanding is that KAW Watts is real power and KAW VA is appearant power (the geometric sum of real + reactive power). I'm not sure what wattcher.py is calculating. It multiplies the voltage sample by current sample from XBee ADC. Is the real power or appearant power?

Thanks!
Drew

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