# calculate power factor

#### jgauthier

Mar 22, 2013
51
This is going to sound like a pretty noob question, and to some degree it is.

I'm building a power monitor, and using a current transducer. I read a similar project here:http://openenergymonitor.org/emon/node/58

I went through his code to learn how he calculate power factor, and it makes sense.
His CT is providing output in mA, so he is doing some calculations to get instant power, etc.

My CT is providing output in volts, which is a direct correlation of current. So, therefore I could process this as current, but I am processing it as power. So, I have instant power.

The challenge is that I need to acquire apparent power and I'm not sure how to achieve that. That math to get the power factor from there is easy.

Thanks for any tips.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,720
Calculating apparent power is easy: calculate RMS Vrmsvoltage and RMS current Irms. Apparent power = S=Vrms*Irms
From the power meter (your other thread) you get real RMS power P.
From S²=P²+Q² you can then compute the reactive power Q.
And since S=P*cos(phi) for sinusoidal waveforms you can compute the power factor (cos(phi)), too.

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