Microchip Energy meter - How it calculates Power (apparent, reactive) and PF

Thread Starter

EL Draco

Joined Feb 13, 2018
I am quite rusty on my theory and hoping you can help me fill in my blanks.

We have a energy meter (i'll stick to 1 phase for the moment, chip details in link below) that allows us to monitor the power consumption of any device worldwide. We provide a supply voltage reference and use a current transformer to get our current reading.

obviously active power is straight forward, but I am trying to understand how the chip is able to calculate PF, reactive power and apparent power in this setup without additional parameters.



Joined Jun 4, 2014
All those can be calculated once you know the magnitude of the voltage and the current and the phase angle between them. Given the inputs available to the chip it has all it needs to do those calculations.


Joined Feb 13, 2018
It all depends on coding.
I guess this is how it might have been done in this case.
  • The chip reads instantaneous values Voltage and Current simultaneously and goes on storing it in an array.
  • Chip will get to know that one cycle is completed when the voltage crosses zero ( in this case, voltage may be shifted)
  • After each cycle it calculates the rms values of current and voltage by accessing the arrays.
  • It can calculate phase shift by measuring the time difference between the occurrence of peaks
  • With rms and phase shift, all the components can be calculated easily
  • This process repeats for every cycle or it may take average of past few cycles
Thank You


Joined Mar 31, 2012
What other information might you need? If you are sampling the instantaneous voltage and the instantaneous current, you can trivially estimate the real power. You can also trivially estimate the RMS voltage and the RMS current. I say "estimate" only because you are working with sampled data -- but if your sampling rate is high enough, the estimate becomes arbitrarily close to the actual value.

So if you now real power, RMS voltage and RMS current, what else do you need in order to determine apparent power, reactive power, and power factor?

This might help.