Error in solar output measurement despite calibration

Hi, I’m using an old EmonTX Shield to measure my power and solar in my new house, and I’m having some issues getting accurate results.

I have an EmoxTX Shield running on an Arduino Uno with an RP12, sending to an RP12 on a RPi 3 running EmonCMS. I have two 100A/50mA CTs connected as per the suggested setup in the documentation.

The data is being received by the RPi and is displayed corrected, however when my 5KW inverter is running at full capacity (according to it’s display, and the website provided by the solar company), the EmonTX says it’s only about 4.5KW.

I have done some isolated calibration as best I can using an electric kettle and a current meter so that the Arduino reports voltage and current results that are within 1% of the separate meter I am using, however when I then place that same CT on the solar system I get the error amount above.

Any advice on how to troubleshoot this would be much appreciated.

This is what EmonCMS is showing right now, while my inverter says about 5.1kW…

Are you using an AC voltage sensor?

Sorry, yes, it’s a 9VAC plug pack. I’ve calibrated that also to within a couple of volts.

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I would have expected calibration at the kettle power (1.5 kW, 2 kW?) to be close enough to the inverter output for the accuracy to have been better than you’re seeing (~ 12% low) so we need to look further.

Did you follow the calibration steps exactly, including setting PHASECAL?

After that, what are the measurement accuracies of the inverter display, the ammeter and voltmeter you used, because if you’re unlucky, all those will add up the wrong way, though I’d doubt they could explain all of the error.

Related to phase error - are the mating faces of the c.t. core clean and seating correctly? If not, you’ll have a massive phase error that will make real power readings quite wrong, yet the amplitude (current & apparent power) varies very little. A cracked core will have much the same effect, because both introduce an air gap.