Inaccurate readings

I’ve had an EmonPi running away for years, and have taken it pretty much as red that it’s giving me accurate results. I’ve got two CT clips and a AC/Voltage adapter for monitoring my solar and mains.

But I’ve been taking a closer look at my power consumption recently, trying to narrow down where all my money keeps going. And I realised I needed to validate that what the Pi is reporting is what is being used. I do not have a pulse meter, as that meter is outside the house where I can’t attach any transmitters etc.

Not having a smart meter, I manually log my meter readings for billing, and helpfully take a photo of the meter. So I converted the time stamps to an epoc and popped it into my import feed in EmonCMS and I’ve got a discrepancy over a month. The Pi is reporting 589kWh, where my meter is reporting 644kWh usage. Which is quite a significant difference.

I’ve got no idea how I go about resolving this. Can somebody point me in the right direction or what to do?

Hello @Dash are you using the emonPi with emonTH temperature sensors or any other radio nodes? If you are not you might like to try the more recently developed continuous sampling firmware for the emonPi which may give better results. I realise this firmware in not showing up in the firmware update list so will look into this shortly.

I assume the CT sensors and Voltage adapter are bought from our shop? Do the voltage readings vary? that would indicate that the voltage adapter is picked up correctly by the emonPi.

The other thing to check is that the cores of the CT sensors are mating together well. A small amount of dust or sideways force can cause the readings to be in quite a significant error.

The other thing to try would be a manual voltage calibration using a multimeter, but it might be worth trying the above first.

What information does your PV Inverter make available to you? Unless you own or have access to reasonably accurate instruments - voltmeter and ammeter, your Inverter is generally likely to the next best thing to use.

The procedure would be to transfer both c.t’s onto the PV infeed. What next depends on how old your emonCMS is. The way that should always work is using emonHub. In emonhub.conf, adjust the “scales = …” numbers (from 1.0 upwards, by around 9 % to 1.09 initially, then home in on the correct setting by trial and error) so that voltage and power read by the emonPi agree with your Inverter. The numbers and the names line up left to right. Note that voltage and power don’t interact in emonhub.

However, if you have a modern emonCMS, you should be able to use the serial interface in Admin → Serial Monitor to edit the calibration settings in the Atmel “emon” part of the emonPi. If you stop emonHub, then send the command “?”, you’ll get the Help text on the screen. In this case, you’ll calibrate voltage and currents, and those do interact to send the power to emonHub and emonCMS. If you can arrange it so that your PV is supporting a largish resistive load, you could try adjusting the phase calibration to give an indicated power factor of 1.0 - this might be a lot easier if you could transfer both c.t’s to your main incomer to do this.