Community
OpenEnergyMonitor

OpenEnergyMonitor Community

CT tracking of solar output doesn't match inverter reading

Installed an OEM system a couple of months ago. Based in NA. My main electrical panel is in the basement with a sub-panel in the garage. I have an 11.3 kW solar array connected to the sub-panel. The system has a reporting device that shows my instantaneous output. I also connected a CT on one of the two leads (red and black) and plugged it into an emonTx. I notice that the power reading on the OEM system is consistently lower by a wide margin than the output from my dedicated solar system tracker. Right now, for instance, the solar tracker says 3081 W while emon says 1251 W. What am I doing wrong?

emonTx v3.4
SCT-013-000, 100 A
50 A, 220 V breaker

Have you set up your emonTx with the correct calibration for your Canadian v.t.?

EmonHub calibration setting is 110 V.

Wait, there are separate calibration settings for each emonTx?

Edit: I read the ‘Use in North America’ page. I realize now that I need to multiply the output of the CT by 2 because I am measuring only one leg of the 220 V circuit. Still leaves me about 10% shy.

What equipment do you have? You mentioned an emonTx, and emonHub - but that is software, normally it runs on an emonPi or an emonBase - what do you have it running on?

The emonTx’s calibration depends on the actual output voltage of your v.t. - the 10% could well come from that. The calibration factor you set it up with is the mains voltage that would give 1 V at the ADC input. Working from your mains, the mains voltage is divided down by the transformer ratio, then by a further factor of 13 inside the emonTx. But don’t believe the number on the transformer, that’s going to be the voltage at full load, whereas in your case it’s effectively working with no load, so the voltage will be 20 - 25% or even more higher. For the US a.c. adapter from our shop, the nominal calibration constant is 130. If you didn’t specify otherwise, your emonTx will have been shipped with the standard UK adapter’s setting of 268.97
So I’d expect you to be reading 48% of the expected value if you have the US adapter but the UK calibration.

Incidentally, the “120 V” setting in emonhub.conf has no impact on the calibration of the emonTx. If emonHub is running on an emonPi, then it will adjust that, and that alone.

You’ll find quite a lot more useful reading about calibration in the ‘Learn’ section here.

Could be. Out of the Box I’ve seen up to 8% on UK measuring.

If you have a good means of determining the actual use over a period, The simplest way to adjust the logged value scale the ‘Input’ in emoncms Input Processing before you log it to a Feed (I log the raw and then log the scaled value).

Until we know what @theScud has got in terms of equipment, and where he’s sending the data and how, we’re guessing.

I just make or use a simple audio Y split then you can use two Cts, to measure both legs since the wire not crossed you just have to place one ct in the opposite direction. also pay attention to your voltage that calibrated right maybe you have it calibrated to display 220v but your actual voltage is 248 as in my case and the nominal in Canada is usually 120v/240v and not usually 110v/220v

If it’s a 240 V circuit that’s either got no neutral connection or whatever - maybe control circuits - that runs on 120 V has a negligible current draw, then two c.t’s are totally unnecessary. But as noted in ‘Learn’, the voltage needs to be doubled IF you’re only measuring the voltage on one leg.

From what few details we have so far, it appears that the inverter is feeding 240 V and it is balanced, hence only one c.t. will be required. What we don’t know is how the voltage is being measured and the no-load voltage of the transformer that’s being used, that’s where the calibration has gone wrong.

Plus, there appears to be an assumption that setting the “120V” flag in the emonPi’s interfacer in emonhub.conf will calibrate an emonTx. It cannot.

Alright, lots of comments so far. Thanks for being willing to help. I’ll try to answer all of your questions in one shot.

I’ve purchased all of the hardware I’m using from this shop, with the exception of a few CTs that I got from Amazon but even then, I bought the same model as ones sold here. I did specify NA options when they were available on the hardware.

I have an emonTX V3.4 at the garage sub-panel with two connected CTs. One CT is monitoring one leg of the 240V solar array. The other CT is monitoring one leg of my 240V EV wall charger. The solar array does have a neutral wire. I put the CT on both the red and black wires and they were the same readings. I also tried the neutral and it was essentially zero, I think as you’d expect.

The emonTX is feeding info wirelessly to an emonPi. The TX is receiving power from a wall adapter, the Pi from a USB cable. I haven’t calibrated anything. I do have a rudimentary multimeter but don’t have the ability to put it in series with any loads. I did measure the voltage across the bus in my sub-panel and got 254 V.

I order to get the solar array output from the CT to match my dedicated solar array monitor, I applied two multipliers in the emoncms software: x2 and x1.236. My guess on being 10% off after doubling the reading was a little off. It was 23.6% off.

I do have a programmer and was using it on the weekend to connect a third emonTX to my emonPi. In addition to the one at the sub-panel, I have two more at the main panel.

Any other info I’m missing?

Thank you for that - it’s now possible to see the overall picture.

As I mentioned, the emonTx does all its processing on-board and sends the power values to the emonPi. That radio link is in that direction only, nothing goes back to the emonTx.

You can probably use your inverter’s display to give you current and power readings.

I take it you have the AC adapter model 77DA-10-09, running off 120 V?
Is the EV charger also 240 V? If so, we can calibrate the emonTx to use the leg-leg voltage rather than the single leg voltage.

There’s little harm in leaving things as you have them, but I’d prefer to calibrate the emonTx on its own so that the correct values are coming out of it. The way to do that, as you have a programmer, is you connect that to the FTDI connector on the front (LED down, GND to the right) and change the calibration to suit your a.c. adapter. You’ll need either the Arduino IDE or some means of using the computer as a terminal sending and receiving serial data via the USB port. The nominal value you need will be 130 × 2 = 260 for k0, but adjust it so that it reads the voltage that your meter or inverter front panel is telling you for the total leg-leg voltage. If your inverter also tells you the current, you can adjust the current calibration k1 (probably) to make that match and finally adjust the phase calibration to give the correct power. If you haven’t got the current from your inverter, keep the phase setting at the default and adjust the current to give the correct power. Then you can go back and put the numbers in emonhub.conf back as they were (because we’ve put the ×2 into the emonTx - or you can calibrate that to 120 V and leave the ×2 in emonhub - as you prefer).
You can adjust the EV current/power likewise, using k2.

There are notes about accessing the calibration with the sketch source on Github: https://github.com/openenergymonitor/EmonTxV3CM

That is rather more than I would expect. The UK and USA adapters aren’t that much different in their voltage ratios (ignoring the ×2), so there must be something else that’s contributing to the error.

You can if you need to, calibrate the other emonTx in the same way. Each c.t. input has its own calibration, and once calibrated, you should keep the same c.t. with its input, as calibration corrects the errors of both c.t. and the components inside the emonTx together.

The emonPi itself has only the values in emonHub accessible for you to calibrate its inputs, and bear in mind with that, calibrating the voltage has no effect on the power readings, because the maths to calculate power is done in the “emon” part, not the “Pi” part - but the “calibration = 110V” bit does, make its way into the “emon” part to change that - but most confusingly, “110V” is a flag, not a number that you can change to finely affect the voltage reading you get. (That’s going to change in the foreseeable future.)