Negative Readings

Hello All

I was wondering if anyone knew how to fit this issue. My power is now reading as a negative value, please see the attached screen shot.

What have you, or somebody there, changed?

Has somebody taken the c.t. off the cable and turned it round?

Has somebody changed a calibration constant to negative - or multiplied by -1 in the processing?

Where are you? - There’s a reason for putting your Country in your profile, it lets us know what your electricity supply is likely to be. And that could have a bearing.

Thanks - turns out the CT had been put back the wrong way when I had an aircon system installed a couple of weeks ago.

Im based in the UK and profile has been updated

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You might be able to correct the negative readings.

Go to Setup → Visualization → EditRealtime and there you can multiply by a constant: -1 in your case.
You’ll need to zoom in a long way so that only the negative part is in the window - and even then you might need to tidy up single readings individually. Or, if you have no export, you can convert all to absolute values.

(Had you been in a country where the sockets allow the a.c. adapter to be reversible, that too would have given you negative readings.)

My power usage went negative a while ago and has been that way for months now looks like. No changes were to anything (not sure if the device update it’s firmware) but no changes made to the wiring. Any clues how I can fix this?

I do not have solar etc, so power should never go -ve. How can I convert them to absolute values and keep then that way?

I don’t believe that - has someone unplugged the a.c. adapter and put it back in the socket the other way round? That would explain it - you’ve reversed the phase of the voltage with respect to the current, so the power has become negative. If nothing else has changed, it’s realistically the only explanation, because your adapter plug, unlike the UK one, is reversible. Had the software changed, everybody would have had the same problem - and nobody else has that I know of.

Nope, AC adapter is hard wired. Can it be due to power utility making changes?

How can I override the power to use the absolute value and I need to apply this change retrospectively to the past values as well. I’d appreciate help here.

Here is what all the readings show:

Here is the graph showing power usage going -ve in February:

Definitely not. What’s happened is either a minus sign has got into the calibration somewhere, or the phase of the current has changed relative to the voltage. There’s no such thing as absolute phase, so if your supplier swapped the wires to the primary of your transformer, or even swapped the legs on the secondary side, the phase of voltage and current in relation to each other would be the same.

How are you measuring the voltages & currents?

I have 2 of “100A/50mA Current Transformer YHDC SCT-013 - 13mm Opening” sensors

Attached the screen shot for calibration values:

Yes, but what are they feeding into - the emonTx/emonPi doesn’t have two voltage inputs, so is it two emonTx’s or an emonTx and an emonPi?

I have a:
Split Single Phase Real Time Whole House Energy Meter Kit - Programmed ESP32

Hmm, I wonder if my calibration values got reset. I think I took a screen shot of them I will try to find the picture.

So it’s sending the data by Wi-Fi to emonCMS?


I noticed the coil type was reset. Restored to correct value for SCT-013, but still have the -ve power issue.

You are probably looking for an additional, or a missing, minus sign in the calibration values. This of course won’t work if those numbers are codes for different devices, rather than true calibration values.

Failing that, in emonCMS, in Setup → Inputs, you can add × -1 as the first step of the input processing for each of the powers.

Having done that, you can edit the values stored in the Feed in Setup → Visualisations → EditRealtime. There, you can scale all the values in the window by -1 again. You’ll probably need to do it in several steps to exactly find the two ends with the editing window.

Thanks Robert. I will have a go at that.

I added a -1 for the power to turn it positive. I have no idea why it went negative. I dug up my original calibration values. Those continue to yield -ve power. The actual power value is accurate (I know from the appliances that are running). But it’s is flipped -ve.

Can you elaborate on how to edit the values in Setup → visualizations? This is what I see there: