CT Accuracy / Calibration

I have a CT clamp measuring my house load connected to an emonTx. I run my Nissan Leaf with a bi-directional (V2H) charger which tries to “Loadmatch” the power to the house to as close to zero as possible. I noticed that the clamp supplied with the V2H charger is showing drawing about 40-60 Watts (i.e. Import from Grid) which is normal. However, my openenergy monitor clamp attached on the same piece of cable is showing an export of around 100 Watts. I then checked on my smart meter and can see a Load value of 40 Watts (import).
Should I be seeing such inaccuracy in the reading from this OpenEnergy CT clamp? Is there any way to calibrate it at all?

If one is showing import and one export then it is likely one of the clamps may be reversed, have you tried either swapping the + and - terminals or rotating the TX CT clamp 180 degrees on the cable?

Thanks for the suggestion, but I have been using the CT clamp for 3 years and it is definitely the correct way round - or at least the readings on the Open Energy Monitor show the correct Import / Export values.

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Given that at higher currents, the power direction is correct, it’s very likely that you haven’t accurately corrected the phase error, either that due that of the c.t itself, that of the v.t. or the apparent phase error due to the way the emonTx works. You’re seeing it in what I assume is the ‘standby’ mode because you’ll have a relatively high current at a very low power factor - the current wave is almost in quadrature with the voltage wave - and this is where a small error of phase can make a huge difference to the real power calculation.

You haven’t mentioned which emonTx you have, so I can’t go into details, but refer you to the ‘Learn’ section where much of this and the general calibration procedure is explained.

But please, don’t call current transformers ‘clamps’ - they are not.

Thanks Robert. I have a Tx3. Yes the error that I am seeing is when there is in theory zero load on the house and the car battery is balancing the house demand, In practice there is always a pull of around 30-60 Watts from the grid. If you could point me to the relevant ‘Learn’ page for the calibration procedure please, that would be great.


Any idea why you couldn’t find it? Was it just “Current and Voltage” was too broad a heading?

Once you’ve got the voltage and current calibration as close as possible, and given your problem is a power reading at a very low power factor, you might find it better to adjust phasecal so as to give a power to match your supplier’s meter - especially if you can do it while it’s showing a very low power factor. This because the power peak with a resistive load is very broad and ill-defined. A better way is to adjust for zero power with a purely reactive load – but most people don’t have access to one of those (although your battery system appears to be close).
When you’ve done it, you should find your apparent power matches the apparent power on your meter and it’s several times bigger than the real power.

When the inverter is matching the house load you’ll probably find the CT is left measuring a very distorted load. The inverter is likely supplying the fundamental and leaving the grid to supply all the harmonics. You can see what my whole-house CT sees under those conditions here. and again in the post immediately after that one.

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