Growatt CT discrepancy.

[user 36398]

Recently I've had a Growatt inverter installed, most things work really nice, one issue that seems to be very common is the reading you get from a Smart meter are not the same as what the Growatt inverter is saying, from what I can gather this is due to CT clamps not been totally accurate.

eg. When Growatt believes it's exporting 0 watts, the Smart meter says about 50 watts usage, this seems pretty consistent too. So of course the outset of this, it that there is a trickle of 50 watts import all day, this of course can add up over a year, I'm on Octopus, so during the night I can charge batteries so I don't want to be exporting any during the day, unless I have surplus from my solar panels.

Now replacing the CT for a Meter really seems overkill, when all that needs to be done is the Growatt inverter just compensates for this 50 watts.. I've searched all over the internet to see how, what seems like a very trivial thing to configure, but Growatts documentation is not very good here .

But during my search I did find this -> https://www.photovoltaicsolar.in/Growatt_Manual/MAX Series Modbus RTU Protocol.pdf

You can see that register 1038 . bCTAdjust CTAdjust enable, looks exactly what I'm after, but of course this is just a flag to say to enable a CTAdjust, but what I can't work out is what register & values do you set for the adjustment. The next 4 registers 1039-1042 look like it might have something to do with it, but with them not saying CT I'm really not sure. Or are these register just not what I think they are?..

 

[johnb2713]

The closer you get to zero the worse the performance of the CT becomes, electrical noise becomes very significant. What you will probably find if you observe it closely is the inverter will be doing a few watts of export and then a few watts of import, they hunt around zero and it will continue to do that no matter what adjustments you make with regards to the CT etc, they cannot get to zero import / export as a steady state. An electricity meter rather than a CT is much better at lower power than a CT BUT, theyre much slower to respond to load changes.
Which model of Growatt do you have?

 

[Murdoch]

50 watts discrepancy - I wouldn't waste any time on it ..................

 

[johnb2713]

50 watts discrepancy - I wouldn't waste any time on it ..................

Spendthrift.............. that's a whole 1200w per day, cant be too careful that's £175.20 per year wasted.
Actually is quite scary when you run the numbers on small loads such as this LOL, now look you've done, jeez, got go round and reduce my loads.

 

[user 36398]

The closer you get to zero the worse the performance of the CT becomes, electrical noise becomes very significant. What you will probably find if you observe it closely is the inverter will be doing a few watts of export and then a few watts of import, they hunt around zero and it will continue to do that no matter what adjustments you make with regards to the CT etc, they cannot get to zero import / export as a steady state. An electricity meter rather than a CT is much better at lower power than a CT BUT, theyre much slower to respond to load changes.
Which model of Growatt do you have?

The inverter is SPA3000

During the summer times I'm going to be generating more than I will consume, so my thinking is that
watts = (CT Watts) - 50. Basically just as simple as that, I understand the CT limitations, but with that simple calculation is should do the trick. I'd even prefer an overshoot here, eg. even (CT Watts) - 100, would be better than the CT Watts that's giving me +50 grid import. IOW: If our smart meter read -50, that would be better than the +50 its doing now.

Btw. That 50 watts is also consistent when a load is used, eg. put the kettle / cooker on and after it settles and I have enough battery / solar, I'm still importing 50 watts. Also worth pointing out, our float wattage (fridge / security cameras etc) for our house is about 250 watts, so will never be near 0..

Do you have any details on those register values?, `bCTAdjust` , "CTAdjust enable", by the naming that sounds exactly what its meant for, but without documentation I'm hesitant to change. I've tested reading those register values and they do line up with the defaults shown inside that PDF, so makes sense from that regard, it's just details what these values are is very limited.

Like I said, I know a meter would be more accurate, but seems an overkill for such an easy fix that can be done in software..

 

[user 36398]

50 watts discrepancy - I wouldn't waste any time on it ..................

Yeah, as pointed out by Murdoch it can add up over the year,. Could buy another Solar panel for that, or money towards another battery etc.. Also my day rate is not the standard protected 34p, as I'm on Intelligent Octopus so my day rate is 43.47p, (18 hrs) so works out a little worse.

 

[johnb2713]

Yeah, as pointed out by Murdoch it can add up over the year,. Could buy another Solar panel for that, or money towards another battery etc.. Also my day rate is not the standard protected 34p, as I'm on Intelligent Octopus so my day rate is 43.47p, (18 hrs) so works out a little worse.

To be fair, it would work out a little better, I rattled the figures through at 40p but of course on intelligent 6 hours are on 10p!

 

[user 36398]

To be fair, it would work out a little better, I rattled the figures through at 40p but of course on intelligent 6 hours are on 10p!

My calcs ->
For 0.34p ((50/1000)*24)*365*0.34 = £148.92

For 18hrs @ 0.4347p and 6hrs @ 0.10p ((50/1000)*18)*365*0.4347 + ((50/1000)*6)*365*0.10 = £153.75

So I'm getting a tad more,. To be fair, the night rate I'll be using to charge car & batteries anyway, so this leakage would not be an issue then..

 

[Murdoch]

But is the 50 watts actually 50 watts AND when the sun is out surely nothing is imported from the grid at all

 

[user 36398]

But is the 50 watts actually 50 watts AND when the sun is out surely nothing is imported from the grid at all

No that's the point, when the sun is out, it's importing 50 watts from the grid, because the CT clamps are reporting incorrectly to the inverter, if I could say to the inverter my usage is what my CT is saying +50 watts, then it would draw from the Solar Panels or the battery as needed instead of the grid. Of course if my batteries are fully charged, and I have excess solar, this makes it go negative..

Is it really 50 watts!, at the end of the day, what I get charged for is what my smart meter says, it saying I'm using 50 watts. You never know, my smart meter might be out by 50 watts , but I'm assuming the CT clamps are the culprit, as looking on the internet it seems to be very common issue.

 

[Murdoch]

Well, I can’t comment about smart meters BUT do you check the readings a few hours apart on a sunny day AND how do you prevent the usage not exceeding the PV generation ?

 

[user 36398]

Well, I can’t comment about smart meters BUT do you check the readings a few hours apart on a sunny day AND how do you prevent the usage not exceeding the PV generation ?

Until my batteries are fully charged, the meter will read 50 watts from the grid.

I don't stop the usage exceeding the PV generation, if I don't have enough solar it will take from batteries first, and then grid second, it's not something I control, the inverter does all this, it's just its out by 50 watts,..

Apart from 11.30 - 5.30, I'm using this as battery & car charge time, so it hardly matters what the CT clamps read then.

I've sent Growatt an email on the "CTAdjust enable", register, hopefully they will come back with some info. I'll post here if I get any helpful info, because it looks like I'm not the first person this has happened too, and replacing the CT clamps with a meter seems a daft thing to do when a simple software solution / setting should fix it.

 

[Murdoch]

Until my batteries are fully charged, the meter will read 50 watts from the grid.

I don't stop the usage exceeding the PV generation, if I don't have enough solar it will take from batteries first, and then grid second, it's not something I control, the inverter does all this, it's just its out by 50 watts,..

Apart from 11.30 - 5.30, I'm using this as battery & car charge time, so it hardly matters what the CT clamps read then.

I've sent Growatt an email on the "CTAdjust enable", register, hopefully they will come back with some info. I'll post here if I get any helpful info, because it looks like I'm not the first person this has happened too, and replacing the CT clamps with a meter seems a daft thing to do when a simple software solution / setting should fix it.

so you haven’t read the meter to see if the 50 watts is real ……

 

[user 36398]

so you haven’t read the meter to see if the 50 watts is real ……

Sorry, not sure what you mean, the smart meter is my meter. But regardless if it's real or not, I'm getting charged on what it says, and I believe it's unlikely incorrect as it's what the electricity company relies on..

 

[Murdoch]

If you turn off the circuit breaker for the inverter does the 50 watts disappear?

 

[user 36398]

If I turn the cir

If you turn off the circuit breaker for the inverter does the 50 watts disappear?

If I turn the circuit breaker off, I'll get at least 250 watts, as now all my energy is coming from the grid.

250 watts is my float energy needs, Fridge / Security cameras & devices in stand by etc.

 

[johnb2713]

I assume your inverter is operating grid tied, does it export to the grid i.e. you dont export limited to 0?

 

[Murdoch]

If I turn the cir


If I turn the circuit breaker off, I'll get at least 250 watts, as now all my energy is coming from the grid.

250 watts is my float energy needs, Fridge / Security cameras & devices in stand by etc.

so 50 watts is far better than 250 watts ..................

 

[user 36398]

I assume your inverter is operating grid tied, does it export to the grid i.e. you dont export limited to 0?

Yes, grid tied. This is working well, apart from the 50 watts under reporting.

And of course when batteries are full, and load is full-filled, it will start exporting.

 

[johnb2713]

And of course when batteries are full, and load is full-filled, it will start exporting.

Well there is no 'of course' about it, it could configured for zero export in which case you end up with a small import all of the time.

Just trying to help.