Solar installation - DNO limit

[BeeJayEff]

Hi, I'm new here, having recently had nearly 13kWp of solar panels installed on a barn roof, feeding into two Solis inverters (covering about 10 kWp and 3kWp respectively). I'm on a single phase supply and was surprised when told that the DNO limited my export to 3.68kWp - if I had known beforehand that there would be such a limit, I would probably have gone for fewer panels, but I didn't. Both inverters feed into a consumer unit in the barn, which in turn is on a sub-main from the property's main meter, consumer unit and fusebox.

I now understand that inverters limit export to the grid by dissipating excess power, and my current question is how does an inverter know how much to export without somehow knowing how much power is being consumed by other circuits on the main CU ? For example, last week I was getting 8kW from the inverters, which was addressing my base load of about 1kW and the rest going into my electric car (on its own feed from the main CU). If the car not been plugged in, there would have been about 7kW available to export to the grid - how would the inverters know to limit their output to 1+3.68 = 4.68kW ?

 

[littlespark]

It may not be a physical limit on how much is being fed back to the grid, but a limit on how much they will pay you.
Even if you feed 6,7 or 8 kW back to the grid, for example, they’ll only pay you for 3.68kW

There has always been a limit on domestic installs.
I’m surprised the installer didn’t explain that.

 

[BeeJayEff]

It may not be a physical limit on how much is being fed back to the grid, but a limit on how much they will pay you.
Even if you feed 6,7 or 8 kW back to the grid, for example, they’ll only pay you for 3.68kW

There has always been a limit on domestic installs.
I’m surprised the installer didn’t explain that.

I'm dismayed they didn't explain it to me; just one of the reasons I have lost confidence in them; they were clearly just keen to sell me as many panels as possible. They are certainly saying that it's the inverters which address the limit by dissipating excess power.

 

[brianmoooore]

I’m surprised the installer didn’t explain that.

And risk losing a sale?

 

[Simon47]

Hi, I'm new here, having recently had nearly 13kWp of solar panels installed on a barn roof, feeding into two Solis inverters (covering about 10 kWp and 3kWp respectively). I'm on a single phase supply and was surprised when told that the DNO limited my export to 3.68kWp - if I had known beforehand that there would be such a limit, I would probably have gone for fewer panels, but I didn't.

"probably". I think it's a safe bet that you wouldn't have bought 3 times the capacity you can export.
I would suggest that you have a claim for fraud by misrepresentation. They've clearly sold you a system that can't be used as-is, so unless they clearly explained that then that's fraud. And I assume that if they did tell you "the system can do 13kW but you won't be able to use it" then you'd have started asking the right questions before signing on the dotted line.

I now understand that inverters limit export to the grid by dissipating excess power

Did the same cowboys who "forgot" to mention the export limit also tell you that ?
They don't - think about how much heat that would be on a sunny day. They simply limit how much power they convert and allow the DC voltage on the panels to rise.

how does an inverter know how much to export without somehow knowing how much power is being consumed by other circuits on the main CU ?

It's not an area I'm familiar with, but I suspect most inverters have no idea - hence they simply limit to 4kW (or whatever).
It would certainly not be difficult to produce an inverter that included power sensing on your meter tails so they could limit the export power. Not by measuring your other loads, but by simply monitoring the current (and phase) in the meter tails. Whether anyone does such a device I have no idea.

There has always been a limit on domestic installs.
I’m surprised the installer didn’t explain that.

Really ? See :

And risk losing a sale?

Exactly. And did they ride off on their horses ?

 

[BeeJayEff]

Thanks for the responses, confirming my suspicions. I think there might be a solution to the DNO limit problem by installing a wireless CT clamp; wired is not and never would have been an option as significant groundworks would have been involved. They will also need to install an Export Power Meter which can take the wireless feed, and I guess I shall be installing a big battery as well. At least I haven't yet paid any of the cowboys' invoice.

 

[Simon47]

It would need to be the inverters that take the CT signal - so they can adjust their output to limit export power. The meter just measures it, it can't control the power at all.

 

[BeeJayEff]

It would need to be the inverters that take the CT signal - so they can adjust their output to limit export power. The meter just measures it, it can't control the power at all.

Ah yes, of course. So do I actually need an EPM, and if so where does it go - by the property's main fuse, meter and CU if it's measuring the total currently being exported, or by the inverters ? Each inverter already feeds an Emlite meter and I already have one CT clamp for my EVSE - could that signal also feed the inverters (though I would still have the groundworks vs WiFi issue) ?

 

[BeeJayEff]

Sorry, duplicate message.

 

[Simon47]

You don't need any meter on your solar output - but if you want to be paid for any export then you'd need a supplier provided meter that could separately count your imported and exported power.
In practical terms, you can't really share a CT between different systems - but then adding a CT isn't any great problem. Your biggest problem is that the inverter needs to be able to use it to control it's output - I have no idea whether they can or not. And it's going to need a cable to connect it.

BTW - have you tried talking to the DNO about it ?

I suspect a few installs have been like this, but the installer has just ticked the "under 4kW" box and said nowt. The user is oblivious to it (assumed the installer was doing everything correctly), and the installer doesn't care as long as he gets paid before riding off into the sunset never to be seen again.