Discuss Solar installation - DNO limit in the Solar PV Forum | Solar Panels Forum area at ElectriciansForums.net

BeeJayEff

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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

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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.
 
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BeeJayEff

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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.
 

Simon47

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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 ?
 
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BeeJayEff

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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

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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.
 
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BeeJayEff

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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) ?
 

Simon47

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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.
 

baldelectrician

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If you have that much spare generation I would suggest a heat store
One of my clients has a massive heat store in a barn (large insulated tank filled with antifreeze) which is used as a dump for the heating during the day (solar heats using immersers for excess capacity).
This stores the heat to be used at night round the house, it is tied in with an oil backup system.

The store also feeds the mixer showers as well
 
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BeeJayEff

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I have had a SMETS2 smart meter installed (since the PV system was installed, so the installers should not have assumed its existence), so I think that covers the ability to measure and charge for export.
I have the DNO visiting tomorrow with a view to investigating upgrading to a 3ph supply, but I suspect that will be extremely expensive.
A thermal store is an interesting option, but presumably the conversion from energy to heat involves some inefficiencies; I'm not sure if that would be more cost-effective than batteries - and in either case there is still the possibility of the battery/store filling up in mid-summer.
 

baldelectrician

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I have had a SMETS2 smart meter installed (since the PV system was installed, so the installers should not have assumed its existence), so I think that covers the ability to measure and charge for export.
I have the DNO visiting tomorrow with a view to investigating upgrading to a 3ph supply, but I suspect that will be extremely expensive.
A thermal store is an interesting option, but presumably the conversion from energy to heat involves some inefficiencies; I'm not sure if that would be more cost-effective than batteries - and in either case there is still the possibility of the battery/store filling up in mid-summer.

Batteries need replaced every 3-4 years at a cost

A large thermal store can take electricity dumped from the PV, the heating can be taken from this (heat exchanger) and used to heat the house.

I would also suggest something like a Tesla power wall (or a different brand), with electricity costs going up there is a cost incentive.
 

Simon47

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Biggest problem with thermal storage, especially if it's a big volume, is the grade of heat. To be useful for heating a building, the fluid (usually water as it's cheap, plentiful, and has a high specific heat capacity) needs to be above anything from 30˚C to 70˚C (or more) depending on the type of heating system employed. Traditional heating systems are really rubbish in that they are designed for high flow rates*, high return temperatures**, and so will return "lukewarm" water back to the store. If you are able to utilise oversized rads, and control the flow well, then you can get your water coming back at not a lot over room temp and put that back into the bottom of the store. If you can maintain stratification, then you can have the top of the store adequately warm to provide useful heat, even if you've only had enough energy recently to heat part of the store.
* To work with boilers designed for 19th century heating systems
** And thus prevent a condensing boiler from condensing much of the time in many installations
You may guess that I'm not a great fan of much of the heating equipment (esp. boilers) currently on the market, and many of the people who "design" systems 🙄 Don't get me started on combi bolers 💩

EDIT: But thinking a bit more, if you are panning to store and use heat rather than lecky, then why not harvest heat in the first place ? Does anyone knwo the relative efficiency of good quality (e.g. evacuated tube) thermal panels v.s. lecky panels and an immersion heater ?
 
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BeeJayEff

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I don't particularly want to store any excess energy as heat rather than lecky, unless costs of installation, operation and maintenance were significantly lower. Nor do I want to have to interfere too much with the existing CH/HW system. The PV panels are already installed; replacing them with thermals would be extravagent. I'm leaning towards a hot water heater in our current HW tank with a battery (Powerwall ?) in the barn with the inverters.
 
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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 ?
The installer should have made a G99 application to the DNO first to install this.

Out of interest, who's told you that the DNO has limited your export?

You'll probably be allowed at lot more export capacity than the 3.68 hard limit under G98 if an application is made to request it.
 

Jim Gough

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I have had a SMETS2 smart meter installed (since the PV system was installed, so the installers should not have assumed its existence), so I think that covers the ability to measure and charge for export.
I have the DNO visiting tomorrow with a view to investigating upgrading to a 3ph supply, but I suspect that will be extremely expensive.
A thermal store is an interesting option, but presumably the conversion from energy to heat involves some inefficiencies; I'm not sure if that would be more cost-effective than batteries - and in either case there is still the possibility of the battery/store filling up in mid-summer.
If you install batteries make sure you adjust the settings so they won't discharge under 20% or charge over 87%. It's worth also considering limiting the charge rate to approx 0.25 times their capacity, but to do this effectively you do need to take into account how much power you can generate to how much battery storage you have. That way the batteries will last a very long time. At the moment you're looking at approx £3.2k per 10kwh of storage for intelligent li-ion batteries. You can use lead acid batteries but these won't last as long, but they are cheaper and much more bulky. I believe you could use your electric car's battery to perform the same thing, but I don't know about it.
 

Jim Gough

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Oh also, with SEG tariff I'm pretty sure that the limit its way higher than the 3.68KW that you quoted. I might be wrong but I think that was the limit for FIT which ended in 2019. (Don't you just love the governments green credentials!) With SEG I believe you can generate up to 5MW with solar panels. Tops you'll only get 5.5p a unit. You need to shop around and see who's best. It can be a right PITA getting a SEG tariff set up as the energy companies don't want to do it.
 

Jim Gough

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Octopus seems to come out no1 for SEG at this exact moment in time, may change in the next month, hour or second.
Subject to the install being MCS / Flex Orb certified.

Not sure what their prices are at the moment because they won't tell you online. I rang them a couple of weeks ago and the prices for electricity and gas were outrageous, and this was pre 1 April prices!
 

snowhead

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Not sure what their prices are at the moment because they won't tell you online. I rang them a couple of weeks ago and the prices for electricity and gas were outrageous, and this was pre 1 April prices!

I think energy prices for the near future will be like stock market share prices, set at the exact moment you sign up for the Energy supply.
 

brianmoooore

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Just ordered a small (1000 litres) quantity of heating oil. Usually the best price is "we'll deliver it sometime in the next month, when we get around to it". This time, I was given a significantly better price for "a semi emergency delivery" in just three days time.
 

jackhammerJIM

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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 ?
How much are getting to export ? 5p per kw ?
 

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