@transparent will have chapter and verse but my understanding is that DNOs care about inverters because they are the gateway to the grid.  So if you plug extra panels into an existing inverter without changing its export limits they probably dont care, but if you add another inverter they definitely do.  Your FIT provider will care though if the output passes through the generation meter and you may have to do an apportionment.  

I suspect a plug in inverter in addition to an existing one would be notifiable to the DNO and if it takes the capacity above 16A per phase (possibly even if it doesn't) would require permission.

As to whether its sensible you need to run the numbers if thats what matters, but overprovision on the DC side is generally recommended (or at worst not deprecated) these days.

Someone else may have a more precise answer referencing the regs, but may basic message is that if you want to be certain of compliance then you will need to do investigation either way.

@jamespa Thanks James. The FIT system is totally separate from the additional system, so any changes to the latter wont change its output in any case. My feeling, like yours, was that as long as the inverter limits the output the grid wont care.

I am a bit curious as to why I seem to have a spare MPPT port though as it would have made sense to me to put EAST on one and WEST on the other, even with the optimisers. But them maybe it makes no difference with them in the frame. 🙂

Posted by: @jamespa

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Your FIT provider will care though if the output passes through the generation meter

I've discussed this with others who have various levels of FIT income.

In practice the Energy Suppliers don't notice if there are changes to the pattern of generation for which they pay you.

They could easily do so in future by passing the generation statistics through AI, but I'd be surprised if most would bother.

Exporting to the grid using the SEG is similar.

Posted by: @bontwoody

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I am a bit curious as to why I seem to have a spare MPPT port though

Just because it uses polarised MC4 plugs doesn't mean its an input.

There are some (Chinese) inverters which use MC4s for a battery connection!

Enter the manufacturer and model name into a search engine and click the "images" option. You should be presented with some diagrams which show the various connections for your model.

Posted by: @jamespa

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I suspect a plug in inverter in addition to an existing one would be notifiable to the DNO and if it takes the capacity above 16A per phase (possibly even if it doesn't) would require permission.

That is indeed so under the current regulations.
You'd need to make a (free) G99 application online because there's more than one generation device at the property.

But the market is rapidly becoming uncontrollable.

None of the uncertified plug-in solar systems which are already being sold in the UK will have received DNO approval of course.
That's because they can't possibly be on the ENA database of G98-approved products.

Posted by: @transparent

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But the market is rapidly becoming uncontrollable.

A slightly maverick part of me says maybe that's ultimately for the better. 

We seem to control new things excessively, whilst allowing old things 'grandfather' rights that would never be allowed if the technology were new.  The regulation attached to renewables is way in excess of the regulation attached to fossil (or just old) equipment and, so far as I can see, that's in large measure because its new.  I can run 9kW+ of electric resistance heating 24*7 without notification, or install a noisy oil boiler without any planning considerations, but as soon as I install a 7kW heat pump, which actually consumes less than 3kW, emits no noxious gases, warms the plant 75% less, and is whisper quiet by comparison to a loud flue, I have to jump through all sorts of hoops.  Im surprised we don't have to have someone walk in front of electric cars with a red flag.

@transparent The fit generation meter does not measure output from the newer system, so adding to that shouldnt affect fit payments.

The inverter is the Huawei

Sun 2000 3.68kW and does indeed connect to the battery in the way you describe (see image), but it does have 2 MPPT connections and only seems to have one used at present.

So could I just connect another PV strind to the vacant connector?

Yes it looks like it @bontwoody 
That spare pair of connectors is certainly marked for PV panel input.

Are you going to buy ready-made leads to connect from the solar panel location by joining each section in series?

Or would you like help to identify and buy the required cable and MC4 connectors yourself?

If anything is likely to go wrong with a PV panel array, then it's most likely at the connection points. So you get a more reliable installation by having as few connectors as possible.

Cable which bought on a reel then fixed on-site, and has MC4 plugs/sockets fitted at the panel and the inverter ends, is more likely to have a long operational life.

@transparent I hadnt really thought that far ahead, but I did buy a crimping tool for when I added an extra second hand battery module (it was a bargain) so I would be up for making the cables to the required lengths.

Thinking about things a bit more, I could actually just put the solar panels on the extension roof, rather than waiting till I build the lean to up against its wall. It would tilt the panels to a better angle.

The Huawei software shows the original panels like this, so it again suggests to me that they are all on one string, with the optimisers sorting everything out.

If its feasible, then I would just need to figure out the best size of array and source suitable panels, so yes please help would be much appreciated. 🙂

1: Is Huawei telling us that their inverter will only operate with PV panels which have their 'optimiser' connected to each panel?

2: Tooling. The definition of MC4 is owned by Stäubli and they own the patent.

All 'MC4-style' plugs and sockets will mate with other, but the tools needed will differ from one manufacturer to another.

Eg the connectors from Amphenol are named Helios M4, and you need a different pair of plastic spanners to do up the housing.

3: Cable. It's most likely that you'll use Double-insulated 4mm² cable.
Best prices are usually from electrical wholesalers rather then the solar industry.

Here's the cable supplied by TLC-Direct.
It must be pure copper, and tinned to counteract oxidisation.

Under no circumstances should you buy lightweight cable marked CCA.
That's Copper-Coated Aluminium and will get hot due to the higher resistance.

Thanks for the link. 🙂

I dont think the panels have to have optimisers, that was the advice of my installer for the roof they were mounted on. Im not sure if it matters that the new string wont have them?

Looking at the output of the new array today whilst sunny, I can see the total output from East and West is maxing out at about 3.5kW. That suggests to me I have some headroom to oversize the system a bit more to improve things in the less sunny days

The original quote is below, so should I try and get the same panels as previously used? Any thoughts on how many?

@transparent @jamespa 

Ive just done some modelling and my newer additional East/West panels were estimated to produce about 2738 kWk per year. Each direction has 6 panels and I think I could comfortably fit an additional 6 on my extensions West roof. Therefore, the maximum output I could expect from them would be about 1369 kWh minus a bit for clipping, so say 1000 kWh. If all that was exported at 12p per kWH then thats a return of £120 per year. It will probably be a bit more as I expect to displace some grid import. With a 25 year life expectancy thats £3000 if everything stays the same.

I can get the panels on ebay for about £55 each so including mountings etc I could probably fit everything for about £700-£900.

The alternative would be to fit another 5kWh battery module. New they are £2500 so not realistic, but I got the last one used for £500 and saw another on ebay for £1000, but they are quite rare. Battery life is about 10 years.

Im not sure how to estimate the batteries payback but if I assume 5kWh of export at 12p-7p=5p (export rate minus overnight import rate), thats 25p per day for 10 x 365 days = £912.50

Intuitively I feel the panels are a better investment, any thoughts?

Posted by: @transparent

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None of the uncertified plug-in solar systems which are already being sold in the UK will have received DNO approval of course.

That's because they can't possibly be on the ENA database of G98-approved products.

Loads of them are in the database and fully compliant. The fact they can also be used in Germany via a plug and socket does not in any way invalidate them being G98 compliant in the UK if wired in according to our regulations.