Adding to an existing FIT tied solar system

[Woodyeyes]

Wow, what a find this site is! I thought I was being quite green/eco until I saw some of the setups you guys have got going! Respect.
My current objective is to increase my solar generating capacity.
Background:
My current FIT PV system was installed in 2013.
12 x 250W Perlight panels on a 32 degree south facing single storey roof (with very easy access). Each panel has a Solar edge optimiser feeding into a solar edge SE3000 inverter. I’ve never had any problems with this set up. The only issue is the main house casts a shadow on the panels from afternoon onwards. I have no batteries, yet.
In the last 8 years I have received a total of £3457 in FIT payments. To date the system has generated 20963 units. My FIT tariff is currently 18.48p and 5.57p for export. In the last 12 months I’ve received a total of £487. The FIT runs for another 11 years. Ignoring the index linking of the FIT rates, that’s worth approx. another £5357
I also have an additional south facing, no shadowing, single storey outbuilding roof with enough space for up to 27 panels to potentially increase my generation capability. The cable run from this roof to my consumer unit would be approx. 35m. I have underground ducting in place.
A single storey garage attached to the house could take a further 3-4 panels. Again, south facing, no shadows. This is about 20m from the consumer unit.
I have a Zappi charge point but no car yet! Still waiting for a delivery date on a MGZS
I installed an Eddi 10 months ago which now diverts excess solar into my 2 immersion tanks.
Mrs Woodyeyes has a hot tub which uses approx. 7-17kWh per day.
The house (large) is gas centrally heated. I rent out one half of the house as a holiday home.
In December 2021 I generated 33.3kWhs, exported 0.2 and fed the immersions with 1.0kwh. The total house consumption was 1208.3kWhs, of which 529 was for the hot tub.
In July 2022 I generated 332kWhs, exported only 6 and fed the immersions with 91kWhs. The total house consumption was 956 of which the hot tub was 211.

Last year I made enquires at Naked Solar to see what they would propose in terms of increasing my solar capacity. They submitted an application to Western Power for a 10kw system with 5kw of batteries and when they got this response they somewhat lost interest.

“…..Generation Installation of a 5kW export limited to 3.68kW system.
That is a gross permissible connection at the property and export limitation across the full installation.
The issue with the property location is that it is at the end of a long section of LV network. Even if we undergrounded all the way from the sub with 300AWC cable it still does not allow for this connection. The only way would be to connect from another sub nearby and underground from there. This would involve over 200m 300AWC in footpaths and roads and so as a ballpark figure would be in excess of £40k. This would also be subject to Traffic Management surveys which may add additional costs. This is based on the single phase request. The customer could have an augmented 3phase supply c£3-4K which would permit 13kW gross generation (export limited to 11kW). Obviously within G99 you would have to ensure no more than 3.68kW difference between phases.
….. All we are interested in is what you have connected in parallel with our network via your AC inverters. You/your installers can connect what they like behind this DC coupled as this has no impact on our network and the other connected customers.”

I’m still not 100% sure what this means or how I can get round it without major extra expense. Any comments would be gratefully received. I feel a bit frustrated that I have an almost ideal roof but adding panels to it seems nowhere near as easy as one might like!

Comments/suggestions would be most welcome. Many thanks.

Main considerations:
How to add further solar generation without losing existing FIT payments.
How to work around DNO’s limits.
Is the long cable run an issue?

[Countrypaul]

Provided you do not change the existing PV Fits system (other than repairs etc.) then you can simply retain that.

Given that your DNO looks to be limting you to 3.68KW export, anything that cannot be used onsite above that 3.68KW cannot be exported. It is not clear to me if you could have an additional single phase system that is limited to 3.68KW export (G99). If so then you could fit a hybrid system with PV panels, batteries and hybrid inverter limited to 3.68KW export. In case of a fault both systems could try and export and exceed the 3.68KW limit and wheter this situation is allowed is not clear to me.
You could also have an off-grid system - so some pv panels feeding an offgrid inverter and batteries (similar to the hybrid system) but completely separate to the grid. This could for instance be used with the hot tub, the HW tanks etc. anythig that could be operated seperate to the grid. You could even charge the batteries in the off grid system from the grid (thinking winter) as that would be DC linked and avoid any possibility of exporting to the grid.

The distance from the roofs to the CU would not be a problem if you fed DC from the panels to an inverter (& batteries?) near the CU, alternatively you could run heavier cable to near the CU from the inverter. Putting the inverer close to the CU (and mains incoming supply?) would also make it easier to put a CT clamp on the mains to avoid excess export.

I don't think it is worth trying to work round the DNO's limits, but there is still plenty you can do without doing so - after all why install more PV to generate electricity and give (export) that to the grid for a posible pittance at most.

I am sure others can give you other ideas, and more detail shortly