I'm seeking community thoughts on the practicality of using solar PV to supplement central heating via an accumulator tank.

Not permitted to put solar thermal on roof or anywhere near house. Solar PV is only practical generation we can install in a field about 80m from the house.

We have 'twin phase' electricity supply (two single phase mains through two separate meters, two consumer units) use average 16kWh/day of electricity. In winter around 18L/day oil and in summer 7L/day on domestic hot water (300L cylinder) and keeping the under-floor slab ticking over.

A neighbour has 17kW PV (the max permitted 1ph grid tie) and in Nov / Dec / Jan it delivers average 10kWh/day. Not much, but enough to run the dishwasher and supplement the hot water. May / Jun / Jul / Aug it's 60 to 70+kWh/day.

My take-away from these figures is go large because PV is relatively inexpensive and aim to export nothing we don't have to. I think we might get permission to grid-tie 8kW to each incoming main, and this is about the max size of domestic 1ph inverters.

So, proposal is 24kW PV array, 3 x 8kW 1ph inverters, two of which hooked into our existing consumer units. 1500L accumulator tank fitted with 9x 2kW immersions. Set up the demand management to favour any domestic load, followed by the lower immersion in the domestic hot water tank. Any additional energy (in 2kW increments, as available) from the two grid-tied inverters to the accumulator, plus all the energy from the third inverter.

On average mid-winter day we should get a chunk of our domestic demand covered, plus most of our hot water. For the other 9 months, and particularly Apr to end Sept there should be 75 to 100kWh/day (on average) Knock 30kWh off for domestic use and hot water and that leaves 45 to 70kWh for the accumulator, saving maybe 4.5 to 7L of oil we'd burn otherwise. The 1500L accumulator should store around 70kWh if the mass of water is heated from 45C to 85C, then extracted to a heat exchanger on the central heating circuit.

No PV installer is much interested in this plan, too much plumbing and a bit left-field, but what does the community think?

P.S. I've considered batteries and calculated that they are no friend of the environment or my wallet, even with electricity at 35p/unit. One of the problems Is that we would need two batteries and charge controllers for each 'phase' of our supply, doubling the cost.

What's your objective: getting a return on your investment? Reducing carbon emissions? Protection against energy price swings?

Where are you located? There are practical and regulatory and economic issues with that scale of PV, but maybe works in other places?

In the UK, oil is about 12p/kWh and solar can be exported for 15-20p/kWh, so it actually costs you money to use PV yourself to displace oil, is better value to export it. But to reduce carbon emissions, it is good to electrify your heating, ideally with a heat pump to vastly reduce the electricity consumption. Then investing in a matching amount of generation/storage would be a way to buffer against price rises or falls.

Something staggering has happened with commercial PV in the UK in the last year as solar farms have broken grid parity and there is now an explosion of utility scale schemes in development (10s of GW) including community-led schemes. These can install many more kW per £ than individuals can domestically, you might be interested to participate in one of those?

Thank you for all your thoughts. My motivations are partly to save money in the long run, partly to be vaguely responsible from a CO2 perspective and partly to elevate the EPC just in case we were ever to let or sell.
There’s no doubt the system is complex. Contactors go wrong, fail to contact and you don’t notice unless you’re monitoring endlessly. We’re based in Berwickshire, sunny side of Scottish Borders.
If we really could get 20p/kWh for export that would be a game-changer. Far better to take the money and buy oil. At least from an economic point of view. I thought the domestic export tariff was only 4 or 5p/unit? Where do I investigate other avenues to sell power to the grid?
The EPC objective would be met with a small domestic installation; we won’t get a better assessment for a bigger PV generation capacity.
It probably would be more grown-up (strictly logical) to install ground source, but our bathrooms would never be properly warm again… We have two ground source systems in other houses, one 240kW (a mad size dictated by RHI) and a domestic 12kW system. These heat at around 40C and stay on 24/7. Cop is around 3.5, so directly comparable cost to oil at £1/L

We are on outgoing since March. Ave has been between 17p and 20p got less as we went into summer. They are taking new customers a friend of mine just signed up. ( Whisper me I can give you a code if you intend applying and we both get £50.)We export quite a bit and we are careful when we use load hungry appliances and so far this Summer we have been 99% self sufficient but the battery makes that work. The export payment goes to pay for the gas (LPG) to the point we are cost neutral. Boiler only used Nov to end Feb beg of March. The secret (not really) is to stuff insulation everywhere eliminate draughts and cold bridging and capture as much solar as you can.

Posted By: Followthemoneyone 240kW (a mad size dictated by RHI)

?? 24Kw?

Sorry a bit off topic. Can you now install more PV than the maximum 4kw as long as the inverter is limited to 13A? If so do you have not notify in advance as the gov site implies its still limited by the array size Sorry to raise newbie query, finally going to look at doing this as we have a huge south facing roof in devon to use.
Thanks

Posted By: andrew_rigamontiSorry a bit off topic. Can you now install more PV than the maximum 4kw as long as the inverter is limited to 13A? If so do you have not notify in advance as the gov site implies its still limited by the array size Sorry to raise newbie query, finally going to look at doing this as we have a huge south facing roof in devon to use.
Thanks

You are allowed to install 10Kw on a single phase electricity supply (30kw on three phase). However you will need to get the consent of the DNO if you install over 4kw. This may or may not be easy to do and may depend on how modern the electrical grid is near you and how many other people have solar in your area. We wanted to increase our solar from 5.5kw and the DNO said that we would have to pay a charge to 'upgrade the system' to cope with it. The charge was quite expensive and would make the ROI on the solar untenable.

An alternative would be to install equipment to limit the export to the grid. Details in the link:

https://www.sma-uk.com/solar-systems/export-limitation.html

Posted By: philedgeIf you want to export more than 16A you need to apply in advance and get approval via the G99 application process

My application was G99/ g100 for a 6kw inverter but restricted to 3.68 export presumably because inverter was capable of exporting more. Included in the application was the manufacturers statement of how the export limitation would be achieved. My inverter is done via software and password protected. In April this year applied for information as how to go about getting permission to export more and was informed that I could go to 5Kw at not cost but to get to 6 would cost £750 to apply and subject to a witness test whatever one of those is. No brainer settled for 5Kw made a big difference to how much I am exporting. All DNO's have different situations it seems .

Our transformer serves 2 premises with 3P do not know what size it is. In my field further down the line there is a pole with another transformer which is new in the last 3 years. This is physically bigger than the one we are tee'd off. The next time maintenance works takes place will quiz the engineers as to their relative sizes and what scope there is for me to expand our PV.

Posted By: philedgeThat £750 charge is about what SPEN charge for a G99/G100 application and site witness test if export limitation is required. Application is £250+vat and witness test is £350+vat. I beleive the witness test is for you to prove to them that your system does actually limit export and that the limitation is fixed in such a way that the user cant increase. Ie engineer/manufacturer password protected, or other "secure" means.

Thanks for the explanation. Looks like I was fortunate I was not charged only mention of payment if I wanted to go over 5kw then £750+ . got the impression that 5Kw was now standard here but could be just my line. We were "rewired" from the transformer and onward about 3 years ago the 5 wires (3P) to the property were replaced with a twisted set so now no wires for me to enjoy the migrating swallows that used to congregate on them in their masses.

AFAIK if you replace like for like then steam on. If you change anything youre obliged to notify the DNO even though export may not increase. If export is or can increase, say with addition of AC coupled batteries, then its a new application as export or potential export has increased.
If theres no or limited spare export capacity in your section of the grid then you may end up having to pay for grid upgrades after your neighbours have used up spare capacity. If youre putting in a G99 application it may well be worth considering any changes youll make in the nearish future and apply for that future capacity now??

Theres no routine witness visit for G99 application, at least not in our case. AIUI the site witness test is for when you have a G100 configurable export limitation system where they want to see the system set for the limit youve declared on the G99 application and that it cant readily be reset by the user.

If the inverter fails after a few years and you change it for a different make and model, then I belive you have to notify them. If its not functionally the same then likely a new application or maybe just another witness test to see that the limitation has been set correctly.