Posted by: @bontwoody

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@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?

2500 quid for a 5kWh extra battery is a complete rip off - 16kWh batteries new are 1700 quid retail checking todays deals (so including vat an installer will get back) so it sounds like it would be cheaper at some point to just switch inverter and batteries. What's your existing battery worth if you flogged it off and changed system 😉 ?

I don't think there is any good answer on battery versus solar. It depends so much on what happens in the next few years. There are a lot of pressures on both solar and overnight tariffs.

- Solar increasingly will be worth money only when time shifted or self consumed. Which is great for aircon/hottubs but more awkward - although eventually we'll reach the point that throwing extra away is cheapest and who cares.

- Overnight tariffs should be under pressure from battery farms (who can in theory do arbitrage better than you can), and from changes away from gas plants.

The house we are currently doing up got insulation and high efficiency windows as the priority. Even that though is at best a finger in the air guess.

If you can fit the panels without scaffolding and complicated roof work that to me sounds like the short term win - hopefully export stays ok for a few more years, and even if it then collapses battery prices are on a sharp downward curve still and solar aint.

@etchedpixels 

Yes, the Luna battery modules are pricey and I had considered whether to try and resale to change them. Picking one up second hand was a lifeline.

Good point about the battery v solar cost trend 😁

So after consideration, I think it makes sense to go ahead with the installation of extra panels. The next consideration is how many and what type? My inverter specs are below:

My understanding is that I can pretty much put any panels on a different MPPT port, although from an aesthetic put of view I might try and match the JA panels used before.

Would these be suitable?

CDN.ENFSOLAR.COM "https://cdn.enfsolar.com/Product/pdf/Crystalline/5b17a468ba397.pdf"

I did notice that, I might have a shading issue. This image was taken at 7.30pm last night, so perhaps it might be better just to put 4 panels as far to the right of the roof as possible and avoid the telegraph pole shadow? Or will it not make that much difference? I dont think it will make financial sense to use optimisers.

I would be mindful of the panel voltages when choosing panels with respect to the startup voltage of the inverter, especially when using a smaller number of panels in the string.

The inverter startup voltage is low at 50V (good) and the panels you link are quite high voltage output (also good), so look like a good match for each other. They should output enough voltage under low irradiance conditions to allow the inverter to power up. Those panels have a higher rated voltage output than either my 405W or 445W JA Solar panels.

@old_scientist Thats good to hear. I may have sourced some cheap optimisers too, so potentially i might be able to have more panels.

This might be escalating a bit. Ive managed to source a bundle of 10 Huawei optimisers for £180 and while my intention was to resell the ones I didnt need, its just occured to me that I can fit another 2 panels on the East roof of the extention too. Even on a perfect sunny May day, the 3.68 inverter is only producing 1750W, so there is definitely room to extra capacity.

I can copy the design of the original string with East and West on the same string with the optimisers controlling everything. The optimisers I bought are P2 and wont work on the same string as the P1 versions on my original string but should work fine on a different MPPT.

The only potential pitfall I can think of is that the 2 panels on the East side might not create enough voltage to operate the inverter early in the morning?

Please consider what action you'd need to take if one or more of the optimisers went faulty.

I have a friend who has three different PV arrays on roofs, and one optimiser in each set showed a fault on the App.
Consequently his inverters received naff all solar input.

He hadn't got a record of which optimiser was placed behind which panel, so he faced removing the panels one at a time to check!

Moreover, when he re-plugged the wiring on the first PV array to bypass the duff optimiser, the APP then reported "too few optimisers in the string to start up"!

@transparent thanks. Will be sure to keep a record of serial numbers with positions 😁

Ive now managed to assemble most of the required items for this installation (£500 so far). In the end I have bought 6 solar panels that I am going to put on the West roof of the extension. Ive gone with Renusol fixings and have been going through the guides and Youtube videos. There are a couple of things that arent clear to me so Im hoping you lot can help 🙂

I will need to extend the 3.6m rails and have bought two rail extenders. The Renusol instructions say to leave a 5mm gap between the rail ends for expansion, but on watching a video the installer inserted the extender completely into the rail and pop riveted it in place. This I imagine wouldnt leave much expansion room?? 🙂 Should the extender just be slotted in and left unfixed?

Secondly, Im unsure about earthing, should the panels be bonded to the rails and the two extended rails bonded with earthing wire and the two separate rails be bonded together? If so should the earth wire be routed through the loft with DC cables? I have to say I can see no evidence of any earthing wires on the two previous solar installations on my house.

Thanks in advance.

Good questions. There's a lot to explain here.

Renusol is a German PV Panel roof-mounting brand, and available in the UK from Midsummer Wholesale.

It's a typical aluminium rail system, with three different rail sizes. Midsummer stock the 41mm x 35mm variant.

The rail has two C-slots and a central box section:

The upper C-slot accepts the brackets which hold the panels themselves, whilst the side C-slot connects to the fixings on the roof (screwed into the rafters).

There are a great number of different fixing methods, depending on the slope of the roof and the preferred orientation of the panels.

To extend the 3600mm lengths of rail, Renusol have a short length of smaller C-section which they call a Splice.

Here's a page from the Installation Guide. It illustrates the parts required, but doesn't show you how they should be fitted together.
The red annotations are mine!

Given it's shape, I'm assuming that the splice is meant to be inserted into the central rectangular box-section of the rail.
A screw/bolt can then be inserted into the hole on the splice, thus allowing the rails either side to slide along the splice as they expand/contract.

I certainly wouldn't be pop-riveting it to both rails!
However, you could secure the splice to one rail only, and leave a few millimeters gap. That allows the 2nd rail to slide along the splice as required.

At £4.62 (+VAT) a C-section for a splice isn't too expensive.

Personally I'd have not bought Renusol's splices. The lengths of rail can just as easily be joined and aligned by using a section of aluminium flat-bar inserted into one or both of the C-section slots.
Ensure that the rail join doesn't coincide with a roof-fixing or panel-fixing for the relevant C-section.

The flat-bar can be drilled and tapped, allowing it to be bolted to one of the rail lengths, whilst the other expands freely.
Here's a length of 5mm-thick flat bar which I just happen to be working on this morning.

You would only need one or two threaded holes of course, not the nine which I'm using here!

Once you've got a system that uses C-section rails, you can devise all sorts of bespoke fittings.
So long as the aluminium is thick enough, it will be amply sturdy.

Earthing of roof-rails and roof-mounted solar panels is not required in the UK.

If you get a shock from touching them, then why are you sitting on the roof?!

Remember that any metalwork at roof level will act as a lightning conductor if you connect it to earth.

In the less-likely event of your un-earthed panels still being struck by lightning, you can protect the inverter by placing surge-suppressor units on each of the two drop-leads.

Those DC-rated Valvetrabs by Phoenix Contact are the top-end of surge suppression against lightning. They are very expensive, and extremely effective.

My house is on top of a hill, and on the South Western upslope to the high ground of Dartmoor.
My evaluation of the associated risks resulted in me buying the best suppression units available!

@bontwoody I'm following your endeavours with interest, as I have an outbuilding with east/west roof, and room for two panels on each aspect in landscape. So far I have purchased two panels for the west aspect, and an ECOflow 800W micro-inverter.

Like you, my next step will be to figure out the mounting system, and I'd been looking at the various Renusol products that would be compatible with my double interlocking concrete roof tiles which look very similar to those installed on your roof. Where did you source your Renusol products?

My (limited) understanding on earthing is that this is only required where the panels are low enough that someone could make contact with them. If they are up on a roof where human contact is not normally possible, then they do not normally need to be earthed. Hopefully someone can confirm or correct this if incorrect.

EDIT: I see @transparent has replied on the subject of earthing whilst I was composing my reply.