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.