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I keep an eye on the battery prices for the Sunsynk CATL 5.12kWh batteries which I currently have 2 of.
Just noticed that ITS have new stock at only £1300 inc vat, which is down nearly £500 from where it was, and nearly £1k below what I paid for mine!
https://www.itstechnologies.shop/produc ... s=e&_v=1.0

First question - It's nearly 2 years since my first battery went in - is it a good idea to add another one at this stage in the battery life?

2nd question - thinking forward to having an ASHP fitted, does it make more sense to go from 10 to 15kWh of battery capacity, to fully use the Flux / Cosy cheap rate charging opportunities, or to spend the money on more PV to boost the amount of actual PV generated, even though more of the ASHP useage will be in the winter, when the PV does nowhere near as much?

I would/have opted for more battery, enough to cover daily consumption in the winter. PV is no use on a dark winter's night when your batteries have expired and you revert to full price power.

Thebeeman wrote: ↑Mon Sep 23, 2024 12:45 pm I would/have opted for more battery, enough to cover daily consumption in the winter. PV is no use on a dark winter's night when your batteries have expired and you revert to full price power.

+1 to all that.

Cosy now gives you 3 bites at the cherry per day but even so 10 kWh is not a lot to run a heat pump (depending on its size).

If you can get an exact match then I would have thought adding a further 5kWh after 2 yrs would be fine. Is there a State of Health readout on your existing two? When I did this after 1 yr I was advised to check the SoC of the new battery and connect it when the existing two modules were at the same SoC, it went very smoothly.

In my case I charged the existing 3 x 2.5 to 90% and then took them out of circuit, inserted a new 5 kw and charged to 90% then reconnected the other 3 with with the new one as master, it all went well with no problems.

The original question was more PV or more batteries. With all the weird tariffs that are now available it depends more on individual circumstances. But the old rule of thumb, I think from memory from SMA is twice the useable battery kWh to the amount of installed PV in kWp.

Bigger battery first, then add more panels via DC charging. If you add the DC charging first then risk the batteries via over charging (if it's a really small pool) and limit the charging amount before have to move to constant export.