@sheriff-fatman We recently added a second Growatt 6.5 kWh battery to our bank. The installer was able to take the VAT from the battery price when they sourced it themselves. It's worth checking whether you are able to buy the battery VAT free if you procure it yourself.

We haven't used the heat pump for heating yet since the battery installation, but from my consumption model, I predict that we should save a bit of money with the addition. Especially in the shoulder months when the upgraded capacity would mean we can stay on Octopus Go instead of moving to Cosy Octopus. That means our EV would get charged on cheaper Octopus Go rates instead of expensive Cosy rates. On the other hand, the added capacity would also significantly reduce battery cycling. Instead of cycling three times on Cosy, it will just charge and discharge the batteries once a day. That should extend the life of the batteries by a few years.

@sheriff-fatman, I'm also quite interested in this. Although we're not in the market yet, I'm doing a bit of early research to get the feel for it.

If we do go in for an upgrade - in a year or so - then I'd be looking to replace the current Growatt inverter (limited to 3kW to or from the battery) with something that can deal with greater throughput. However, like you I have been looking at Fogstar for the battery side. Importantly, our current battery is outside and that's what I'd want for any future purchases so Fogstar's external enclosure is what particularly gets my interest. At the moment, that's £4,000 for a half-populated enclosure (4 modular batteries to a total of 20kWh) or £7,000 for a fully populated enclosure (all 8 slots taken up giving a total of 40kWh). What I don't know yet is details of which inverter or how to get the two playing nicely together.

Posted by: @majordennisbloodnok

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@sheriff-fatman, I'm also quite interested in this. Although we're not in the market yet, I'm doing a bit of early research to get the feel for it.

If we do go in for an upgrade - in a year or so - then I'd be looking to replace the current Growatt inverter (limited to 3kW to or from the battery) with something that can deal with greater throughput. However, like you I have been looking at Fogstar for the battery side. Importantly, our current battery is outside and that's what I'd want for any future purchases so Fogstar's external enclosure is what particularly gets my interest. At the moment, that's £4,000 for a half-populated enclosure (4 modular batteries to a total of 20kWh) or £7,000 for a fully populated enclosure (all 8 slots taken up giving a total of 40kWh). What I don't know yet is details of which inverter or how to get the two playing nicely together.

Ours is a loft installation, which is where both inverter and existing batteries are located.  I don't have the same issues as you regarding an external installation, but my understanding is that loft installations are now 'discouraged' at the very least, and potentially no longer allowed as new installations, so I'm not sure how those regulations impact anything done to upgrade them.  There's also the practicality aspect of physically getting the units into the loft if a straight swap was done.  By comparison, the Sunsynk units are around 75kg, I believe, which the installer coped with easily enough.  However, the lightest Fogstar option would be 125kg, and I've assumed that 2 of these will be more practical than the single 230kg behemoth, given that they're the same overall cost.

Potentially, any change might present an opportunity to relocate the system, if we were forced down that route, albeit that would change the overall scale of the job to one where it's no longer the 'simple' change of batteries originally envisaged.  The 5kW Sunsynk inverter does all that we require from it, so I have no particular desire to move away from it.

I'm aware that some of the higher end batteries, such as the Sigenstore ones, have their own inverter on them, and had looked into what might be feasible adding one of those as a separate installation, but presumably only one of them can be linked to the existing solar array, so I'm not sure how feasible an option this would be in any event, and disregarding the additional cost required for it.

Thanks also to @chandykris for the input re VAT considerations, which is a very pertinent point for consideration, regarding whether a supplier would opt to supply the batteries directly.  They'd presumably look to recover a margin in doing so, but if their base price is 20% lower than mine to begin with then it is logically a sensible consideration, although it perhaps restricts the choice of installer if only certain ones are certified with Fogstar (their approved supplier list appears largely to be companies specialising in motor-home conversions, which is presumably an area where they're well established already).