Feasible solutions to expand existing Sunsynk battery capacity
I have a 6.3kWp solar array with a 5kW Sunsynk inverter and 2 x 5.1kWh Sunsynk batteries, which were installed in December 2022 and which have performed exactly as hoped since installation. We have mature trees, with TPOs in place, around our property so the solar generation was always going to be compromised by shading, so each panel has a Tigo optimiser fitted to it enabling me to see the individual generation from each. The generation to date has been more than adequate to make the entire system a good investment for us.
Currently, we utilise the Octopus Intelligent Go tariff, so the batteries are fully charged overnight at the 7p rate and this then maximises any solar export during the day, for which we get Octopus' 15p rate, in addition to our own usage. The battery storage is sufficient to avoid peak grid usage for most of the year, but is currently insufficient to do so in the winter months. We're about to have a heat pump installed, so this will only exacerbate the problem further, hence I've started to look at options to increase the battery storage.
Sunsynk batteries come in a fixed 5kWh(ish) size (there are various versions of these, so you have to make sure any new ones added are compatible with the existing ones) and I've been quoted figures of £2,700 and £4,100 by our existing installers to add 1 and 2 batteries to the setup respectively.
Battery storage costs have fallen significantly in the period since our original installation and it seems to me that this might not be the most cost-effective way to add further storage. As far as I'm aware, the existing Sunsynk batteries can't be mixed with other manufacturers' options, but it still might be more cost effective to replace them with another solution to upgrade the storage beyond 20kWh, and then potentially look to recoup some of the costs of such by selling the existing batteries after removal.
From a bit of research, I'm particularly interested in the Fogstar batteries, as a value for money alternative. I'm aware of new installations which use Sunsynk inverters paired with Fogstar batteries, and they are currently advertising a single 32kWh battery for £3,499 or a 16.1kWh battery at £1,750, so buying two of these would equate to the same overall cost as the 32kWh option, but with additional practicality of being easier to physically handle (120kg weight per unit vs 233kg for the 32kWh option). The batteries themselves are built up of a series of smaller cells, as there are some DIY build options, but I'd be very much looking at the pre-built versions if purchasing directly.
I assume that the installation process, from an electrical perspective, would be relatively straightforward for a suitably qualified electrician, as it could presumably utilise the existing connection points to some extent, but I'm also conscious that I am most likely over-simplifying things, and perhaps missing some fundamental considerations. There's also presumably a configuration process to get the batteries set up within the Sunsynk system and 'talking to' the inverter, so would require familiarity with the Sunsynk inverter.
The question for the hive mind is whether this sounds like a sensible solution, or whether there's another feasible option I should be considering. Also, is it realistic for me to seek to purchase the batteries myself and seek quotes for them to be installed on this basis, or would the expectation of any electrician be to supply the batteries themselves, whether from a warranty or other perspective? Also, what would be the typical cost for carrying out work of this type (ballpark expectations only). Every renewable company I've looked into so far either wants to generate a default quote for a full PV installation or to retrofit a battery to an existing system without one using whichever default solutions they sell.
I'm happy to commence a process of speaking to local electricians to get quotes on this basis, but am just trying to pre-arm myself with understanding all the relevant considerations involved before doing so. A 32kWh battery system would more than meet our requirements and, based on the above, there looks to be an option to achieve this that, on face value, might be broadly similar in overall cost terms to adding 2 further Sunsynk batteries to achieve 'only' 20kWh of storage.
@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.
16 * 435 watts PV
13 kWh Growatt battery
1 EV - Mercedes EQB
6 kW Aira Heat Pump
@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.
105 m2 bungalow in South East England
Mitsubishi Ecodan 8.5 kW air source heat pump
18 x 360W solar panels
1 x 6 kW GroWatt battery and SPH5000 inverter
1 x Myenergi Zappi
1 x VW ID3
Raised beds for home-grown veg and chickens for eggs
"Semper in excretia; sumus solum profundum variat"
Posted by: @majordennisbloodnok@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).
- 22 Forums
- 2,078 Topics
- 45.5 K Posts
- 53 Online
- 3,355 Members
Join Us!
Trusted Installers
Struggling to find a reliable heat pump installer? A poor installation can lead to inefficiencies and high running costs. We now connect homeowners with top-rated installers who deliver quality work and excellent service.
✅ Verified, trusted & experienced installers
✅ Nationwide coverage expanding
✅ Special offers available
Latest Posts
-
RE: Renewable Heating Hub Homeowners' Q&A Podcast
In our latest episode, we hand the mic to four homeowne...
By Mars , 11 hours ago
-
RE: Forum updates, announcements & issues
@andrewj I’m currently away and only back this weekend....
By Mars , 11 hours ago
-
This is very interesting - I have just had my ASHP spli...
By heat-pump-newbie , 14 hours ago
-
RE: Help Stop the MCS Heat Pump Monopoly
Response sent. Thank you for highlighting the document...
By Sheriff Fatman , 18 hours ago
-
RE: I worry I've messed my Samsung settings up somehow
Yes , this time of year . Unfortuneately I dont have ...
By Alfapat , 20 hours ago
-
RE: Feasible solutions to expand existing Sunsynk battery capacity
Ours is a loft installation, which is where both invert...
By Sheriff Fatman , 21 hours ago
-
All your heat pump problems solved by -- a plate heat exchanger in a plinth!
The cut away appears to shows that its a plate heat exc...
By JamesPa , 22 hours ago
-
RE: 5 Star Service from Havenwise
@sheriff-fatman Exactly this is why a lot of our users ...
By HCas , 2 days ago
-
RE: Solar Thermal DHW and ASHP
I'm beginning to think solar thermal is something that ...
By Alex_N , 2 days ago
-
RE: Commencing on an ASHP Installation Process
I received the BUS Grant permission email yesterday eve...
By Sheriff Fatman , 2 days ago
-
RE: Solar Power Output – Let’s Compare Generation Figures
510kWh of solar PV from my 4kW Hyundai panels and SMA S...
By JulianC , 4 days ago
-
RE: Is my Samsung heat pump working too well?
Thats in itself is interesting. I have a Vaillant heat...
By JamesPa , 4 days ago
-
RE: Jokes and fun posts about heat pumps and renewables
@transparent Not a swell season then ☹️
By Toodles , 4 days ago
-
RE: Air Changes per Hour - ACH and the MCS requirement
update - my old installer insisted on a heat loss of 11...
By MatWin , 5 days ago
-
I agree it's not very helpful. Generally, the max heat ...
By Old_Scientist , 5 days ago
-
RE: Why Your MCS-Certified Installer Might Not Be As Competent As You Think
Hive and most other smart thermostats don't play well w...
By JamesPa , 6 days ago
-
RE: Enabling WiFi: Samsung ASHP on Smart Things App
Thanks for the input. I've been running various experim...
By Grahamh-uk , 6 days ago
-
RE: Radiators downstairs are cooler at the bottom after ASHP install
@melonbuffet Hi, this sounds great is you are getting 5...
By ASHP-BOBBA , 6 days ago
-
RE: Ripped Off: How UK Homeowners Are Paying Gas Prices for Wind Energy
I believe there's another reason that network upgrades ...
By Transparent , 7 days ago