I have just had a my inverter and battery installed, about a week ago, and I think it is very similar to @batpred system. I have a Solis S6-EH3P10K02-NV-YD-L with 16.1kWh Fogstar battery pack. I have a seperate CU for 'back up load' which the inverter switches over to run from the battery if there is a power cut.  It isn't full islanding, I don't have everything running through this board, but it is enough to keep the lights and some of the sockets powered in the event of a power cut.  We rarely have power cuts and they don't last for long so that wasn't a big consideration, but as the inverter has the option and it is a new build I have made use of it. If I really want to I think the invert is just about large enough to run the house but I was advised not to run everything through this board. The electrician has set it up so it is fairly easy to move circuits between boards if I want to change things in the future.  This was all installed as part of the solar install so I don't know an exact cost but the inverter, battery and extra wiring has cost around £4-4.5k. This is a 10kW 3 phase inverter and it was picked as it works with 48V batteries, so not the cheapest option.  Most 8/10kW 3ph inverters only work with high voltage batteries and I couldn't find those at a price I was prepared to pay. I expect the battery to pay for itself within 5 years, but time will tell.  With 2 EV's we can use everything we can store during the summer months so with that and being able to run mostly from the battery during the day I expect a lot lower electric bill.

I think the inverter will run without internet connection but I haven't tried it yet.  It does have an AI powered setting in the app that will set the charge and discharge times according to the house needs, weather forecast and tariff, including Agile. This does need an internet connection. I used it the first night but that was it as I have been trying out different settings myself first.  I like to play around with the settings to start with to find out how it works but after that I'm hoping the AI will keep things ticking over without much involvment from me.

In terms of safety the latest Fogstar Solar batteries have integrated fire suppression built in them so a lot of people must be asking about the fire risk.  I bought the batteries in the black Friday sale, just before they put the latest batteries with fire supression on the website.  I bought Fogstar because of the price compared to most other batteries. If it doesn't last quite as long then that is OK.  From everything I have read LiFePO is very unlikely to have an issue with runaway so I am comfortable with them in the house.

All the charge and discharge rates, times etc. are controlled by the inverter. The battery pack feeds the SOC back to the inverter and the inverter uses that to stop the charge when the set SOC is reached.  The battery pack has a screen with a few basic settings and the SOC and cell voltages. I like having the cell voltages as I can see that the SOC% and the cell voltages on the battery display don't match the charts I can find online showing LiFePO4 cell voltages and SOC. Fogstar are being careful with the readings and protecting the battery, both low and high.  It also displays state of health, which I would love to have on my EV batteries!

I am finding that with a new build house and a 5kW heat pump we have been able to run all day from the battery charged overnight for most of the last week, but we don't use too much energy to heat the house.

Posted by: @johnnyb

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In terms of safety the latest Fogstar Solar batteries have integrated fire suppression built in them so a lot of people must be asking about the fire risk. 

However, see what I've just posted here about these (cheap and functional) inbuilt aerosol fire suppressor units.

Posted by: @johnnyb

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I think the inverter will run without internet connection but I haven't tried it yet.  It does have an AI powered setting in the app that will set the charge and discharge times according to the house needs, weather forecast and tariff, including Agile. This does need an internet connection. I used it the first night but that was it as I have been trying out different settings myself first.  I like to play around with the settings to start with to find out how it works but after that I'm hoping the AI will keep things ticking over without much involvment from me.

I can confirm that not only does it work without internet, it also works without the datalogger dongle! 

As for Solis AI, I posted a summary of my findings on the Solis inverter thread. There´s not much they would need to sort to perfect it, but they may have other priorities..

I agree it could give that hands off experience. In my case, I doubt it would add 5%. 

We have a Tesla Powerwall 3 and Samsung 12kW heat pump, and we are on the Octopus Cosy tariff.

Our highest usage this winter was around 45kWh and we just about managed with a single 13.5kWh PW3, so I would say a single PW3 unit would be about right for up to 50kWh per day usage on a tariff such as Cosy where you can recharge 3 times per day with a little sensible load shifting (e.g, we always reheat the DHW from grid in a cheap slot, run the washing machine and dishwasher, and try to cook with electric oven from grid rather than on battery). We only ever use cheap rate Cosy electricity at 14.8p/kWh

The evening slot between 4pm and 10pm is probably hardest. Our heat pump ticks over at 1kW on minimum so the heat pump and background usage alone is going to consume most of the battery capacity. Cooking a evening roast dinner in the electric oven on the coldest of winter evenings would be a struggle, so you do need to use a little common sense and load shift where able (either that, or buy a bigger battery). As you have a smaller heat pump that us, yours may consume a little less which would help.

We too thought long and hard about Musk/Tesla before making our purchase. One of the biggest considerations for us was the thermal management of the PW3, which is industry leading, and was important for us given it is cited outside in the cold. Many batteries struggle to charge/discharge in cold conditions, which kind of makes them useless for running a heat pump off peak if you can't charge them up when it's -2C outside. If you plan on installing outdoors (or in an unheated shed or garage) make sure your choice of system has good thermal management.

Overall the PW3 is a no-brainer. It's set and forget operation, specs which are very hard to match (11.04kW inverter/load capacity, 13.5kWh storage, stackable expansion packs, 100% DoD, 10 year warranty) and reputation all make it very hard to beat.

We had existing solar with AC inverter, but asked our installer to connect the existing solar into the PW3's inverter which has given more flexibility and makes charging the battery from solar more efficient.

There are significantly cheaper options, but you are not always comparing apples with apples.

Posted by: @old_scientist

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We had existing solar with AC inverter, but asked our installer to connect the existing solar into the PW3's inverter which has given more flexibility and makes charging the battery from solar more efficient.

Thank you for sharing! There seem to be so many PV inverters not able to help green the grid when it needs it, even if it seems simple to rewire..

It makes sense to let the hybrid inverter manage everything and these days even ai is being packaged in for free. 

With 8kw, I am considering adding a bit more storage and start delivering 24kwh/day...

Posted by: @old_scientist

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Many batteries struggle to charge/discharge in cold conditions, which kind of makes them useless for running a heat pump off peak if you can't charge them up when it's -2C outside.

Correct.

LiFePO₄ cells mustn't be charged if their temperature is 0°C or below.
That would cause permanent damage to the Lithium electrode plates.

If you want to locate a LiFePO₄ battery outside the house, then it should have a BMS with a thermostat and foil plate heater.

@majordennisbloodnok, what was your motivation to down the Growatt route and do their products offer islanding for power outages?

Posted by: @editor

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@majordennisbloodnok, what was your motivation to down the Growatt route and do their products offer islanding for power outages?

Our Growatt decision was rather decided for us, @editor. Our local county council had an initiative in partnership with Solar Together - basically a bulk-buy scheme with installation then put out to tender. We were given literature about the kit they intended to use, but there was no choice per se on brand; merely how many panels and how big a battery. The cost involved was, at the time, too good to ignore and the savings from the bulk negotiated price meant our quote effectively provided the battery for free. With 20:20 hindsight, I would now approach it differently, not least of all since technology has moved on. I've also heard mixed reviews about the Solar Together schemes although for us it worked fine and, let's face it, a typical install of solar panels, an inverter and a battery is very much more straightforward to do than installing a heat pump.

As for the inverter itself, it is a hybrid and can provide some islanding functionality although we don't have it installed that way. Nor would I want to install it with islanding functionality without having a significantly larger battery - the only I actually wish we had done differently at the time. However, I would not just put a larger battery into our existing installation since whilst the inverter is a 5kW unit it can only move 3kW to or from the battery. If I were to get the battery currently at the top of my wish list (the Fogstar outdoor rated cabinet with either 4 or 8 rack batteries - giving either 20 or 40kWh), that'd mean either 6 hours plus or just over 13 hours depending on configuration to do a full charge, and if in Winter the ASHP was chewing 4kW I still wouldn't be able to draw enough just for the heating. In fairness, this is precisely why I'm watching the various inverter threads quite closely, since I suspect upgrade time will come around reasonably soon and we might well repurpose the Growatt inverter and battery as a hand up for our son and his first house.

There are certainly newer Growatt units that have much better rates of current to and from the battery, and I don't have any big concerns about the Growatt product quality per se. I would, however, say that their online control (the Shine portal and/or app) is way short of where I'd want it to be, hence why I went for a straight modbus connection and linked it into Home Assistant. Quite a few of the current models appear to support generator input too, which may be a consideration if you want an islanded solution for short to medium power cuts with the option of filling up the battery or running the home from generator if the power cut persists. None of these are features I've investigated yet with Growatt; I merely saw them mentioned, so please don't take it as anything more than a knot in a handkerchief for further thought.

I’ve started looking at a few brands (admittedly just a quick browse so far) and I can definitely see the appeal of all-in-one solutions like the Powerwall. The simplicity is a big draw... everything is designed to work together, it’s well packaged and there’s less decision fatigue.

That said, you’re clearly paying a premium for that convenience and brand confidence.

What I’m finding genuinely confusing at this stage is understanding what’s actually required with more modular systems. Battery pack, inverter, gateway, monitoring, backup functionality... it’s not always obvious what’s essential, what’s optional and what’s included versus what’s an add-on.

Posted by: @editor

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[Tesla Powerwall] you’re clearly paying a premium for that convenience and brand confidence.

Not necessarily.
You need to compare storage battery products based on cost per kWh of capacity.

When I checked a dozen brands about 5 years ago, the Powerwall was the most expensive outright,
but the 2nd cheapest per kWh.

Realistically, @editor, I think the best option for you is to forget brands for the moment and simply draw up a list of wants, then split that list into essentials and nice-to-haves. The only nuance on that is with regard to the islanding, since there may only be some things you want to keep running and some you're happy to live without. I know it sounds a little patronising to simplify it like this, but once you can see the shape of the problem you're trying to fix, I'm sure you'll have a much better understanding that you can then use to assess different products and, of course, understand the compromises you're accepting when you plump for one particular solution.

@editor 

More kWh for my hard earned cash was the priority. 

Our 45kwh of storage should meet our HP needs and the rest of the house load, other than the very coldest of days.

40kwh of Tesla storage would be at least £15,000.

I'd doubt you'd get your money back, certainly not within 10yrs, possibly even 15.