Adding a battery (whichever you decide on) is likely to be a good idea as it opens up your options to TOU tariffs (such as Cosy or Agile if you are with O.E.) I take advantage of the Cosy tariff and ensure my Powerwalls are charged at the lowest tariff allowing me to go ‘off grid’ at all other times. I have solar PV but of course when the ASHP is needed most, the PV is supplying the least; the storage therefore makes the most of the lowest tariff rate to keep my grid running costs down. I don’t have a FIT but do export for some 8 months of the year @ 15p. per kWh to Octopus (Outgoing Fixed). FWIIW, I have 27kWh’s of Powerwall 2’s and know that is an expensive option but am very pleased with it nevertheless. Regards, Toodles. 

@toodles thanks! The powerwalls do look to be the ideal complement, plus heatable do a quite tempting interest free finance option currently. Good to know yours are working well.

"Stoke on Trent in the north of the country"! - just saying...

I have a GE AIO and I'm very happy with it.  The notable disadvantage is the absence of battery expansion, meaning another inverter which is unnecessary and another DNO application. Personally, I would prefer a little more storage capacity although - at the price - I can easily live without.

To me, a hybrid inverter means a single point of failure, which is why I'm happy with an AC-coupled battery (and solar microinverters).  A negative issue for the Powerwall when I bought my AIO was the 'closed' access via the app only.  I use my AIO with its API (GivTCP) and HomeAssistant and it integrates well with my Octopus Agile Import and Octopus Fixed Export tariffs. Even the standard GE automations would allow an AIO to charge every day on the more usual ToU tariffs without need of HomeAssistant..

@ianmk13 In more recent times, an app called NetZero has come to the aid of the party. I use some of the facilities in NetZero to automate functions on my Tesla kit and I know there is at least one other RHH reader who uses NetZero to automate his Agile tariff charging programme.

I carried out a daily ‘update’ once the next day’s Agile rates were released but am now on Cosy which provides fixed hours for the TOU tariff which simplifies it for me. Regards, Toodles.

We have a 4kW array and a Sunnyboy inverter (still working very well from original) and FIT. You need to ensure you get an AC coupled inverter into the battery, imho to not disturb your FIT set up.

There is a very convoluted gov document on what circumstances and configurations you can keep your FIT tariff and we decided to play it safe and not touch it physically. You can keep the 50% deemed usage or go on an  export tariff, generation tariff remains the same.

We added 2kW more PV and a separate Growatt inverter (also AC) to ensure we didn’t pay VAT on the battery. You don’t need to do that since batteries are VAT free now.

We wanted a Tesla for the capacity but they were not available that year so went for the next size available that could go outside, a 9.5kWh Givenergy. We’ve had very little trouble with our battery but a few people have had, in particular with the software. 
We had an emergency power supply included in the system which we’ve never used. 
If we had to select again it would be a Tesla or Givenergy AIO with a gateway.

The main issue with Givenergy software seems to be they claim the systems can be extended with further batteries but then they don’t work well together. Since we only have one battery that’s not an issue.

Thanks all. I am definitely keen not to do anything to compromise the FIT as that provides over 1.6k a year in payments.

What is slightly putting me off the Givenergy 9.5 is that the chap (who doesn't install powerwalls as he is not an approved installer) said the Sunnyboy would go and a new separate hybrid inverter would go in it's place- thus we have the cost of the inverter and the cost of the battery. He advised that the Sunnyboy would be nearing the end of it's useful life anyway as it was installed in 2011 from looking at our old paperwork that came with the house.

If I understand this correctly the powerwall has the hybrid inverter built in, and thus I understand IanMK13s point I think that separating the components might be preferable from a servicing and repairs point of view....would the Sunnyboy stay or go then with the powerwall?

So....I've just had the first quote through which is for the Givenergy 9.5kW battery. 2 x 9.5kW batteries just under 7k. Single battery for just under 4k. This includes a 5kW inverter.

The powerwall (single) I've been given a ballpark of 7.5k to 8k.

To make sure I understand this correctly, is the main benefit of the AIO and powerwall that there is no need for separate inverter, and for off grid supply in case of power cut? Are there any other benefits or is it a case of weighing up 19kW versus 13.5kw, with the former being cheaper as well even with the separate inverter?

@benson  - I'd recommend that you check the functionality first, and the price afterwards.

The GivEnergy product range is split into

• All-in-One units, which have an inverter and storage battery in the same enclosure
• separate inverters to which storage batteries are connected

When I checked a couple of months ago, they were about to release a solution which would allow two or three All-in-One boxes to operate together in parallel.
The parallel operation is achieved by the All-in-One units being linked via a separate controller.
By inference, that parallel operation is not (yet) available for the 'separate' inverters.

You need to have sufficient power available from inverter(s) to be able to start your heat pump.
Typically that will require 30A-ish or about 7.5kW.
You can't achieve that with a single 5kW inverter.

If you have a larger inverter, but already have other loads running it, then that 30A may not be available.
Check what happens in those circumstances.

Also check the specification for what output power is available from a GivEnergy product when there is a power-outage.

When I last read their specifications, there is an off-grid output port which remains live, but it only operates at about two-thirds power.

GivEnergy is a dynamic British company, and will continue to develop more functionality across their product range.
So you need to verify what is actually available from the specific devices you are being quoted for.
The model numbers and firmware versions matter.

Im going through a similar process, working out a sensible battery setup with ASHP, electric car and 4kWp of PV.  I still cant get the finances to stack up.  Looking at agile prices by time of day with a bit of a bias towards the winter, it seems like I could perhaps get to an average unit cost of ~17p, compared to 23.4p which I get on a fixed price tarriff, provided I could store about 12hrs worth of energy.  With my heat pump working at 4kW output, which is where it will sit most of the time, that's 16kWh at a COP of 3.  So its the 19.5kWh battery at 7k for a saving of about 6.5p per kWh, perhaps £600 saving per year based on 10MWh/year which is a reasonable estimate including car, heat pump and base load. 

The payback for the capital is thus 12 years, and if I want a rate of return of say 3% on top (which I could get by putting the money in the bank) then its even longer, and anyway greater than the claimed battery lifetime. 

I've run the numbers with more detail extracted from meter reads, but get essentially the same result (so it's more transparent if expressed crudely).

If I had an environmental argument the payback is good enough for me and Id go for it, but I haven't heard one (yet). 

Have I got it wrong or do batteries just not make sense unless you buy cheaply and self install?

@jamespa if you are factoring in electric car then you want a charger that is compatible with Intelligent Octopus Go, then you can import at 7p most of the year you can run the house from the battery and export all solar at 15p, have you tried running it again with those numbers it may look better?

@gary If no EV, a half-way measure might work? Cosy (8 hours available as 3,3 and 2 hour slots) at the lowest rate [currently approx 12.2 pence per kWh.] This works for me as I have no ‘V’s at all. Regards, Toodles.