If you can afford the larger capacity NOW, you would get the benefit of zero VAT if purchased as part of your Solar PV installation - bought later, you would incur VAT. The larger the capacity of storage, the more flexible you can be about when you ‘download’ your energy from the grid to charge your battery. I am with OE and am on the Agile tariff which works for me as a retired nerd! I have the time to check the following days half hourly rates and set my Powerwalls to charge accordingly. Only having had the ASHP since February, I don’t know my typical consumption yet for real winter weather; having 27 kW/h of storage will enable me to buy much of my consumption at the cheapest times. I have perused the Cosy offerings but I think I can still do better if I keep up my membership to the Retired Nerds Club for now! Regards, Toodles.

@toodles thank you- really helpful! I thought that would probably be the case… being an all electric household it makes sense! 
Enjoy the Winter with your ASHP!
katie

@curlykatie We are also an all-electric house and with 8.1kWp of solar, we do export some kW/h per day to OE under the Fixed Export tariff of 15pence per kW/h. The Eddi energy diverter ensures the DHW immersion element has anything it requires during the day (Sunamp Thermino) and that the Tesla battery is filled if the Sun makes sufficient an appearance! All the rest then goes to export to help pay the ASHP use in the winter. We have a condensing tumble dryer that uses a heat pump and along with an induction hob, helps to keep the power consumption down. (Average daily use is 12-13 kW;h with the DHW included. Regards, Toodles.

Hi @curlykatie, consider also the Cosy Octopus tariff. https://octopus.energy/smart/cosy-octopus/ If offers two low rate periods each day 04:00 - 07:00 and 16:00 - 19:00. That's 2x3 hours during which, at typical max charge rate of 3kW you can charge up a 9kW battery twice a day. I currently charge a 6.5kW battery up twice a day in the heating season at around 90% round trip efficiency. This alone knocked 30% of my electricity bill overnight!

The battery powers my ASHP and other consumables during standard and peak rate tariffs. It falls short of heavy use days when the ASHP is working hard, so based on what I saved last winter, I've ordered a second 6.5kW Growatt battery to increase my storage to 13kW. That's fitted next week. Cost with VAT for the Growatt is £2,690. The first battery, an afterthought on my solar PV purchased last December, was £3,400. Li-ion battery costs have come down quite notably in the last 9 months. That could be Chinese pricing policy, they have cornered most of the market, and also that there's much more solar PV in the UK now getting installed.

I would go for the larger battery storage capacity, it'll provide a real benefit and cost saving in winter set to scheduled daily charging based on flexible time of use tariffs. Which tariff you want to go for is up to you, but consider your daily kWh usage and the time & rate of low tariff when deciding. 

Be careful where the batteries and inverter are located. There's good guidance on this, which some installers pay little notice of. You want to avoid under-stairs and egress routes from your house. Garage, external, or airing cupboard not opening onto a landing is fine. Avoid loft spaces where the insulation is above ceiling as they can get very hot in summer and inverters will start to limit their power output on internal thermal protection if the ambient temperature gets too warm.

@allyfish thank you- that’s really useful. I have just spoken to the Solar company and they do up to 21kW batteries so will see what they can do on price. 
they are planning on installing the battery in the lof- and it gets roasting in there!! we have a garage so will speak to them about that too!

Thanks again ☺️

@curlykatie a pleasure to help. Installers tend to want to locate solar PV equipment in a place that's most convenient & quickest for them, not necessarily the safest or most convenient place for you. I would avoid a loft space where the summer temperature can get really hot. Solar PV battery systems are very well protected, with metal casings, and advanced battery management system monitoring, but the inverters kick out heat, and they'll do this less efficiently in a hot space than a cooler one. The inverter waste heat could be useful in a garage, utility space or similar, not normally heated space to keep a bit of background heat and lower humidity. Our solar PV inverter & battery is in an airing cupboard at the end of the en-suite along with the hot water cylinder & ASHP controls. It's not on an escape route, and I've taken the precaution of fire proofing the loft space attic hatch and fitting a smoke alarm in there. In the very unlikely event something went wrong, it will buy us time to get safely out.

@allyfish Our solar wiring, switches,Tesla Gateway and Powerwalls are all in our unheated extension that is just a fire door away from access from our hallway; the ambient temperature ranges from approx 12 degrees C to 25 degrees C, winter to summer. We ‘mollycoddle’ our battery but it should last longer that way and it is easy to access should there be a need at any time. Regards, Toodles.

@allyfish great advice, thank you. The garage can get quite cold, assume this is not an issue?!

@allyfish another probably silly question! With octopus cosy how does it work to import/ export to and from the grid?

@curlykatie garage is fine. Most inverters are designed for an outdoor/unheated installation location. Better to have the inverter and battery a little cooler than too warm. 

Octopus Cosy is an import tariff. You can sign up for a separate fixed or variable rate export tariff with Octopus or another provider. You’ll need your MCS certificate details to do this once the install is done. I went for Octopus fixed outgoing at 15p/kWh. Surplus export in summer builds a little credit and pays for the annual standing charge even on our modest 3.6kW system.

@allyfish thank you for your help ☺️