I don't think its worth investing in more battery power if you are getting an ASHP. It would be more worthwhile to invest in a solar PV to hot water converter like an iBoost to make use of any excess solar (if you don't already have one). During the winter the ASHP will run most of the time and the reduced solar production would all go towards powering the heat pump.  

I already have a PV to immersion diverter, however with an ASHP this isn't really worth it. the ASHP will be much more efficient.

On the battery the driver is the cheap rates at night Octopus Go is 5p for a 3 hour period. With a COP at 2.8 this is 1.8p per kilowatt of heating. Being able to store this proves a massive advantage in terms of cost vs 10-14p unit rates on a normal tariff. At 14p I've estimated the heating might cost me roughly £1,500 per year, at 5p its going to be just less than £600. The trick will be eeaking this out in the winter (Dec, Jan and Feb) where the demand is highest.

I’ve seen a few YouTube videos about the Midea - looks a good bit of kit. 
I hadn’t thought about batteries feeding the ASHP. I was discussing batteries elsewhere on this site and thinking they weren’t for me. But as I install my ASHP..... But I’m thinking you will need massive batteries to drive the ASHP through the cold day time, and fill them at night. But it is definitely something to investigate. But I’d like to see the maths. And wonder how I will fund 😊

Posted by: @batalto

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As an FYI we used 36,000kwh of gas last year for heating and hot water - taken from gas use in ft3 converted.

We are due to get a 12kw Midea system to replace the current 32kw boiler.

The rule of thumb I’ve seen has been total yearly kWh/2000 gives the peak power in kW.  It works for us- but would give 36000/2000=18kW.  Are you sure 12kW is enough?

@robl I have a Daikin EDL series 8kW/h monobloc for heating only (DHW handled by a Sunamp Thermino ePV210) and my short exposure of winter weather from mid-Feb. onwards this year leads me to think our daily need will be for 25-45 kW/h for milder to cold conditions. Using Octopus Agile, I’ll be looking for most of this energy at the cheapest hours and decanting it into 27 kW/h of battery storage. Two periods pf charging a day might make this feasible. I realise that oodles of Tesla expenditure is not in itself an economic proposition but, I have other reasons to want the security of power when we need it as we have experienced long power outages when we could not heat or eat! Regards, Toodles.

If I may add to my previous posting: I am ‘sticking’ with Agile at the moment as I think the average pence per kW/h works out lower than the fixed low cost periods offered by the other tariffs in the Octopus ‘Stable’ (I like mixed metaphors!) as I am a nerd who in his retirement has time to peruse the 48 daily rates to select the cheapest and thus greenest. Regards, Toodles.