@julianc and all other replies. Have read your posts and really chuffed I got your feed in so thank you. I was aware there were nieve assumptions your expertise would fill in.

Answering logically off grid only really replies to summer, based on electrical 10kwh consumption and solar top up with 10kwh battery. 

18kwh daikin was recommended by the specialist cinergi in Hampshire, but does seem overly large as they also recommended the 12kwh samsung ultra quiet unit.

@batalto cheers, there was some greatly optimistic number crunching my end with a hand full or fantasy. However, just found out about the octopus cosy tariff. Theoretically, if I can run ashp on the low peak times and top up the battery then that would mean the 9.6kw battery would 6 hour gaps between charging. I'm thinking that's realistic.

@derek-m just waiting on heat calcs. Will post as soon as I have them.

@robl really appreciate your reply and its you are very similar to my situation with the exception I need the ashp. My consumption was roughly 10kwh elec per day rising to 15 in winter. Usual 30kwh gas for heating in winter months pre build. Therefore I'm assuming my figures will be broadly similar to yours.

I suspect the 18kwh daikin is a oversized but will see from the calcs. The spec of givenergy batteries is supposed to be modular and easier to add to. I've realised octopus cosy is going to be the best bet. Would allow me to stick to 20p kwh using the off peak periods through the winter.

Are you tempted to add a battery to your set up? How do the controls work? Is it set up on a priority basis of solar iboost and then export? 

V2g on car is very interesting. Essentially the set up I have outlined is in essence a sticking plaster until I get a v2h compatible car. That would then add 70kw battery to the mix. 

@julianc when your heatpump is heating your water does the mixergy heat the tank using the same stratification method it does when being heated by electric?

I have read somewhere that it feeds into the bottom like a standard cylinder when being fed by the heat pump...? 

I just wonder whether the cost benefit of a mixergy is only at it's best when you are already running off electric, or have an gas or other inefficient boiler setup.

@squeakysim morning. When the HP is heating the h/w, it does so via the plate exchange, which as you suggest, is at the bottom of the cylinder. But the stratification still applies as the warmed water is “piped” to the top. And a mixer keeps moving cold water down. Seems to work well

Update:  we’ve now had our ASHP for 2 years (22nd April actually, sorry for delay)

We achieved SCOP of 3.72 this year, running the system on weather compensation. 
This is slightly down on the first year 3.80, using Nest control. 
I think the reason is a colder winter. But this is a guess. 
We are pleased with the results. This has roughly maintained parity with the costs of our previous oil boiler. Good news. Given the costs of electricity. 
My Daikin has been off since early May, with water heated by solar PV & our Solic 200 electric diverter. 
Finally we had the second annual service for the HP, through Daikin. Although it was a subcontractor this time. He did a good job. I had an issue where the pump in the hydrobox seemed to run on a long time, when the HP or the internal circulation pump were not running. He adjusted a setting. We shall see if this resolves the problem. 

Reading some of the above ....a small reminder that the batteries can only charge/discharge at a maximum rate. It may need to be a factor in the design & calcs for usage. In my case with GivEnergy Gen1 5kw hybrid the maximum is 2.6kw. 

So even though I have 3 x 8.2kw batteries I can't fill them, if they're empty, in the windows currently offered by Octopus for cheap rates. Similarly with a 16kw LG ashp in winter I will need to draw at high rates than batteries can supply - meaning I'm obliged to import from grid.

I've decided it's much easier to be on Octopus Tracker for imports (in recent times circa 17p) and taken the standard fixed for exports (currently 15p, 1 year fix).  Chances are very high the standard export tariff will be cut in the not distant future - business model can't work if import Tracker is so close to export rate? Wholesale cost of elec within the Tracker is under 8p. 

Standard Fixed export was increased last Sept from 7.5p to 15p when wholesale rates were through the roof. I would expect a cut back to somewhere between 7.5p to 10p. And changes to Flux likely too IMHO.

@tim441 high Tim. Sorry for slow response. I don’t have separate batteries. I’m waiting for Vehicle to Grid or Home as I have 100kWh of batteries across 2 EVs in my garage. Just need a CCS V2G charger 👍

Had my ASHP switched off since May. Hot water heated by solar PV and electric diverter (Solic 200). H/w heated for free. 
Switched ASHP on yesterday, 12th October for  heating. 
Will be interesting to track the SCOP this year

Just a quick thank you Julian and your thread. Based on your shared experiance I now have a 12 kw Daikin Altherma 3 hht. I coupled the system with 14 REC 420 solar panels, mixergy 200 litre tank, giv energy 5 kw inverter and 19.5 kw giv energy batteries. I've now signed up for Octopus cosy ready for the winter. I've done some trial runs on heating but not yet turned on as I have a EPC rated A house so will need to drop nearer 0 before I will need to run heat pump in earnest. 

@p-c-brown Great news PCB!  I hope it all works out. Good choices 😊. 
Interested to hear how you get on / set up etc. Best wishes