You can probably plug your own numbers in to what I have done. I have a 12 kW GSHP .

[edit ops. Slipped a digit. Figures corrected]

For my house when it is cold like for instance now it's around -1.7ºC. This is sometimes for large periods of time. My GSHP COP is at 3.5 with an input of 1.4Kw and output of 5.33kW. That's 127kWh for the day. The radiators are running at 42.4 out and 35.4 return for an average of 38.9. So to get the same out put you need your flow to be at least 42.4 to get the same results.

So without tank losses you are looking at 127000/1.16/17 hrs=7470 litres to able to drive the radiators. If you store at double the temperature ie 49 degrees then you will need approx 3235 litres of storage. However, the COP plummets as you go nearer 50 degrees. Looking at my DHW tank the COP is 2.46 at 49 top/45 bottom degrees. The temperature in reality will need to be a lot higher to counter mixing as you circulate the water at some speed. From what I can find it is really hard to counter mixing.

Now, assuming your non peak times are 7 hours then you will need 127000/2.7 (an average cop) 47kWh input or 6.7kW from the grid to your HP to charge the tank let alone service your heating. Single phase wouldn't be able to recharge in the time required.

A 7800 litre tank is not cheap and will need the fittings for a very large coil.

My battery of 38kWh (about the same foot print as 280liter tank) will do the 17hrs* 1.4kW and 23.8kWh and is easily charged in the 7hours. I am looking to increase it soon. There is then another 8 usable kWh for the house. Any sunshine also goes directly to the house/heat pump before charging the batteries. I went the battery route which also gave me power cut resilience and more savings throughout the year. I'd recommend the home brew battery thread which gets the batteries cheap which can then be coupled with a Victron Multiplus/Quattro or similar. It's also got the ability to service the house all year which saves significant sums of money.

https://camelot-forum.co.uk/phpBB3/view ... &start=830