Solar storage - How can I work out what sized batteries I need?

Hi all, my installation begins tomorrow in time for the end of the RHI payments. I have opted for 12.5kW of solar PV, 2 Tesla batteries and a Vaillant Arotherm plus ASHP. I currently use 40 kWh/day, which is expected to rise to 60kWh/day when the ASHP becomes active. My calculations are based upon the following - I am on a variable tariff of 28p/kWh due to an inoperable smart meter (that’s another story for another thread!). I currently have an oil boiler which has cost me £1,850 for 2600L over the past 12 months for CH and hot water. However oil costs are currently £1,500/1000L so if that remains for the foreseeable future my projected oil costs will be £3,900/year. Based on my current 40kWh/day at 28p/kWh and the oil costs my annual energy bill would be £4,088 + £3,900 = £7,988. Switching to the ASHP and buying from the grid at an average of 60kWh/day would result in an energy cost of £6,132/year, so simply moving to the ASHP will save me £1,856/year (provided it does what it says on tin and electricity costs and oil costs remain as they are now). The 12.5kW of solar PV is projected to produce 10,299kWh/year. Now I appreciate that this is not linear over the year and this is where it begins to become guess work until my 1st year of experimentation has been completed. However, my purchase from the grid will be reduced whatever happens and even if this results in me being able to use 80% of what I generate due to the 2 Tesla batteries then my energy costs will be 60kWh/day x 365 days = 21,900kWh/year - 8,239kWh/year (utilised 80% of generated solar PV) = 13,661 x 28p = £3,825. So on current prices my new system will save me £4,163/year over my current system. The entire new system is costing me £38,000 with a RHI payment of £11,600, so an actual outlay of £26,400, which will take me approx 6.5 years to repay, then I’m saving £4k/year in energy costs. In the future I may install further solar PV and another Tesla battery if that takes up some of the £3,825 from the grid, or alternatively a wind turbine to produce more of the winter requirement and level the annual curve. I will let you know how I get on but my entire system should be installed and active within the next 2 weeks (the solar PV install is now delayed until after 1/4/22 so i get 0% VAT after today's budget!).