god, you're thick.
what is the cost, including installation, of (let's say) a 6kWh store (A)
what is the cost, including installation, of (let's say) a 4kW solar array (B)
Given that, in the UK, there is no significant generation from a domestic solar array for the 6 months of the year when energy need is highest:
How many days of a year will a household be able to store 6kWh of electricity? (C)
What is the cost of electricity per kWh? (D)
Let's suppose that the interest rate on a loan is 8%
and that the opportunity cost of money not invested elsewhere is 8%
and that the installation has a useful working life of 20 years, after which it is worthless scrap
Can your scheme generate a return of over 8% p.a., ON TOP OF returning the original cost (A) + (B), using a DCF rate of 8%, over a 20-year period?
No it can't.