I agree its a no brainer but in the opposite way to you.
I have a 14kW standalone battery and inverter system. I use Octopus Intelligent tariff which works out like this:-
Cost of system £4200
Peak Rate 29.2p
Off Peak 7.5p
To fully charge the battery 14 x 7.5p = £1.05
Using the battery instead of peak energy saves (29.2-7.5) x 14 = £3.03 saving
Break even point = £4200 / £3.03 = 1386 days or 3.79 years
There is a slight loss during charging and discharging but it's nothing like 20 years. The Battery Life is around 6000 cycles (1 cycle = fully charged to fully discharged) if you did one cycle every day that's 16.5 years. They're deemed to be end of life when their capacity drops to 80%, even at that level my battery will still have a capacity of 11.2kWh, I'll be gone before my batteries die