Would you happen to know the expected payoff for a standalone battery system, by chance? I don't doubt that car batteries would probably be more expensive, but they may perhaps be easier to get people to purchase.

On a related note, Tesla is also investing in constructing utility scale battery banks for solar PV installations to help with demand side management (the same is being applied for wind power within the UK). You may be interested in this short article describing such a system in Australia: https://www.bbc.com/news/world-australia-42190358