I agree local electrical storage is pointless, unless use for security reasons, hence why i would prefer to use a battery of an electric car for energy storage, in case of emergency purpose… or as I do now a very small storage for essential operations… 200ah 12v ( runs my water pumps for domestic water and circulates water through thermal storage solar systems …)
but you forgot to mention absorption rate for lead acid is only 80% so your saving is mute as it negligible – and why I use thermal storage over electrical storage as is it ~100% efficient in those respects and generally 60% of house energy usage is heat related anyways-- so instead of batteries put in a second hotwater tank in series and cascade dump your excess into there . it will give extra hotwater when needed and reduce your over all heating needs at a much lower cost then batteries - especial so in a cool climate like the UK where average summer temps are quite cool…