Sounds like a terrible waste, spending potentially tens of thousands for a battery bank just to sit there the vast majority of the time, but I guess it’s not really any worse than doing the same thing for a whole house generator. Not sure what type of lithium batteries Tesla uses, I know Lithium Iron Phosphate will hang in there quite happily unused. Others don’t.
Tesla uses their own 7071 lithium ion, just a tad bigger in dimensions but with 30 to 50% more capacity than the 18680 in common usage