Doesn’t have to be a grid tied system.
Wire it the same way a UPS back up system, automatic shut off of mains circuit with backup kicking in. Mains circuit stays off until manual reconnect.
Surely data centers and others have such in place, so maybe Duke has objection to the feed back into the grid part only.
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.