If it's not in the policy "we don't cover homemade lithium projects", your covered. Very few things a home policy won't cover like nuclear war, terrorist acts for example. If your operating as a home business building these then that could be an issue, otherwise you can be an idiot and build whatever you want and your covered.

We don't cover !@#$ing with the utilities in ways that are not standard is probably in there somewhere..

The tie-in to the utility can easily be professional or professionally verified. That's quite separate from the battery bank itself.