Surely it only needs to be signed off if you intend to sell the property with them or sell excess back to the grid. If youre just using the batteries how is anyone going to know?
If your house burns down for any reason, not necessarily the DYI batteries, the insurance company will know anyway.
shrug if you can rely on nobody noticing, or non-enforcement, sure, but it is actually a criminal offence not just an administrative requirement.
I'd assume your fire insurance covers nothing if illegally installed batteries are found inside after a house fire.