Find an electrician who will inspect and sign off on your work. It's not illegal as licensed master electricians have mechanics do all the grunt work who are unlicensed. The electrician vouches for his employees work which legitimizes it. No different than you doing the work and having the electrician inspect, call out issues to fix, and inspect again until no issues are found and sign off.

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?