I'm trying to do a battery backup setup. I've contacted the county for compliance requirements. I'd do it without permits and possibly without UL certified equipment. My main hesitation is due to insurance denying a claim because of no permit or UL certification.
I am in Florida and installed off-grid inverters and server rack batteries, all permitted. The process was super easy and the permitting fees were nothing compared to the cost of the system. For more details, keep reading.
I did the work myself with help from a friend. This falls under the Florida owner-builder exemption. You just have to sign a waiver saying you are aware of a bunch of things but still want to do it yourself. Also you are not allowed to pay anyone for help, but family or friends can help as long as you are supervising the work.
I called the city and they told me I needed a building permit and an electrical sub-permit. A survey map marking the location of the equipment was required. Other than that, I described the project as installing a battery based *manual* backup generator, drew laughable connection diagram, printed the UL certifications and manuals for inverters and batteries, Finally printed 2 copies and delivered them in person. They filled a notice of commencement and i needed to go to the county to get it stamped and bring it back once the permit was approved. i paid $13 to the county, including $1 for an extra copy of the NoC. To my surprise, the permit was approved the following morning. I brought the stamped NoC and paid $58 or $68 and got my permit. You have 6 months to complete the project.
About a month later we were done: new subpanel with most of the house circuits, a manual interlock for choosing between grid and off-grid power, and then the inverters and batteries.
I set up the inspection online and the inspector came the following day. He asked a bunch of questions, seemed to care a lot about grounding, wire sizes, and how the system would be turned on/off. If the system kicked in automatically, I would have needed an external disconnect, but since it was manual... Also, for the new panel, i had to use AFCI breakers for all 15A/20A circuits that go to any outlet inside the house. Got the approval signed on the spot!
Overall, permitting was a frictionless, straightforward experience, and the few people i dealt with were super helpful. All these rules are there to protect me and my house, and the people involved made it easy to comply with the rules.
As for going completely off-grid now, no one can prevent me from stopping DEF service and opening the main breaker in the entrance panel...
YMMV.