Solar battery backup on ADU while connected to main house

[Justin F.]

Hello!

Hoping to get some advice on powering a detached 860 sq ft 2bd/2ba ADU (accessory dwelling unit) in California.

My primary house has a 100A electrical panel supplied by underground powerline. When I installed solar on the primary house (with no battery backup), I had to replace my existing electrical panels (main and sub-panel) because they were outdated. I wanted to upgrade the electrical panel to 200A but PG&E said I had to trench to upgrade the power lines to handle the additional power. They also said I had to move the existing electrical panel 36 inches away from the gas meter if I wanted to upgrade to 200A or higher. Some contractors have mentioned costs of ~$20K to trench. I was also told it'd be pretty expensive to relocate the main electrical panel because they would have to run new lines.

That being said, I called Alameda county and asked if it was a requirement for me to upgrade my 100A electrical panel if I were to build a detached ADU. They said there wasn't a requirement but the only requirement was the ADU had to be connected to the main house. I believe the other option if for the ADU to have it's own power but that would actually involve more trenching since the ADU is in the backyard so that's also cost prohibitive.

I doubt a 100A panel will be sufficient to power the main house and an ADU. ADU design/build firms and ADU contractors all mention I need to upgrade the main electrical panel to 200A to power the ADU.

My question is...rather than spending $20K on upgrading the main electrical panel to 200A, can I spend that money to install solar panels on the ADU (which I'm required to for detached/non-manufactured home) with battery backup? Essentially making the ADU self-sufficient but still connected to the main house. The ADU would run off the solar batteries and if necessary pull electricity from the main house.

Is this a viable solution?  If so, I'm surprised none of the contractors have mentioned this.

Thanks in advance!