Lawyers argued that Title 24 required that residential buildings be interconnected with the grid [0] [1], but 2022 updates to the code seem to have clarified that standalone solar PV systems with storage are OK (though they should still be meet the same standards as grid-connected systems would) [2]

In any case the small number of people who can afford to do so and want to do so seem to be doing so [3]

[0] https://www.cailaw.org/media/files/IEL/Publications/2016/gri...

[1] https://klinedinstlaw.com/wp-content/uploads/2016/02/how-tit...

[2] https://www.energy.ca.gov/programs-and-topics/programs/build...

[3] https://www.reddit.com/r/solar/comments/168mlxw/possiblelega...