How much of a dent, percentage-wise?
All new construction. It really could make a difference, especially when you consider how many panels are actually required. When the AC is most needed is when the cells will also be producing the most electricity. Tens of thousands of homes producing enough (or almost enough) of their AC needs would matter.
Of course it’s still a dumb idea. It will help California the same way a Pay Day Loan helps the lendee. It can get you out of a tight jam, but you will pay dearly for it. And it won’t fix the systemic problem.