1) It's not enough power. Yet. So you'll need to draw from the grid.
2) Sometimes it's cloudy and you need to draw from the grid.
Given you need to still draw from the grid, but aren't paying as much... what does that do to the quality or maintenance cost per use for the grid? Do people who aren't on solar gets charged more per kW/h? A move to push them to get solar? Do people with solar get hit with a "grid maintenance" fee for not paying their fair share of grid maintenance (previously built in to their electric bill)? Do we raise taxes? Do we let infrastructure crumble?
I think solar is great, no question. But better if capacity is built through solar plants, that use the existing grid, rather than mandated as part of individual home construction.
Plus... and this is the part that bugs me... does this mean we can't build houses in the shade? Will we have to tear down old trees, or build taller houses than our neighbors have, in order to meet some quota for energy production... what if I want to have a roof-top patio? What if a new technology comes along... let's call it "Solar 2"... and it's better. Are we forbidden to use it until the laws are changed? What if it's just better in my opinion for my life... can I still use it, or am I locked in to whatever standards the State of California deems is best for everyone?
Yeah this law is going to suck.