There’s an Elephant in the room:
The sun doesn’t shine at night.
Solar power is not a complete solution.
As long as renewables are a fraction of the total power, it’s no big deal. Hitting 60% is fine, but remember that you have to have power plants you can shut off when that happens. The ability to turn on and shut down is called “dispatchable” Nuclear power is the least dispatchable. It takes a long time to ramp a power plant up, and back down. Coal isn’t much better, taking about 48 hours to go from cold to full power. But you can run it at a fraction of the power keeping everything hot. Then it doesn’t take nearly as long. Natural gas is probably next.
Now California, being in a sunny warm climate, the demand for electricity tends to go up during the daylight to run airconditioners, and down at night. So it would be reasonable for California to plan on covering all of it’s air conditioning power needs with solar power.
Light industrial power tends to have a daytime cycle too. Overall, power demand is lower at night. But night demand is not zero.
Wind is more complex and tends to be harder to predict. (You know that the sun is going to rise.)
And if everyone in California gets and electric vehicle, the night time demand is going to get a lot bigger.
What’s the answer?
A: More research into better batteries. For this purpose, they don’t have to store a lot of energy per pound. They have to store a lot of energy per dollar. Cheap massive batteries will work.
B: We need to figure out a way to make a high energy chemical (gasoline, hydrogen, methane, methanol) with a process that is dispatchable. That is, we can turn on the production line when we’ve got surplus energy, turn it off (or at least down to simmer) when energy is short.
This would allow over provisioning the system: You need 20 GW during the day. You install 35 GW, and use the 15 to make hydrogen. Run turbines at night with hydrogen. In essence you are storing energy as a chemical.