The solar ramp-down is faster than the AC ramp-down. If you have a dumb thermostat, it runs the AC a fraction of the time which is a function of the outside tem

The solar ramp-down is faster than the AC ramp-down. If you have a dumb thermostat, it runs the AC a fraction of the time which is a function of the outside temperature. However, the temperature stays elevated some hours after sunset, but the solar supply drops off quickly some hours before sunset. Solar supply is pretty well matched to AC demand except for a big mismatch a few hours around sunset. Consider California. Currently, this mismatch is taken up by fossil fuel plants and imports, but the solar demand drops off so fast that they have real difficulty in ramping up as needed. Batteries are a major part of the solution, but another part is improved transmission of wind from states to the east. This will cost less than you might think. California might use 800GWh daily; and yes, installing 800GWh of battery capacity would be prohibitively expensive. However, the wind picks up in the evening just a few hours after the sun goes down, so the batteries really just need to shift some excess power from midday to the evening. When I last calculated this, the additional transmission + batteries would cost around $40 billion. Interestingly, smart thermostats can make this worse: devices with a home/away mode allow the house to warm up when no one is inside because the owner doesn't want to pay for cooling that they won't enjoy. However, as all of these owners return home after work and school, this creates a demand spike; also, the AC units on these houses, which have been allowed to warm up, now need to run far longer into the evening. Really, the home/away feature is a little bit of a hack: the real goal is to minimize cost with some constraints on the temperature, and the utility ought to pay owners to shift their demand forward from evening to midday. This is like getting paid to store energy temporarily. So some combination of demand shifting---either with smart-er thermostats or batteries---adding other generation sources like wind, and some additional transmission, mean that California really can get to 100%. It will not be free, but it seems like the cost is less than $100 billion. Now in Texas, the situation is somewhat reversed. There is too much wind generation, and not enough solar, but the upshot is similar.