In California at least, solar power is peaking power, since hot summer afternoons are both when power demand and solar generation are at their maxima.
Yes, solar is
usually a good match for A/C on hot days, but if the sun gets blocked by clouds on a hot day, even for a few seconds/minutes, they need to spin up the NG turbines to make up the difference. You don't want a brown-out on a hot summer day. It isn't unusual in the Midwest to be very overcast/cloudy, and still be hot/muggy with lots of A/C load. But even if it happens only occasionally in a certain area, you still need to be prepared for those times.
But I think your point is that when solar becomes a really big component of the system, it will require either batteries or alternate base load capacity to carry the load at night.
No. We already have plenty of capacity for night (coal, hydro, nukes, and wind - but wind is intermittent too, but better at night). What I'm saying is since solar is intermittent, for every unit of solar power you put in for the daytime peaks, you need an equal amount of some sort of backup that can spin up/down fast to keep the grid up when the sun goes behind a cloud.
That
could be done with batteries, but AFAIK, NG turbines are far more cost effective, so I don't expect that to be the first choice for the majority of peaking power.
We don't need to store solar power at all, until we are actually routinely
overproducing during the day. Even then, if storage costs more than running the alternatives, the economical thing to do is let that power go to waste. But that is a hypothetical - as I said, we are so far from routinely overproducing solar power, it isn't an issue. If we get there in 20 years, there will likely be options that are not available now.
-ERD50