[Retric]

Solar is the economic equivalent of base load power in that it’s cheap, and doesn’t follow the demand curve. Batteries then fill the role of peaking power plants as long as they can be filled cheaply. How you get from there to a balanced grid isn’t to have exactly as much generation on average as you need. Instead you install about twice as much as you need on average because it’s just that cheap vs any other source that even half of all generated solar is wasted it’s still cheaper than any other alternative.

At that point you are still going to get multiple day stretches where wind and solar only cover ~1/2 of daily demand but hydro can make up the difference on such occasions even if it’s only supplying 6.6% of annual US demand. Basically you get 1-2% hydro on most days and on 5% of days you a lot of energy stored.

As to high costs, because of the excess solar you’re generally filling batteries with nearly free electricity. Average nighttime wholesale prices therefore end up at ~10c/kWh or whatever the battery storage costs settle on, but daytime rates when most demand actually takes place are going to tank. That’s a net reduction in average prices. Trying to make a grid from Nuclear + batteries on the other hand means your paying Nuclear prices at night, but nuclear + battery prices in the daytime which is the opposite of what you want. Nuclear + fossil fuels on the other hand simply doesn’t go far enough.

Now in a mostly solar world a very low percentage of electricity may end up generated by fossil fuels, but a 99.X% solution is success by any reasonable metric.

PS: As a sanity check you can look at what people are paying when their off grid and then realize that’s very much a worst case.