They can't though -- the next day could always be cloudy and there's a very strong chance that one day in 90 will be.

So solar panels can't stock a reliable X day supply, even if over any (moderate) period of time, they have a very guaranteed supply.

They can with batteries, but that is a slightly different situation.

In answer to the question below, this isn't practical with modern battery technology.

Is there a battery technology that can store 90 days worth of full-plant production (which doesn't require a second generator consuming a fuel later)?

All the ways I know of on that scale actually require a second plant/turbine -- in which case, you didn't store 90 days of solar production capacity, you actually stored 90 days of the second plant's fuel, which you produced intermitently with solar capacity. (Which can still be a very good and environmentally friendly thing to do! But isn't what we're talking about as a technical point.)

Ways of the second variety:

- Hydro pumping

- Hydrocarbon synthesis

- Hydrogen separation

- Compressing gasses

And there's probably more that I'm forgetting.