Ah ok thanks for correcting me

Perhaps they don't turn off but merely down? Is that a day? Is there even such a thing as different levels of operation?

Even if not a day, maybe still not responsive enough.

Then again, maybe it gets you close enough that now you only need a smaller more doable version of some otherwise unrealistic gap filler like batteries or thermal mass or gravity mass etc.

That is not correct. Coal and gas fired power plants have turndown capabilities where they can decrease their power genration very quickly by 75%-90% or so depending on the plant.

https://www.babcockpower.com/wp-content/uploads/2018/02/exte...

Is it common for those huge old plants to use pulverized coal?

I imagine all the new ones use it, and thus all the new ones can change their outputs in a matter of minutes, instead of hours. But that still makes coal one of the slowest sources to react to grid volatility, and sill basically useless to compensate wind oscillations, only useful for the solar cycle.

I've seen some really old plants and they've all used pulverized coal, since it burns more consistently. It basically explodes when it ignites due to the temperatures involved. Coal and gas are both really good at responding quickly to varying load requirements.