Comment by Animats

Full startup and shutdown of a coal plant can take days, but throttling down to about half power during low-load periods is normal. Power plants do this every day. Late at night, load decreases and plants throttle down. Every morning, between 0600 and 0900 local time, there's "morning load take-up" as load increases.

The big issue is what's called "ramp rate" - how fast can output power be increased or decreased? This tends to be around 1% to 3% of capacity per minute. See Figure B-2 in [1], which is a detailed discussion of how to improve your coal-fired power plant's ramp rate.

Natural gas plants have much higher ramp rates, and for batteries, it's under a second. Nuclear plants can be ramped up and down, slowly, but since it's all capital cost, cutting output raises the price of the power.

[1] https://www.epri.com/research/products/3002012979