Coal plants can't dynamically ramp up or down to partial capacity but coal plants can be activated and deactivated according to demand - there is, essentially,

Coal plants can't dynamically ramp up or down to partial capacity but coal plants can be activated and deactivated according to demand - there is, essentially, a priority list of plants in the US dictating the order in which to turn them on in times when demand is exceeded - there are also some bi-modal plants out there (some hydro plants on lakes which can pump water into the lake during low demand times and run their turbines when demand is high). Coal plants tend to be quite low on this list due to their poor scaling and obvious environmental impact - in fact for a while (I'm not certain if this is still accurate - it's about a decade out of date) one of the last entries on that list was a rarely activated coal power plant in downtown boston that had an extremely inefficient conversion ratio and was deemed exceptionally dangerous due to being located in a population center - still, in times of extreme demand it was standing ready to be activated.