It depends on the type of plant.
There's a concept in power generation of "base load", https://en.wikipedia.org/wiki/Base_load , which is the minimum usage. Some plants are expensive and slow to start up and shut down. If they're cheaper per kwH, then it makes sense to use them, but only for base load.
Then there's https://en.wikipedia.org/wiki/Load_following_power_plant , which are halfway compromises. They spin up and shut down perhaps twice a day. If load starts falling (or grows more slowly than predicted), these'll definitely get spun up less often. If load rises (faster than predicted), more of these will be used while plans are made to bring up more base load power plants.
Then there're peaker plants, https://en.wikipedia.org/wiki/Peaking_power_plant . These are expensive but responsive. They spin up and shut down within seconds of a change in demand.
This is all controlled by auctions, at various timescales.
A example that might be popular among the Musk fanboys of HN (e.g. me) would be https://www.theguardian.com/technology/2018/feb/23/tesla-bat...