Basicly you have 3 kinds of power plant. Base loads large coal or nuclear plants that run full power at all times. Next are variable load plants like hydro electric that can increase and decrease their power ouput hourly as needed. Finalnly you have your peak loads plants like this. They can fire up quickly on e a a time or in groups and in a matter of seconds they can provide power to keep the grid stable.
Still a cool picture.....
Doug