This brings up a very important point. Electricity supply must be carefully balanced with demand and so that's why most regions have ISO (independent system operators) to facilitate the coordination needed to maintain system balance.

Typically coal and nuclear power plants are "base" load stations, providing always-on capacity. It's not always possible to vary their output to match demand, so typically (natural) gas power plants are used to match the remaining variable load. The start up/shut down cycle for these plants is far less costly.