It's not practical to turn on and off any steam based power plant. Coal and nuclear fuel are simply used to boil water. The steam created in both plants is the

It's not practical to turn on and off any steam based power plant. Coal and nuclear fuel are simply used to boil water. The steam created in both plants is then passed through a steam turbine, which turns a generator. The turbine is finely tuned to a specific set of steam characteristics. These are generally related to temperature and pressure. The process of getting the steam loop up to the right temperature and pressure takes a significant amount of time and energy. Once it is at steady state, it is very efficient. Natural gas turbines work differently. There is no steam. The natural gas is burnt directly in the turbine, much like a jet engine. Thus you can throttle it just like an airplane. There is some lag, but it is relatively responsive.