Mid-merit power plant

On a hot summer day, the demand for electricity can vary considerably from day to night. It also exhibits weekly patterns, with diminished demands on weekends, as well as seasonally, with some utilities seeing their highest annual loads on hot summer days and others on cold winter mornings.
These fluctuations in demand suggest that during the peak demand, most of a utility’s power plants will be operating, while in the valleys, many are likely to be idle or shut off entirely. In other words, many power plants don’t operate with a schedule anything like full output all of the time. It has also been mentioned that some power plants, especially large coal-fired plants as well as hydroelectric plants, are expensive to build but relatively cheap to operate, so they should be run more or less continuously as baseload plants.
Others, such as simple-cycle gas turbines, are relatively inexpensive to build but expensive to operate. They are most appropriately used as peaking power plants, turned on only during periods of highest demand.
Other plants have characteristics that are somewhere in between; these intermediate load plants are often run for most of the daytime and then cycled as necessary to follow the evening load. The following figure suggests these designations of baseload, intermediate, and peaking power plants applied to a weeklong demand curve.