It is not always compulsory that all the units of a plant are available to share a load. Due to maintenance some of the unit may be stop and it is also not necessary that the less efficient units are switched off during off peak hours. There is a certain amount of shut down and start up costs associated with shutting down a unit during the off peak hours and servicing it back on-line during the peak hours. To complicate the problem further, it may take about eight hours or more to restore the boiler of a unit and synchronizing the unit with the bus.To meet the sudden change in the power demand, it may therefore be necessary to keep more units than it necessary to meet the load demand during that time.This safety margin in generation is called spinning reserve. The optimal load dispatch problem must then incorporate this startup and shut down cost for without endangering the system security