# HOMER Grid 1.1

The required operating reserve is the minimum amount of operating reserve that the system must be capable of providing. HOMER calculates the required operating reserve for each time step based on the values that you enter on the Constraints page. Whenever possible, HOMER ensures that enough dispatchable capacity is available to keep the operating reserve equal to or greater than the required operating reserve. HOMER records any shortfall as a capacity shortage.

Because operating reserve guards against increases in the load or decreases in the renewable power output, the required operating reserve is a function of both the load and the renewable power output (specifically, the solar and wind power output, because the hydro power output typically experiences little short-term variability). The amount of required operating reserve, therefore, typically changes from one time step to the next. In each time step, HOMER calculates the required operating reserve on the AC and DC buses using the following equations:

 where: Lres,AC = the required operating reserve on the AC bus Lres,DC = the required operating reserve on the DC bus rload = the Operating reserve as a percent of load in the current time step variable Lprim,AC = the average AC primary load in the current time step Lprim,DC = the average DC primary load in the current time step rpeakload = the Operating reserve as a percent of annual peak load variable = the highest AC primary load experienced by the system during the year = the highest DC primary load experienced by the system during the year rwind = the Operating reserve as a percent of wind power output variable Pwind,AC = the average AC wind power output in the current time step Pwind,DC = the average DC wind power output in the current time step rsolar = the Operating reserve as a percent of solar power output variable PPV = the average PV array output in the current time step