The operating capacity is the total amount of electrical generation capacity that is operating (and ready to produce electricity) at any one time. It is therefore the maximum amount of electrical load that the system could serve at a moment's notice.
To ensure reliable supply, the operating capacity should be greater than the electric load. The difference between the operating capacity and the electric load is the operating reserve. As it simulates the power system, HOMER attempts to keep the operating reserve equal to or greater than the required operating reserve.
In HOMER, both dispatchable power sources (generators, grid, storage bank) and renewable power sources (wind, solar, hydro) provide operating capacity. The operating capacity of a dispatchable source is equal to the maximum amount of power it could produce at a moment's notice. For example:
•A generator that is not currently operating provides no operating capacity because it cannot be counted on to provide power at a moment's notice. It must first be started, allowed to warm up, and synchronized.
•A 50 kW generator that is operating provides 50 kW of operating capacity, regardless of the actual amount of power it is producing at any time.
•The operating capacity provided by the grid is equal to the maximum grid demand.
•The operating capacity provided by the storage bank is equal to the maximum amount of power it could discharge at a particular time. It therefore depends on the storage bank's state of charge and its recent charge and discharge history. For more information, see the article on the kinetic storage model.
The operating capacity provided by a non-dispatchable renewable source (like a PV array or a wind turbine) is equal to the amount of power the source is currently producing, not the maximum amount of power it could produce. Since a renewable power source cannot be controlled like a dispatchable source can, its maximum capacity is not relevant in this context. So a wind turbine with a rated capacity of 50 kW that is only producing 13 kW provides only 13 kW of operating capacity.
HOMER keeps track operating capacity and operating reserve separately for the AC and DC buses. For more information, see operating reserve.