The generating capability that is “standing by” ready for service in the event that something happens on the power system.
Reserve
Reserves represent the generating capability that is “standing by” ready for service in the event that something happens on the power system, such as the loss of a large generator. (Reference NERC Performance Standard BAL-002-1, Disturbance Control Performance, and PJM Manual 12, Attachment D). The severity of the event determines how quickly the reserves have to be picked up. Exhibit 6 illustrates how PJM classifies the different types of reserve services.