Operating reserve describes the supply resource capacity that is available to the system operator within a short interval of time to maintain the balance of supply and demand on the bulk electric system. Definitions of operating reserve vary from region to region, but operating reserve always includes some form of spinning and non-spinning reserve. Some regions also include regulating reserve. And some also have other defined reserve services such as flexible ramp capability, supplemental reserve, or replacement reserve. The services that provide operating reserve are a part of the broader category of ancillary services, although not all ancillary services provide operating reserve.
Operating reserve is used by the system operator when supply is short relative to demand due to load forecasting error, when renewable output is different from forecast, and/or during power plant or transmission outages.