Operating reserve

In electricity networks, the operating reserve is the generating capacity available to the system operator within a short interval of time to meet demand in case a generator goes down or there is another disruption to the supply. Most power systems are designed so that, under normal conditions, the operating reserve is always at least the capacity of the largest supplier plus a fraction of the peak load.
Types of operating reserve.
The operating reserve is made up of the spinning reserve as well as the non-spinning or supplemental reserve:
Generators that intend to provide either spinning and non-spinning reserve should be able to reach their promised capacity within roughly ten minutes. Most power system guidelines require a significant fraction of their operating reserve to come from spinning reserve. This is because the spinning reserve is slightly more reliable (it doesn't suffer from start-up issues) and can respond immediately whereas with non-spinning reserve generators there is a delay as the generator starts-up offline.
Centrally controlled air conditioners and thermostats that are used in large residential areas can be used as a fast and considerable operating reserve. Advantages of this technology are under studies.
Operating reserve is a crucial concept for ensuring that the day-ahead planning of generators' schedule can withstand the uncertainty due to unforeseen variations in the load profile or equipment (generators, transformers, transmission links) faults.
The California Independent System Operator has an operating reserve at 6% of the metered load. Included in that is a spinning reserve at 3% of the metered load.
Other types of reserve.
In addition, there are two other kinds of reserve power that are often discussed in combination with the operating reserve: the frequency-response reserve and the replacement reserve.