Spinning Reserve

Reliability is also of great concern to the system dispatcher, who must plan for the possibility that one or more generators may fail to produce power at any time. This is done by ensuring that other generators are available on short notice. But, most generating units require a 12 to 18 hour warm-up period. Large utility systems operate several units partially loaded at all times, so they can be brought up to full load quickly if and when they are needed.

This rolling reserve or spinning reserve is the remaining capacity that the generator could produce should the need for power arise. It is common to have a 15 to 20 percent spinning reserve to provide reliable service in case some generating units drop off line, break down, or customer demand suddenly surges.