Unavailability is when an electricity generating plant is temporarily unable to perform at its full capacity.
The amount of unavailability is deducted from the plant’s total nominal generating capacity. Nominal capacity is the theoretically highest amount of energy that a power station or turbine can provide on a lasting basis under ideal conditions.

Unscheduled unavailability

Power stations can break down at short notice for various (usually  technical) reasons. This is referred to as unscheduled unavailability. Regardless of the type of power station and the energy source, individual system components can also fail without necessarily resulting in complete unavailability.

For example, if there is a defect in an individual condenser (in which the steam is condensed from the turbine) in a steam power station, the turbine affected usually breaks down. For a power plant with only one turbine the result will then be complete unavailability. At a power station with several turbines, the unaffected turbines will be able to continue generating electricity and the unavailability will only be partial.

Technical disruptions account for the majority of unscheduled unavailability but external factors can also trigger unavailability. For example, run-of-river power plants cannot be operated at their full capacity when water levels are very low or very high.  Power plant operation can also be disrupted by extreme weather conditions and, depending on the circumstances, result in either unscheduled or scheduled unavailability.

Unscheduled unavailability usually occurs at short notice and unpredictably.

Scheduled unavailability

Most system components are regularly serviced without having to shut down the entire facility. This can result in limited but scheduled unavailability.

Even with a temporary, scheduled complete shutdown, power stations are usually not shut down immediately but rather over a certain period of time. The unavailability in such cases is initially low and then increases over time.
The same applies the other way around when a power station is powered up. At coal power stations the process takes around one day.

Overhauls are scheduled inspection processes that are planned long-term and are more extensive than regular maintenance work. For an overhaul an entire power generation unit or turbine is usually shut down, inspected and serviced. This process may take several weeks.

In summer, restrictions on the amount of warmth inflow into rivers and streams can also result in a power plant being shut down or not being allowed to run at full capacity since only smaller amounts of cooling water may be discharged.

Unlike with unscheduled unavailability, operators are aware of scheduled unavailability in advance. For relatively large power stations, scheduled unavailability is announced ahead of time on certain transparency sites. This makes it possible for other market participants to prepare for the unavailability.