Capacity credit

Capacity credit (CC, also capacity value or de-rating factor) is the fraction of the installed capacity of a power plant which can be relied upon at a given time (typically during system stress), frequently expressed as a percentage of the nameplate capacity. A conventional (dispatchable) power plant can typically provide the electricity at full power as long as it has a sufficient amount of fuel and is operational, therefore the capacity credit of such a plant is close to 100%; it is exactly 100% for some definitions of the capacity credit (see below). The output of a variable renewable energy (VRE) plant depends on the state of an uncontrolled natural resource (usually the sun or wind), therefore a mechanically and electrically sound VRE plant might not be able to generate at the rated capacity (neither at the nameplate, nor at the capacity factor level) when needed, so its CC is much lower than 100%. The capacity credit is useful for a rough estimate of the firm power a system with weather-dependent generation can reliably provide. For example, with a low, but realistic (cf. Ensslin et al.) wind power capacity credit of 5%, 20 gigawatts (GW) worth of wind power needs to be added to the system in order to permanently retire a 1 GW fossil fuel plant while keeping the electrical grid reliability at the same level.

Definitions

There are a few similar definitions of the capacity credit:

effective load carrying capability (ELCC) defines the capacity value as the extra load that can be added to the system once the plant is added without degrading a chosen reliability index (usually the loss of load probability). Unlike the dimensionless CC, ELCC is expressed in power units (megawatts). California regulators, in their resource adequacy calculations, use different term, qualifying capacity (QC). For a dispatchable plant, QC is self-assessed and might go as high as the maximum power of the unit.