De-rating
Also known as: Performance Limitation
The reduction applied to a BESS plant's capacity credit in a capacity market, based on its discharge duration. A 1-hour plant is de-rated more than a 4-hour plant because it can sustain output for a shorter period. De-rating factors are set by the system operator.
In energy storage
De-rating directly determines capacity market revenue: only the de-rated MW, not nameplate MW, earns the clearing price. In the UK Capacity Market, de-rating factors are calculated as Equivalent Firm Capacity (EFC) multiplied by technical availability — a 1-hour battery receives roughly 10–14% and a 2-hour system roughly 21–27%.
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