Capacity Margin
How much more electricity the Great Britain grid can supply than it expects to need at the coldest winter peak. De-rated means each source is discounted for the chance it is unavailable, so this is a cautious figure. A wider margin means less risk of running short. It is a forecast made before each winter, not an outturn.
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Source: National Energy System Operator (NESO) · Department for Energy Security and Net Zero
Coverage: Great Britain