Value chain · CLM-1470
In plain terms
Britain's capacity market does not count gas or nuclear power stations at their full size. It counts a gas plant at about 88 to 92 per cent of its capacity, and a nuclear plant at about 75 to 85 per cent. Government advisers say the nuclear figure now rests on too few plants to be reliable.
The claim as recorded
In Great Britain's Capacity Market, NESO's de-rating factors for combined cycle gas turbines range from 0.876 to 0.9159 across the auctions for delivery years 2016-17 to 2030-31, and for nuclear plants from 0.7544 to 0.8524; in the latest auction listed, the 2030-31 four-year-ahead auction, they are 0.9048 for gas and 0.7566 for nuclear. One minus the factor is 8 to 12 per cent for gas and 15 to 25 per cent for nuclear (our arithmetic). The Panel of Technical Experts says the nuclear factor now rests on data from only five operating plants, most near the end of their lives, which it judges too few to be reliable.
Evidence behind this: T3: system operator data (SRC-790) and an independent expert panel report to government (SRC-792).
Scope and limits
A de-rating factor is not an outage rate. No held document defines it or gives the method: the panel says it is set by the Capacity Market Rules, which are not held. Reading one minus the factor as the share of capacity the market does not count on is ours. Whether the factor reflects breakdowns alone, or also planned outages and behaviour at times of system stress, is not shown here, so it gives no planned and unplanned split. The factors are for future delivery years set at each auction, not measured availability in those years. Great Britain only. The source URL of the CSV was not recorded when it was fetched.
What it assumes
No assumptions are recorded against this claim.
Where it comes from
, CCGT rows: lowest 0.876 (2017-18 T-1), highest 0.9159 (2024-25 T-1 and 2027-28 T-4), 2030-31 T-4 0.9048. Nuclear rows: lowest 0.7544 (2025-26 T-1 and 2028-29 T-4), highest 0.8524 (2018-19 T-1 and 2021-22 T-4), 2030-31 T-4 0.7566. One minus the factor: gas 8.4 to 12.4 per cent, nuclear 14.8 to 24.6 per cent. SRC-792 PDF p.20, paragraph 58, read at a zoom 3 crop: "The methodology for conventional plant de-rating factors is prescribed in the Capacity Market Rules." Paragraph 60: "There are now only 5 operational nuclear power plants, most nearing the end of their lives, and this is making the data set for establishing the de-rating factors too small to be sufficiently reliable." PDF p.21: Recommendation 101, that NESO and DESNZ reconsider the nuclear de-rating method (paraphrase).
Basis: dimensionless de-rating factor per Capacity Market auction, delivery years 2016-17 to 2030-31; percentages are one minus the factor (our arithmetic).
Related records
- Answers question
- QST-021 not published in this release
Cite this
energyaperture.com/c/clm-1470
CLM-1470, Energy Aperture, as of 2026-10.
Connected records
Other published records linked to this one. A list, not a judgement.
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