Negative prices (weekend)
Sat 6 Sep · Sun 7 Sep
Research notes
Figures are for London unless they say otherwise.
What happened
- Sat 6 Sep: a single negative half-hour at 23:30 (London −0.5p), average 12.5p.
- Sun 7 Sep: below zero for 5.5 hours, longest spell 4.5 hours from 10:00, London low of −4.4p at 12:00, average 7.0p. All 14 regions went negative on both days.
Likely causes
- Wind built through Saturday and stayed high all Sunday. Elexon-metered wind rose from 2.9GW early on Sat to 15.9GW late that evening, then averaged 12.8GW on Sun. Gas plants averaged only 3.4GW on Sun. (confirmed by source: Elexon FUELHH, 6 to 7 Sep)
- The Met Office forecast for that weekend had the west "turning increasingly windy" on Sat, and rain with "coastal gales possible around the Irish Sea" on Sun as low pressure moved in. (confirmed by source: Met Office daily weather, 6 Sep 2025)
- Sunday demand was low: total transmission supply averaged about 21.9GW, against 25 to 27GW on nearby weekdays, and GB was exporting up to 5.4GW over the interconnectors at times. (confirmed by source: same Elexon query)
- Solar added to the midday surplus on Sun (peak about 6.2GW), which fits the negative spell running 10:00 to 14:30. Sat was sunnier (10.2GW peak) but wind was still low at midday, so prices stayed positive until late evening. (figures confirmed by source: PVLive; the link to the price shape is inference)
- Montel notes that negative prices on 7 Sep were long enough to trigger the rule that suspends Contracts for Difference top-up payments to wind farms. (confirmed by source: Montel Q3 2025 summary)
Wider context that week
- Ofgem had announced on 27 Aug that the price cap would rise 2% to £1,755 from 1 Oct. (Ofgem)
- Nuclear output was weak all quarter: Montel says Q3 2025 nuclear generation (7.8TWh) was the lowest for a third quarter since 2014. (Montel)
- In early Sep EDF extended the lives of Heysham 1 and Hartlepool by a year, to Mar 2028. (EDF fleet update)
Confidence
High for "windy weekend, low Sunday demand, some solar". The generation data is unambiguous and Montel independently singles out 7 Sep.
Gaps
No news report specific to GB prices on 6 or 7 Sep. I did not find the actual Epex or N2EX day-ahead figures for the day.