Negative prices (weekdays)
Wed 10 Sep · Thu 11 Sep · Fri 12 Sep
Research notes
Figures are for London unless they say otherwise.
What happened
- Wed 10 Sep: one negative half-hour at 23:30 (−1.3p).
- Thu 11 Sep: below zero for 2 hours in scattered half-hours, low −2.9p at 23:30, average 9.9p.
- Fri 12 Sep: below zero for 5 hours, low −2.1p at 12:30, average 7.9p. All 14 regions negative each day. Evening peaks still reached about 36p.
Likely causes
- Three consecutive very windy days. Elexon-metered wind averaged 13.5GW (Wed), 14.6GW (Thu) and 14.0GW (Fri), peaking at 16.0GW. Gas averaged 3.4 to 3.8GW. (confirmed by source: Elexon FUELHH, 10 to 12 Sep)
- The Met Office described Fri 12 Sep as the windier day of a "breezy and gusty" spell, part of a run of Atlantic lows. (confirmed by source: Met Office weekend weather, published 12 Sep)
- Solar was strong on Thu and Fri (peaks of 9.2GW and 9.4GW), which lines up with Friday's negative half-hours falling around midday on top of the overnight ones. (figures confirmed by source: PVLive; link to price shape is inference)
- GB exported as much as 5.1GW on Fri, a sign of more power than the home market could use. (confirmed by source: same Elexon query)
- These were working days, so this was supply-driven rather than a low-demand weekend effect. (inference)
Wider context that week
- The Met Office summed up the month as "a succession of low-pressure systems" with strong winds at times, and the second week as notably cooler. (Met Office Sep 2025 stats)
Confidence
High for the cause (sustained strong wind plus midday solar). Medium on detail, since no market report for these days was found.
Gaps
No news coverage found for these dates. No named storm was involved (the 2025/26 storm list had only just started and Amy was the first name used, in Oct).