Negative prices
Research notes
Figures are for London unless they say otherwise.
What happened
- London min -0.9p at 14:00, avg 4.7p, max 22.1p at 18:30. Below zero for 2.5h, with an early spell from 06:00 and another around 14:00. All 14 regions at or below zero. Unusually the highest price anywhere was in the East Midlands (24.1p) rather than Northern Scotland. The daily average, 4.7p, was almost as low as Floris Tuesday.
Likely causes
- A windy Sunday. The Met Office weekend blog described "a distinctly autumnal turn", with low pressure bringing rain and blustery winds, and meteorological summer ending "on a wet and windy note for many". (confirmed by source) Met Office weekend weather, 30 to 31 Aug 2025, Met Office 10-day trend, 28 Aug 2025
- Wind averaged 12.5GW, the third windiest day of the summer, peaking at 14.6GW. The Saturday before had also been windy (peak 15.7GW). (confirmed by source) Elexon FUELHH 30 to 31 Aug
- Britain was a net exporter across the day (average 2.0GW of exports, up to 4.7GW), gas averaged 3.5GW, and solar still reached about 9.8GW at 11:30. Transmission demand fell to 15.5GW around midday. (confirmed by source) Elexon FUELHH 31 Aug, PV_Live 31 Aug, INDO 31 Aug
- NESO needed trades for "Downward Regulation". Balancing cost £15.37m. (confirmed by source) NESO daily balancing costs, Aug 2025
- High wind all day on Sunday demand, with the 06:00 dip coming from wind alone at the quietest time of the week and the 14:00 dip from wind plus solar. (inference)
Wider context that week
- It was the last day of what the Met Office provisionally confirmed as the UK's warmest summer on record (mean 16.10°C, beating 2018's 15.76°C). Met Office summer 2025 breakdown, via Wired-Gov
- Ofgem announced on Wed 27 Aug that the price cap would rise 2% to £1,755 from 1 Oct. Ofgem
- The previous Sunday, 24 Aug, had been the opposite: almost no wind (1.1GW average) and, according to Drax, interconnectors "importing more than 30% of Britain's demand over the day, the highest on record". A week later Britain was exporting all day. Drax Electric Insights Q3 2025, Elexon FUELHH 24 Aug
- 31 Aug was not a bank holiday weekend; the late summer bank holiday was Mon 25 Aug.
- The pattern continued into the next period: Drax records a new low of -£99/MWh on 6 Sep 2025. Drax Electric Insights Q3 2025
Confidence
High that wind on a Sunday was the cause (forecast, data and NESO agree). No press report specific to the day.
Gaps
- No news article on GB prices for 31 Aug 2025.
- I did not establish whether the low was named or linked to the remains of Hurricane Erin, which affected UK weather in late Aug. The Met Office pieces I read do not say so for this weekend, so do not claim it.