Negative prices (weekend)
Sat 20 Sep · Sun 21 Sep
Research notes
Figures are for London unless they say otherwise.
What happened
- Sat 20 Sep: one negative half-hour at 23:30 (−2.1p). Otherwise an ordinary day, average 17.7p.
- Sun 21 Sep: below zero for 3 hours in short spells (overnight and at 06:00), low −1.1p, average 8.1p. All 14 regions negative.
Likely causes
- Wind picked up late on Sat and blew through Sun. Elexon-metered wind was as low as 3.5GW on Sat, reached 13.9GW by late evening, and averaged 11.1GW on Sun. (confirmed by source: Elexon FUELHH, 20 to 21 Sep)
- Sun 21 Sep was one of the sunniest days of the month for solar: a 10.4GW peak, fourth highest of Sep. Sat was dull (4.3GW peak). (confirmed by source: PVLive)
- Sunday demand was low: total transmission supply averaged about 22.3GW. (confirmed by source: same Elexon query)
- So: a breezy, sunny Sunday with weekend demand. (inference, but well supported)
Wider context that week
- The Met Office notes a short dry, settled spell followed from 22 to 26 Sep. In the Elexon data wind then fell away sharply, to a 2.2GW daily average by 29 Sep, which is why there were no more negative prices until Oct. (Met Office Sep 2025 stats)
Confidence
Medium to high. The data fits well, but wind was not exceptional, so this is the least dramatic of the Sep events.
Gaps
No weather warning, storm or news report tied to these dates.