Negative prices
Thu 17 Sep · Fri 18 Sep · Sat 19 Sep
Research notes
Figures are for London unless they say otherwise.
What happened. Thu: −2.1p at 12:30, below zero 4 hours, average 20.2p. Fri: just below zero (−0.7p) for 1.5 hours mid-afternoon. Sat 19 Sep: below zero for 15 hours, including 8 hours overnight from 01:00 (low −4.3p at 03:00), average 3.9p, the cheapest day of the period by a long way.
Likely causes
- Three days of strong, steady wind. Transmission wind averaged 14.2 GW, 14.8 GW and 15.2 GW, the three windiest days of the period, and on 19 Sep it never dropped below 13.5 GW. Britain exported throughout, 5.6 GW at 03:00 on Saturday (confirmed by data) Elexon FUELHH 17 to 19 Sep
- Catalyst, 17 Sep: wind was "the dominant factor"; combined wind and solar forecast "near 15,200 MWh today against a seasonal norm of about 10,200", and day-ahead baseload fell £59.93 (35%) to £111.24, the peak contract by 56% to £77.88. For 18 Sep baseload was £103.95. Its 18 Sep report expected "strong wind Saturday 19 September" with a sharp drop afterwards (confirmed by source) Catalyst 17 Sep, Catalyst 18 Sep
- Saturday's negative prices came overnight, which is the wind pattern rather than the summer solar pattern: low weekend night-time demand with 16 GW of wind and only 3 GW of gas plant left on (confirmed by data for the generation; the link to price is inference).
- Gas also eased over these sessions, from 207p to 191.60p, "a three-session decline" (confirmed by source, Catalyst 18 Sep).
Wider context that week. No named storm: the 2026–27 storm season list shows none named as of 20 Sep 2026 Wikipedia: 2026–27 European windstorm season. Gas was still about 198p on 18 Sep Trading Economics, which is why Thursday and Friday still averaged about 20p despite the wind. Norway's Troll maintenance was extended to 25 Sep. Catalyst expected wind to fall below normal from about 21 Sep.
Confidence. High.
Gaps. I found no Met Office write-up of the weather system behind the wind, and no report yet covering 19 Sep after the event (it was yesterday). Worth checking NESO or press this week for any wind record on 18 or 19 Sep; I found none, and transmission wind peaked at 17.4 GW on 18 Sep, which I can't compare against the all-time record from this data alone.