Negative prices (bank holiday weekend, record low demand)
Sat 24 May · Sun 25 May
Research notes
Figures are for London unless they say otherwise.
What happened
24 May: one negative half-hour at 23:30 (−1.3p London), the start of the run. 25 May: the standout day of the period. London averaged just 1.3p, fell to −8.0p at 15:00, and was below zero for 14 hours (longest spell 10.5h from 05:30). Lowest anywhere −9.6p in Northern Scotland; the evening peak reached only 18.9p.
Likely causes
- Argus: the N2EX day-ahead market had 17 consecutive negative hours on 25 May, equalling the record from 16 Jul 2023, with hourly prices down to −£35.18/MWh, the lowest since that date. A further four negative hours followed on 26 May. May had 21 negative hours against 4 in April and none in May 2024. (confirmed by source) Argus, 27 May 2025
- Argus on the causes: wind reached 12.2GW, 47% of the mix and the highest Sunday share since 22 Dec 2024; solar averaged 7GW in peak solar hours; gas was only 3.1GW across the day; national demand was 16.5GW, the lowest daily figure on record; and GB exported a net 3.3GW, the most since Nov 2022. (confirmed by source, same link)
- NESO confirms 2025's lowest transmission demand was 12,912MW at 13:30 on 25 May. That beats the 13.4GW lockdown record NESO had warned about in April. (confirmed by source) NESO 2025 review · NESO, 15 Apr 2025
- Grid data agrees: transmission-metered wind averaged 11.7GW and never fell below 10.1GW, solar peaked at about 11.8GW, and GB exported all day, up to 6.2GW at 07:30. On 24 May wind built through the day to 12.5GW by 23:30, exactly when the first negative half-hour appears. (confirmed by source: grid data) Elexon FUELHH 24 to 25 May 2025 · PV_Live 25 May
- Weather: Atlantic lows ended a long settled spell that weekend. The Met Office forecast strengthening wind and coastal gales in the north on Saturday, and said wind would be "a watchpoint for Sunday, especially across Scotland", with the south-east staying largely dry and bright. Not a named storm. (confirmed by source) Met Office, 21 May 2025
- 25 May was the Sunday of the late May bank holiday weekend (the holiday itself was Mon 26 May) and the start of half-term, so demand was unusually weak. (inference on the holiday effect; the record low demand itself is confirmed above)
Wider context that week
- On Fri 23 May Ofgem announced the cap would fall 7% to £1,720 from 1 Jul, with falling wholesale prices accounting for about 90% of the cut. Ofgem, 23 May 2025
- Argus, 22 May: May was on course for the widest peak-below-baseload discount on record on N2EX, with solar out-generating gas in daytime hours and base-load demand at 22GW, the lowest for May apart from 2020. Argus, 22 May 2025
- Spring 2025 was the UK's sunniest on record, and solar generation for Jan to May was up 42% on 2024. Carbon Brief, 4 Jun 2025
Confidence
High. A specialist report on the exact date, NESO's own record, and grid data all agree.
Gaps
Did not find an Octopus post about plunge pricing that day. A search-engine summary of the Argus piece claimed solar was 45% of the mix at midday; the article text I read says 24% during peak solar hours, so use 24% or re-check the article. Argus uses "national demand 16.5GW" (a daily average) and NESO uses 12,912MW (a half-hour transmission minimum); they are different measures, not a conflict.