Negative prices (pre-Christmas gales)
Sat 21 Dec · Sun 22 Dec · Mon 23 Dec
Research notes
Figures are for London unless they say otherwise.
What happened
- Sat 21 Dec: London averaged 6.4p, level with 24 Nov as the lowest daily average of any day in the data file, with three scattered negative half-hours (min −0.5p at 01:30).
- Sun 22 Dec: below zero for 6.5 hours, including a 5.5-hour run from 01:00; min −1.4p; average 6.8p.
- Mon 23 Dec: touched 0.0p at 03:30, then a normal working-day shape (37.4p peak).
Likely causes
- A deep low passed north of the UK on Saturday. The Met Office had wind warnings out for both days: gusts over 80mph possible in the far north of Scotland on Saturday, then 50 to 60mph widely on Sunday and 60 to 70mph on exposed coasts and hills. Source: Met Office, 20 Dec. (confirmed by source)
- Sun 22 Dec was the windiest day of the whole quarter on NESO's data: wind averaged 21.7GW (61% of supply) and never fell below 20.9GW. Saturday averaged 20.4GW. Wind was 21.3GW (67%) at 01:30 Sunday, the lowest-priced half-hour, with imports near zero. Source: NESO historic generation mix. Figures (confirmed by source); link to price (inference).
- Monday's zero at 03:30 was the tail of the same spell: wind was 21.7GW at midnight and 18.9GW at 03:30, when total supply was only 28.4GW and gas was down to 1.9GW. Wind's share peaked at 70.5% at 00:30 on 23 Dec in this dataset, higher than the 69.2% "record" RenewableUK had quoted for 24 Nov, though I found no announcement of a new share record. The wind then eased to 10.8GW by 18:00. Source: NESO historic generation mix. (inference from confirmed figures)
- Weekend before Christmas, so demand was low, with many workplaces already shut. (inference)
Wider context that week
- The warnings were mainly a travel story: ferry cancellations and disruption on the busiest getaway weekend. Met Office, 20 Dec
- This was the third windy spell in a week, after the records on 15 and 18 Dec (see previous section). December was the windiest month of 2024, with 38.9% of electricity from wind. Solar Power Portal
- The Elexon Market Index Price averaged £41.60/MWh on 22 Dec and dipped to −£2.92 at 01:30, the same half-hour as Agile's low. Elexon market index, 22 Dec
Confidence
Medium to high. Met Office warnings for exactly these days plus the windiest day of the quarter in the generation data. No press report links the two for this weekend.
Gaps
The low was not named by the Met Office group. Found no market coverage of day-ahead prices for 21 to 23 Dec, and no curtailment cost figure for the weekend.