Negative prices (Storm Ashley)
Sun 20 Oct · Mon 21 Oct
Research notes
Figures are for London unless they say otherwise.
What happened
- Sun 20 Oct: London averaged just 7.1p, with four separate negative half-hours (2h in total), min −1.5p at 23:30.
- Mon 21 Oct: below zero for 5.5 hours, including a 4-hour run from 02:00, min −1.5p at 05:00. The price recovered to 41.2p by 18:00.
Likely causes
- Storm Ashley, the first named storm of the 2024/25 season, crossed the UK on 20 and 21 Oct with the strongest winds in the north-west. Gusts of 82mph at Aberdaron, 81mph at Killowen, 78mph on Tiree; over 100mph at mountain stations. The low deepened from 988hPa to 952hPa in 24 hours. Source: Met Office October 2024 summary, and Met Office Storm Ashley report (PDF). (confirmed by source)
- Bloomberg reported on Fri 18 Oct that UK wind output was "poised for record" as Ashley approached, with Sunday's generation forecast to pass 22GW for the first time. Source: Bloomberg, 18 Oct. (confirmed by source, as a forecast)
- The record did not happen on NESO's figures: wind peaked at 20.4GW at 12:30 on Sunday and averaged 16.9GW (56% of supply). Even so, wind was 17.4GW to 18.3GW (65% to 66% of supply) through the negative half-hours, imports fell to zero, and Sunday's total supply bottomed at 22.0GW at 03:00. Source: NESO historic generation mix. Figures (confirmed by source); link to prices (inference).
- Sunday was a weekend, so demand was low. Monday's negative spell was overnight, before the working day started. (inference)
- Wind farms were paid to switch off because the grid could not move all the power south: about £11.7m of constraint payments over 20 and 21 Oct, nearly all to Scottish wind farms. Source: Scottish Daily Express using Renewable Energy Foundation figures. (confirmed by source; REF is a campaigning charity critical of wind subsidies, and the paper did the adding up)
- The Elexon Market Index Price averaged £17.60/MWh on the Sunday and touched −£9.74. Source: Elexon market index, 20 Oct. (confirmed by source)
Wider context that week
- Storm Ashley was named by Met Éireann. It disrupted ferries, flights and rail in Scotland and Northern Ireland. Wikipedia: Storm Ashley
- The clocks went back the following Sunday, 27 Oct (not an event day).
Confidence
High. A named storm, a Bloomberg forecast of record wind for that exact day, and generation data matching the negative hours.
Gaps
Could not read the Bloomberg article body (paywalled), so only its headline claim is used. Found no report of the actual day-ahead auction prices for 20 or 21 Oct.