Price spike
Wed 12 Aug · Thu 13 Aug
Research notes
Figures are for London unless they say otherwise.
What happened. Wed: London max 57.7p at 18:30, average 26.6p, 67.4p in Northern Scotland. Thu: max 68.6p at 18:30 (the highest half-hour of the period), average 30.1p, 80.6p in Northern Scotland.
Likely causes
- Heatwave. The mid-Aug heatwave ran 9 to 15 Aug and peaked on 13 Aug at 38.2°C at Hartpury College, Gloucestershire, joint fourth hottest UK temperature on record; Kew reached 38.1°C (confirmed by source) Wikipedia: 2026 UK heatwaves, Met Office
- Wind collapsed. Transmission wind fell from about 9.5 GW in the early hours of 12 Aug to 5.4 GW by the evening, and to 1 to 2 GW for almost all of 13 Aug. Gas plant ran at 13 to 14 GW through both evening peaks (confirmed by data) Elexon FUELHH 12 to 13 Aug
- Day-ahead prices. Baseload went £91.07 (7 Aug) to £125.82 (10 Aug) to £133.24 (11 Aug, for delivery 12 Aug) to £149.15 (12 Aug, for delivery 13 Aug), which was the month's high. Catalyst's explanation: "Forecasts pointed to 36 to 37°C, wind output fell sharply", plus the eclipse, plus France, where nine reactors were constrained "by high river temperatures, low water levels or jellyfish", cutting what Britain could import at the evening peak (confirmed by source) Catalyst 13 Aug, Catalyst 12 Aug, Catalyst 11 Aug
- Bloomberg's 11 Aug headline was "UK Power Price Hits Highest Since June as Temperature Soars"; the snippet says £133.24 was the highest since 23 Jun. Page would not open, so headline and snippet only (confirmed by source, partly) Bloomberg, 11 Aug 2026
- Solar eclipse, 12 Aug. A partial eclipse covered 91% of the sun over Britain from 18:07 to 20:08 BST, peaking at 19:12, the deepest since 1999. Modo Energy expected up to about 2.2 GW of solar to drop out on a clear evening (confirmed by source) Modo Energy. NESO issued an Electricity Margin Notice for the eclipse evening; IBTimes reports an initial shortfall estimate of up to 1,700 MW, later about 1,200 MW, and some backup power bought at up to £875/MWh (confirmed by source) ECIU, 12 Aug 2026, IBTimes UK, 13 Aug 2026
- The eclipse was probably a small part of the price story. London's 12 Aug peak (57.7p at 18:30) does fall inside the eclipse window, but the next evening's peak was higher (68.6p at the same time) with no eclipse, just less wind and more heat (inference).
- Gas. NBP rose from 136p to about 149p over these days; Norway's Kollsnes plant had an unplanned outage on 13 Aug, and the US threatened a naval blockade of Iran (confirmed by source) Catalyst 14 Aug
- Nuclear. Hartlepool 2 off since 7 Aug; Heysham 1 reactor 1 running below full power (confirmed by source, Catalyst 11 Aug as above).
Wider context that week. Houthi and US reports of attacks on shipping on 11 Aug. EU storage about 60%, 12 points down on last year. French heat-related nuclear curtailment reached about 11.6 GW by 14 Aug Catalyst 17 Aug. The balancing market hit £349/MWh at 21:18 on 13 Aug (Catalyst 14 Aug).
Confidence. High. Several independent sources and the data agree.
Gaps and conflicts. (1) Sources disagree on when the margin notice was issued. ECIU's release dated 12 Aug says it was for "tomorrow evening's peak"; Catalyst says NESO issued it for the evening peak of 12 Aug and "cancelled it before midday". I could not find NESO's own notice. Safest wording: "NESO issued an Electricity Margin Notice ahead of the eclipse evening". (2) Catalyst says the eclipse removed "midday generation"; that is wrong, it was an evening eclipse. (3) I did not find a NESO figure for how much solar actually dropped.