IN Brief:
- Ofgem will oversee an independent review of system operation during the 22–26 June heatwave.
- Low wind, reduced gas availability, sustained demand, interconnector flows, and network constraints tightened operating margins.
- Frequency remained within statutory limits and no customer demand was disconnected, but governance and operational decisions will be examined.
Ofgem will oversee an independent review of the National Energy System Operator’s management of Great Britain’s electricity system during the extreme heat experienced between 22 and 26 June.
Across the five-day period, low wind generation coincided with reduced availability from gas-fired plant, sustained electricity demand, adverse interconnector movements, and transmission constraints. NESO’s control room used reserves, market trading, demand flexibility, and operational instructions to maintain system balance as available margins tightened.
No customer demand was disconnected, while system frequency remained within the statutory range of 49.5Hz to 50.5Hz, reaching a reported minimum of 49.66Hz and maximum of 50.23Hz. Voltage stayed within required limits, and no transmission line or cable was overloaded.
Maintaining those limits did not remove the need to examine how decisions were reached, documented, and authorised. The independent review is expected to consider the management of the event, the quality of associated records, and whether existing procedures provided a sufficiently robust basis for intervention during an unusually tight operating period.
Hot-weather system stress differs from the winter peaks traditionally associated with security of supply. Cooling demand can remain elevated into the evening, wind output may be low, and thermal generating plant can face restrictions or reduced efficiency. High ambient temperatures also affect conductor ratings, transformer cooling, cable performance, and auxiliary systems at generation and network sites.
Interconnectors add another layer of uncertainty. Imports can provide valuable capacity when neighbouring systems face different conditions, yet widespread European heat may tighten several markets simultaneously. Commercial flows respond to price, while physical capability depends on the operating state of connected networks and any constraints affecting cross-border transmission.
Network congestion can leave enough generation available nationally but unable to reach the area where it is required without exceeding transmission limits. Redispatch, reserve instructions, interconnector adjustment, and demand response may then be needed even when total installed capacity appears adequate. The location and response time of available resources become as important as the aggregate reserve margin.
The episode also tests the resilience responsibilities that expanded when NESO became the independent system operator in October 2024. Ofgem regulates its licences and has set requirements covering energy resilience, reporting, emergency preparation, and the assessment of future system risks, creating a framework against which the June decisions can be examined.
Climate resilience is becoming a larger part of network regulation as operators consider hazards beyond historical weather patterns. The ED3 resilience debate has already exposed the difficulty of valuing investment that may be used infrequently but becomes critical during sustained heat, flooding, storms, or other extreme conditions.
Greater demand flexibility could provide additional operating options during future events. Industrial loads, commercial refrigeration, batteries, electric vehicles, and other controllable equipment can alter consumption over short periods when suitable communications, contracts, and settlement arrangements exist. The UK flexibility roadmap is intended to expand that capability, although system operators need tested and measurable response rather than theoretical capacity.
Any review must distinguish decisions made with incomplete forecasts from conclusions reached with hindsight. Control-room conditions change continuously as generators, interconnectors, demand, and constraints move away from forecast values. The relevant questions concern the quality of information available, the procedures followed, the options considered, and whether the resulting record supports effective scrutiny.
The June heatwave passed without supply disconnection, statutory frequency breach, or network overload, but it exposed the increasingly varied conditions under which Great Britain’s electricity system must operate. The independent examination should establish how individual risks approached operational limits and whether technical, procedural, or governance changes are required before another sustained period of extreme weather.
