GB grid cut unsupplied energy in 2025-26 as availability hit a five-year low
Serguey Asael Shinder2 min readThe National Energy System Operator has published its annual report to Ofgem on the performance of Great Britain's electricity transmission system for 2025-26, the year to the end of March. The report is required by the transmission licence and covers availability, security of supply and quality of service on the onshore and offshore networks.
On the measure customers feel, the year was good. Total estimated unsupplied energy fell to 39.20 MWh, from 422.42 MWh in 2024-25. Overall reliability of supply was 99.999983%, against 99.999818% a year earlier. There were 601 events in which transmission circuits were disconnected automatically or by urgent manual switching, down from 644; ten of them led to a loss of supply to customers, compared with 28 incidents the year before.
Availability moved the other way. Annual system availability, the share of the network in service, was 93.66%, the lowest of the five years shown in the report, which started at 94.99% in 2021-22. Winter peak availability was 95.72%, also the lowest in that run. Monthly figures dipped below 92% in June, July and September. Availability of reactive compensation equipment, which helps control voltage, was 75.11% for the year.
Quality of service had one change. NESO recorded two reportable voltage excursions, both on National Grid's network in England and Wales, after none in the previous four years. On 29 June 2025, five substations in the South West reached 423.41 kV on a 400 kV system for 168 minutes; on 9 September 2025, Pembroke reached 422.30 kV for 16.8 minutes. There were no reportable frequency excursions; the last ones were in 2019-20 and 2008-09.
Why it matters
Availability counts planned outages as well as faults, and a grid in the middle of a large build-out takes circuits out to connect new ones. Fewer customer losses with more of the network out of service is the result operators want. The two early-morning summer voltage excursions point to a newer problem: keeping voltage down when demand is low.