The T-4 auction for the 2028/2029 Delivery Year concluded on 11 March 2025 and secured 43.1GW of de-rated capacity1. The result is set out in the Statutory Security of Supply Report 2025, published on 17 December 2025 by the Department for Energy Security and Net Zero and the Gas and Electricity Markets Authority1.
The same report records that the T-1 auction for the Delivery Year 2025/2026 concluded on 5 March 2025 and secured 7.9GW of de-rated capacity, at a clearing price of £20/kW per year1. The report states that £1.25 billion was paid out to Capacity Market agreement holders for the 2024/25 Delivery Year1.
The Capacity Market is described in the report as GB's key mechanism for ensuring there continues to be sufficient electricity capacity margin to meet peak demand, and as technology neutral, with the purpose of ensuring security of GB's electricity supply at least cost to consumers1. Most of the required capacity for the next four years has been procured through four-year-ahead auctions, with any remainder procured through "top up" auctions held in the year before delivery1. The report says most of this capacity comes from existing units, while new build projects able to enter multi-year agreements have also been successful, including a pipeline of electricity storage and interconnector projects1.
"The T-4 auction for the delivery year 2028/2029 concluded on 11 March 2025 and secured 43.1GW of de-rated capacity"
For this winter, 2025/26, the National Energy System Operator forecasts sufficient electricity capacity to meet demand, with a derated margin of 6.1GW (10.0%) of the Average Cold Spell peak demand, an increase from 5.2GW (8.8% of peak demand) last winter and the highest predicted margin since 20191. The report also refers to a Loss of Load Expectation of three hours per year as a standard, and states that a LOLE of below 0.1 hours/year applies in this case1.
| Auction | Delivery Year | Concluded | De-rated capacity secured |
|---|---|---|---|
| T-1 | 2025/2026 | 5 March 2025 | 7.9GW |
| T-4 | 2028/2029 | 11 March 2025 | 43.1GW |
Why it matters for households
Capacity Market auctions determine how much firm capacity is contracted to be available years ahead, and the cost of those agreements is recovered through levies on electricity bills. The report records £1.25 billion paid to agreement holders for the 2024/25 Delivery Year1. The 43.1GW secured for 2028/2029 is the volume contracted to underpin supply in that year, alongside interconnector imports and the wider generation mix1. For a household, the practical link is between the margin of capacity held above expected peak demand and the reduced likelihood of supply interruptions; the report frames the national supply position as one where GB is expected to have the ability to access and secure sufficient supplies of electricity and gas to meet consumers' demands over the short and long term1.
What happens next
The report states that the next Capacity Market auctions will be held in March 20261.