​Transmission constraint costs in Great Britain hit £1.9 billion in 2024/25, accounting for 71% of total balancing costs. That share has risen from 43 9% in 2022/23 and is on course to reach £4-8 billion annually by 2030 without major intervention. The bottleneck is Scotland.

In 2025, turning down Scottish wind cost around £350 million. Replacement generation (mostly in the form of gas) costs over £1 billion. Curtailment is now the primary driver of Great Britain's national balancing costs.

Wind buildout is outpacing transmission capacity, and the infrastructure needed to correct that imbalance does not arrive until the end of the decade. The current rates of curtailment of 30-40% greatly exceed the accepted 3-5% of curtailment on an efficient grid

Key takeaways

• Constraint costs reached £1.9bn in 2024/25, representing 71% of GB's total balancing bill, up from 43% in 2022/23. Without reform, costs are projected to reach £4-8bn annually by 2030.
• Key boundaries, B4 and B6, were constrained 42% and 34%, respectively, in 2025.
• North-of-Scotland wind accounts for 60-62% of total system-bid volume. Sea Green is the single most curtailed asset at 2.7 TWh in 2025, followed by Moray East at 1.7 TWh and Moray West at 1.4 TWh.
• Scottish curtailment offers 3-5 years of Balancing Mechanism income for batteries in constrained zones, depending on market rules. Post-2030 transmission upgrades will reduce that balancing work.
• ETYS projects B4 capacity nearly doubling to 7.3 GW and EC5 tripling to 13.5 GW by 2030, but Clean Power 2030 targets require 20 GW of Scottish wind generation.
• Near-term tools, including the Constraint Management Intertrip Service (CMIS) and the SO:TO Optimisation Trial, have already delivered £119m and £268m in consumer savings, respectively, but network relief requires the transmission upgrades arriving late this decade.

Part 1: Constraints across Great Britain

Scotland's B4 and B6 boundaries are the primary source of Great Britain's constraint problem

Great Britain's high-voltage transmission network is divided into zones. NESO tracks flows and limits across a set of defined constraint boundaries, thermal thresholds beyond which power cannot flow safely without risking line overload.