Curtailment is an instruction to a power station to reduce its output, not because nobody wants the electricity but because the network cannot carry it to where it is needed. Constraint payments are what the system operator pays generators to make those changes.
How it works
Great Britain has one wholesale price, but the wires have limits. The tightest bottlenecks are between Scotland, where much of the wind capacity is, and the big demand centres in England. When more power is generated north of a bottleneck than the lines can move south, the National Energy System Operator (NESO) uses the Balancing Mechanism to:
- pay generators behind the bottleneck, mostly wind farms, to turn down, and
- pay other generators closer to demand, often gas plants, to turn up and replace that power.
Both sides of the trade are paid, so a constraint costs twice. The cost is passed on to bill payers through balancing charges.
The cost in Great Britain
| Financial year | All constraint costs | Of which thermal constraints |
|---|---|---|
| 2024/25 | about £1.9bn | about £1.7bn |
| 2025/26 | about £2.2bn | about £2.0bn |
Our own totals, summed from NESO’s daily constraint data; NESO’s published figure for 2024/25 thermal constraints is about £1.7bn. Thermal constraints, where lines would overheat, are the main cost. The rest pays for voltage control and keeping enough spinning generators on the system for stability.
Why it matters
Curtailment means clean electricity is thrown away while fossil plants are paid to run, so emissions and bills are both higher than they need to be. The long-term fix is more network capacity, particularly new north–south transmission links. In the meantime, batteries and flexible demand placed behind bottlenecks can soak up power that would otherwise be curtailed. NESO now also buys some of these actions through its Demand Flexibility Service.