Grid Congestion and Wind Curtailment: Why Britain Pays to Switch Wind Off
- Curtailment means turning down wind output because the grid cannot export it.
- Constraint costs combine payments to turn wind down and payments to turn gas up elsewhere.
- The main bottlenecks are on the England-Scotland boundary and within Scotland.
- UKERC estimated constraint costs of roughly £1.65bn over 15 months from January 2024 to April 2025.
- Grid upgrades take years, which is why storage near the bottleneck is attractive.
Britain sometimes pays wind farms to produce less because the wires cannot carry all of their power south. This is called curtailment, and it explains a lot about where batteries are being built.
What curtailment is
Much of Britain's wind is generated in Scotland, while a lot of demand is in England. The wires between them have a limit. When more power is available than the wires can carry, the system operator, NESO, asks some wind farms to reduce output. That is curtailment.
A UKERC analysis of curtailment costs describes the mechanism. Constraint costs are the sum of bids to turn wind down and offers to turn gas up elsewhere in the Balancing Mechanism, and NESO picks the cheapest options. The curtailed wind farms still keep the income from their market trades and get further payments to cover lost support income.
Where the bottlenecks are
UKERC points to thermal limits on the transmission lines as the largest driver. The boundary between England and Scotland has a nominal secure transfer limit of about 4.5 GW, raised to about 6.7 GW by the 2018 Western HVDC Link. A boundary within Scotland has a secure capacity of about 3.4 GW and is often the tightest point because so much new wind sits in the far north.
The analysis also notes that real-time capacity is often well below the maximum. In 2024, average available transfer capability on the northern boundaries was about 60%, partly because lines are taken out of service for upgrade work.
What it costs
UKERC estimated constraint costs of roughly £1.65bn over the 15 months from January 2024 to the end of April 2025. It also found that adding boundary capacity cuts those costs: up to about a quarter for an extra 500 MW and about 45% for an extra 1,000 MW, in its modelling.
Those are estimates from a research group, not official forecasts, and costs move with gas prices because turning gas up is a large part of the bill. They do show why the system operator and the government care about the problem. We also link to the home batteries and the grid deep dive for the wider picture.
Good to know
UKERC estimates that the delivery dates for new Scotland-to-England links run to 2029 and beyond, so the bottleneck is not going away overnight.
Where batteries fit
Batteries cannot fix a transmission bottleneck on their own, but they can soak up power that would otherwise be curtailed when they sit on the constrained side of it, and release it later. Large sites also connect at the transmission level, as in the Eku Energy purchase of a 300 MW site at Didcot, which shows how developers are positioning for grid services.
For households, the link is indirect. Home batteries are small and spread out, and they sit behind local networks. Policy papers such as the UK clean flexibility roadmap update look at how that spread of flexible kit could be used. How much it helps with a Scotland-to-England bottleneck is something we could not confirm.
Common questions
What is curtailment?
It is turning down a generator, usually wind, because the grid cannot carry its output. The generator is paid to reduce.
What are constraint costs?
They are the combined cost of paying some generators to turn down and others, often gas plants, to turn up, so supply and demand still balance.
Does curtailment raise my bill?
Balancing costs are ultimately recovered from energy users, but how much lands on a household bill is not something we could confirm here.
Can a home battery reduce curtailment?
Not by itself in any large way. Home batteries are small, but widespread flexible demand and storage may help over time.
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