NESO spent £221.2 million managing electricity constraints across Great Britain in July. Its monthly analysis assigns 47 per cent of that cost to its Scotland category, while a changed operating arrangement at a Scottish 275kV substation increased a nearby transfer limit by about 500MW and produced an estimated £39 million saving in the same month.
A change in the way a single Scottish electricity substation was operated saved an estimated £39 million in July, according to the National Energy System Operator, providing an unusually clear measure of how much Scotland’s transmission bottlenecks can cost before a new power line has been built.
NESO’s July balancing report records £302 million of total balancing costs across Great Britain during the month. Of that, £221.2 million was attributed to constraints: situations in which the electricity system could not simply move or use power in the pattern the market would otherwise produce.
NESO’s accompanying chart assigns 47 per cent of July constraint costs to its “Constraints – Scotland” category. Applied to the £221.2 million monthly total, that represents approximately £104 million.
The figure is not a bill issued to Scotland, nor does it mean that Scottish consumers alone pay it. NESO operates the electricity system across Great Britain, and balancing costs are recovered through the wider electricity-market arrangements. The Scottish category identifies where a substantial part of the physical system restriction arose.
July also demonstrates that the size of the constraint problem is not fixed. A new running arrangement proposed at a 275kV substation in Scotland increased the transfer limit of a nearby thermal constraint by roughly 500 megawatts. NESO estimates that the single operational change avoided approximately £39 million of costs during the month.
The electricity exists. The network has to move it.
Scotland’s transmission problem is increasingly one of geography and capacity.
Large volumes of renewable generation are located in the north and east of the country, while much of Britain’s demand lies farther south. Electricity does not move across the network without limits. Transmission circuits have maximum operating capabilities, and outages for maintenance or reinforcement can reduce them further.
When the amount of electricity seeking to cross a constrained part of the network exceeds the secure transfer capability, NESO has to intervene. Generation on one side may be reduced while generation or other services on the other side are increased. Those actions carry costs.
The Scottish transmission network is divided by NESO into seven principal boundaries running from the far north through the central belt and ultimately across the Anglo-Scottish connection.
At the Upper North boundary, B0, capability is currently limited to 1.1GW by the Shin–Beauly 132kV circuit. B1a, covering north-west transmission, is limited to 2GW by the Kintore–Tealing 275kV circuit. B2 is limited to 2.8GW by Killin–Errochty. Farther south, B4 is limited to 4GW by Tealing–Westfield, B5 to 3.9GW by Denny North–Lambhill, and B6, the main Scotland-to-England boundary, to 6.7GW by the Harker–Moffat 400kV circuit.
These figures are not the amount of renewable generation installed behind each boundary. They are the present transfer capabilities identified by NESO for the relevant network sections. The difference between generation behind a boundary and the amount that can securely cross it is central to the constraint problem.
A system being built while it is running
NESO expects the pressure to increase as further generation connects.
Under its Holistic Transition pathway, one of the future-energy scenarios used for network planning, Scotland could have around 38GW of generation capacity against gross demand of approximately 6GW by 2030. That is a scenario rather than a prediction of a single certain future, but it illustrates the scale of the potential export requirement.
NESO says offshore wind is the principal driver of the rapid increase in north-to-south transmission requirements. It expects higher transfers across the Scottish boundaries and is planning reinforcement accordingly.
Building the reinforcements can temporarily add another complication. Major transmission works require existing equipment to be taken out of service at times. NESO specifically warns that work around the B4 boundary may itself produce significant constraint costs while assets are unavailable for construction and reinforcement.
That creates a difficult operational sequence. The network needs additional capacity because more generation is connecting, but providing that capacity can require outages on parts of the network that are already heavily used.
July’s wind and constraint numbers
Wind generation across Great Britain produced 4TWh in July, down from 4.6TWh in June. Scottish wind output fell from 1.7TWh to 1.5TWh.
Across Britain, 614GWh of wind was curtailed during July. That was lower than June’s 710GWh but, according to NESO, remained the highest July curtailment volume in the previous five Julys it examined. The 614GWh figure is a Great Britain total and should not be treated as Scottish curtailment alone.
The most difficult day was 2 July. NESO recorded 122GWh of wind curtailment and £25.6 million of balancing costs. Around 95 per cent of that day’s cost came from constraint management, and 97 per cent of the constraint element was associated with thermal constraints. NESO said exceptionally high wind levels coincided with outages that reduced southbound transfer capability.
For July as a whole, thermal constraint system costs reached £167.8 million. That was £13.8 million below June but £75.6 million higher than in July 2025.
The figures also show why constraint costs cannot be read solely as a measure of how much wind was curtailed. Wholesale electricity prices, network outages, the location and price of alternative generation, voltage requirements, inertia and the precise configuration of the network all affect the final balancing bill.
The £39 million change
Against those large system costs, NESO’s account of the Scottish 275kV substation is unusually specific.
The operator said outage-optimisation measures produced approximately £105 million of savings across Britain during July. The most valuable individual action was the new Scottish substation running arrangement. By increasing the transfer limit of a nearby thermal constraint by roughly 500MW, NESO estimates it saved approximately £39 million.
No new transmission corridor had to be completed to obtain that saving. It came from changing the usable capability of the existing system.
That does not remove the need for major reinforcement. The underlying Scottish boundaries remain constrained and additional generation is expected to increase transfer requirements. It does demonstrate the financial scale of relatively small changes in available network capacity when the system is operating close to a boundary limit.
NESO is developing further non-build measures alongside physical reinforcement. Its Scottish Constraint Management Intertrip Service is intended to contract with generators so that output can be rapidly disconnected following particular faults, allowing the network to carry more power securely before the fault occurs. Delivery is planned from the fourth quarter of 2027.
The system operator is also developing arrangements intended to increase electricity demand close to constrained parts of the network at times when generation is abundant, rather than reducing generation solely because the power cannot be transported farther south.
The July report places those future mechanisms against a current financial baseline. Great Britain incurred £221.2 million of constraint costs in one month; NESO assigned 47 per cent of the total to its Scotland category. In the same month, approximately 500MW of additional usable transfer capability around one Scottish substation was worth an estimated £39 million in avoided costs.
As the next wave of Scottish generation seeks connection, the condition and operating capability of the wires between the turbines and the eventual demand are becoming measurable in hundreds of millions of pounds as well as megawatts.
Sources
July 2026 Monthly Balancing Cost Report
National Energy System Operator, August 2026
https://www.neso.energy/document/386501/download
Balancing costs
National Energy System Operator, current reporting page accessed 23 September 2026
https://www.neso.energy/industry-information/balancing-costs
Scottish boundaries
National Energy System Operator, Electricity Ten Year Statement current page accessed 23 September 2026
https://www.neso.energy/publications/electricity-ten-year-statement-etys/electricity-transmission-network-requirements/scottish-boundaries
Constraints Collaboration Project
National Energy System Operator, current project page accessed 23 September 2026
https://www.neso.energy/industry-information/balancing-services/constraints-collaboration-project
