Methodology

Data sources

• Elexon Insights (BMRS): indicative bid volumes and cashflows for every Balancing Mechanism unit, per half-hour settlement period, plus the BM unit reference list (fuel type, region).
• NESO data portal: daily thermal constraint costs by constraint boundary, used as context.

Contains BMRS data © Elexon Limited copyright and database right 2026 (BMRS licence). Supported by National Energy SO Open Data (NESO Open Data Licence).

Definitions

• Switched off (curtailed) energy for a wind unit on a day is the volume of its accepted bids, the instructions to reduce output, that the grid operator flags as system actions, in MWh. Unflagged bids, used to balance overall supply and demand, are counted separately and not shown as curtailment.
• Paid is that unit's bid cashflow in £, multiplied by the flagged share of its bid volume in each half-hour. Elexon publishes cashflow per unit per half-hour, not per instruction, so this split is an approximation. Positive means the grid paid the wind farm to reduce output.
• Only units Elexon classifies as wind are included.
• Where Elexon publishes a revised figure for a half-hour, only the latest version is used.
• Settlement days run midnight to midnight UK time, so clock-change days have 46 or 50 half-hours.

Why some payments are negative

Each generator sets its own bid price. Many older wind farms earn subsidy per MWh generated, so they bid negative prices: the grid pays them to stop. Some newer farms on Contracts for Difference bid positive prices instead, which means they pay the grid when they're turned down. Both show up as curtailed energy; only the first shows up as a payment to the farm.

Grid boundaries

Power can only flow south as fast as the transmission lines allow. This tracker groups Scottish sites around two major transmission boundaries: B4, between the north of Scotland and the Central Belt, and B6, between Scotland and England. Connection location matters, but broad reporting zones do not establish that a particular site can relieve a particular constraint.

Each wind unit is assigned a zone using Elexon's region code where it has one, and the wind farm's location otherwise. These assignments are provisional until checked against grid connection records; the tracker shows how many are verified.

Checks before publishing

• Annual check (the publish gate): our calendar-year totals are compared with Montel's published figures for wind curtailment volumes and payments, for Great Britain, Scotland and northern Scotland. If any differs by more than 10%, nothing is published. Montel’s GB payment benchmark covers renewable curtailment, predominantly wind; we use it as an approximate cross-check of our wind-only total, rather than an exact like-for-like reconciliation.
• Monthly reconciliation: where published monthly figures exist, our totals must agree within 10%.
• Sign check: a month where wind farms are, on balance, paying the grid is flagged for investigation.

Limitations

• Elexon's indicative figures are published before final settlement and can be revised.
• Payments are what wind farms received for reducing output. They exclude the cost of the replacement generation (usually gas) switched on elsewhere, which headline "constraint cost" figures often include.
• Boundary assignments are provisional, as above.