UK Grid Report methodology

Generation mix and carbon intensity (2009 to now)

Source: the NESO Data Portal's half-hourly historic generation mix dataset (transmission generation plus NESO's estimate of embedded wind and solar), aggregated by calendar year. Shares are energy-weighted: each fuel's half-hourly megawatts are summed across the year and divided by the summed total, never averaged as percentages.

NESO's renewable rollup counts wind, solar and hydro. Biomass is reported separately, so we also publish a biomass-inclusive share, which is the basis official statistics (DUKES) use. Carbon intensity uses the methodology built by the National Energy System Operator with the University of Oxford Department of Computer Science.

Wholesale prices (2017 to now)

Source: Elexon's Market Index (MID), the traded half-hourly reference price used in imbalance pricing. Prices from the two market index data providers (APX and N2EX) are combined volume-weighted per settlement period; zero-volume submissions are skipped. The yearly average is volume-weighted across all periods; negative-price hours count half hours whose combined price is below £0.

MID history on the Elexon Insights API begins in early 2017, so earlier report years carry no price statistics. Years where market index data covers less than 90% of half hours say so on the page; below 60% coverage we publish nothing.

Demand (2022 to now)

Source: Elexon's initial national demand outturn, daily (INDOD) for annual totals and highest and lowest demand days, and half-hourly (INDO) for the peak half hour of the year. This basis is GB national demand as initially settled; history on the Insights API begins in 2022. The long-run series shown back to 2009 is electricity supplied on the generation basis described above, because no demand-basis series reaches that far back.

Balancing cost (review-gated)

Definition used: what it cost to balance the system through the Balancing Mechanism and balancing services adjustment actions on a given day. Computed as the net of all indicative Balancing Mechanism cashflows (Elexon EBOCF, offers plus bids, every BM unit) plus the net cost of balancing services adjustment actions (Elexon DISBSAD).

Limitations, stated plainly: EBOCF is the indicative settlement run, not the final reconciliation, so figures can move slightly as settlement matures. The Insights API exposes DISBSAD actions per period only as average price and total volume, so that component multiplies the two (an approximation of the exact per-action sum). This measure covers BM and DISBSAD actions; it is not the same as NESO's fuller monthly balancing-cost reporting (which includes further service categories), and will typically read somewhat lower. Daily figures swing widely: a constraint-heavy day (Scottish wind curtailed behind the B6 boundary while gas is re-dispatched south of it) can cost several times the annual daily average.

Wind curtailment (review-gated estimate)

Definition used: wind output bought off through the Balancing Mechanism. We identify every wind BM unit from Elexon's BM unit reference list, then sum accepted bid volumes on those units (made positive) as curtailed energy, and the negated bid cashflows as the cost paid for that reduction.

Limitations, stated plainly: the settlement path we use does not expose system-operator flags, so ALL accepted wind bids are counted, not only system-flagged constraint actions; trading-driven wind bids inflate the volume. Embedded (non-BM) wind cannot be curtailed through this mechanism and is invisible here. Every figure derived this way is labelled an estimate and links to this page.

Validation: our computed curtailment cost matches independently published constraint-payment figures within about 1% for 2023 (£311m vs around £310m) and 2024 (£395m vs £393m, Renewable Energy Foundation). Our volume reads higher than constraint-flagged published figures (10.5 TWh vs 8.3 TWh for 2024) for the reason above, which is why the cost is the headline figure and the volume carries its basis in the label.