Transmission Headroom

Overview

This page shows assessed transmission import/export headroom at two levels of detail: individual ETYS boundary and ETYS transmission zone. Headroom here means the difference between maximum available transmission capacity and the reference flow — current or historical baseline power transfer.

Interpretation: positive values indicate available capacity margin; negative values indicate a transmission constraint (further growth would need reinforcement or trigger curtailment).

Each map's colour scale is symmetric around zero (0 = amber), so red always means a constraint and green always means margin — regardless of how the underlying values happen to be skewed for the current filter selection. Colour intensity is only comparable within a single map at a single filter selection: not across different scenario/year choices, and not between maps. Areas shown in grey have no assessed figure for the current filter selection — this is a coverage gap, not a zero value.

The two views below define "import" and "export" differently — they are not the same measurement at different resolutions.

Which view to trust

The two views answer genuinely different questions, so "which is right" depends on what you're trying to learn:

• For "how much headroom does this zone/area have, in a direction that matters to that zone" — use the ETYS transmission zone view. Its zone-relative import/export labelling is the physically meaningful one, because import/export only means something once you fix a point of view — the zone — and this view does exactly that. It also has materially better coverage, since it only needs one of a zone's contributing boundaries to have an assessed figure in the relevant direction.
• For "what has NESO actually published for this specific boundary" — use the individual ETYS boundary view. It reports NESO's assessed figures directly, with no directional re-interpretation layered on top, so it's the ground truth to cite if you need to trace a number back to the ETYS source tables.

The zone view is more informative for area-level analysis, but it is a derived interpretation — working out each boundary's direction relative to each zone — rather than a raw published figure (see How we work out import vs export for each zone). Treat it as our best current reading of NESO's data, not as an alternative NESO source.

Confidence in that reading varies by boundary. For most boundaries, NESO's report states the flow direction explicitly. For a handful, we've inferred it from a general regional pattern rather than a boundary-specific statement. For two boundaries, we've made a manual judgement call that hasn't yet been confirmed against zone-specific generation portfolios. That's the part still subject to expert review — not the geometry or the arithmetic, but whether we've read NESO's narrative correctly for those boundaries.

ETYS Transmission Zone

Where these zones come from: NESO publishes headroom for individual named boundaries (lines on the map) — it does not publish headroom for areas. To show a single figure per area, we built these zones ourselves: we cut the map along the lines of the boundaries that have a published capacity figure, together with the GB coastline. Boundaries with no published capacity figure aren't used as a cut line, so they don't create further subdivisions. Treat the zone outlines on this map as our own construction for this analysis, not an official NESO zone map.

How we work out import vs export for each zone: NESO's report describes, in prose, which way each boundary typically flows (e.g. "flows north to south", "power moves out of East Anglia") rather than publishing a ready-made import/export label. We read that narrative for every boundary and recorded it in a reference table: each boundary's typical compass direction of flow, our confidence in that reading, and any known exceptions.

Confidence falls into three tiers: NESO stated the direction explicitly; we inferred it from a general regional pattern rather than a boundary-specific statement; or we made a manual judgement call pending expert confirmation. Known exceptions include a boundary that can run in reverse during low-generation conditions, and one whose published figures use a reversed sign convention — its "primary" published number actually represents import rather than export, so we flip it before use.

We then work out, geometrically, which compass direction each zone sits relative to every boundary it touches, and combine that with the reference table to label that boundary as the zone's import side or its export side.

How to read this map: we work out import and export relative to each zone's own point of view, using the approach above. A given boundary can be the export side for the zone on one side of it, and the import side for the zone on the other — the direction is assigned per zone, not fixed to the boundary itself. Because of this, a zone can show an assessed export figure even where the boundary it touches has never had an export figure separately published for it — the zone map is using whichever of that boundary's two published figures matches the zone's own side (see the boundary map below).

Each zone's total is also the sum of every boundary that touches it, not just its largest boundary. A zone appears on the map once at least one of its boundaries has an assessed figure, so zone-level figures tend to be larger and more complete than the equivalent boundary figures.

Individual ETYS Boundary

How to read this map: here, import and export are not relative to any particular zone. They follow NESO's own fixed convention for each boundary — the direction NESO originally assessed as primary counts as import, the reverse as export — regardless of which zones sit on either side. Many boundaries have never had a reverse-direction figure assessed by NESO at all, so their export value is genuinely blank here — but that same boundary's primary figure may still be feeding an export value in the zone map above, if that direction happens to be the export side for a zone next to it. Don't expect this map's export figure to match the zone map's — or even to be populated wherever the zone map's export figure is populated for zones touching that boundary.

Data sources

• Assessed headroom: NESO's Electricity Ten Year Statement (ETYS) boundary capability assessments, which project available transmission capacity against a reference power flow for each scenario and year.
• Scenario pathway: each pathway (Holistic Transition, Electric Engagement, or Hydrogen Evolution) represents a distinct, published assumption about how quickly the electricity system decarbonises, driving a different assessed flow and therefore different headroom.
• ETYS transmission zone construction: zones are not a NESO-published geography — we construct them by cutting the map along the lines of the boundaries that have a published capacity figure, together with the GB coastline, so each resulting area becomes one zone. See Where these zones come from above.
• ETYS transmission zone attribution: each ETYS transmission zone's figure is the full sum of every boundary's assessed headroom that crosses it — NESO's own ETYS methodology assesses each boundary's capability independently, so summing whole boundary figures doesn't double-count the underlying assessment.
• Export headroom gaps: not every boundary has a published reverse (export) capability assessment — where NESO hasn't assessed a direction, no figure is shown for it, rather than a misleading zero or estimate.