| Year | Basis | Mean | P50 | P95 | Read |
|---|---|---|---|---|---|
| 2022 | synthetic | 2.8% | 3.7% | 7.0% | in-sample — all pass (crisis year) |
| 2023 | synthetic | 16.8% | 1.2% | 44.5% | P95 hot (scarcity knee) |
| 2024 | synthetic | 10.4% | 10.0% | 55.8% | mean/P95 hot |
| 2025 | synthetic | 12.0% | 8.2% | 59.4% | HELD-OUT — FAIL on P95 |
| 2025 | realised | 1.6% | 24.0% | 55.4% | HELD-OUT — best mean of any year; FAIL on P50/P95 |
| 2026H1 | synthetic | 1.4% | 23.0% | 55.6% | HELD-OUT WINDOW — first held-out mean PASS; FAIL on P50/P95 |
| 2026H1 | realised | NOT OBTAINABLE at generation — NESO's Historic Demand Data 2026 ended at its 2026-06-13 publication frontier: 90.3% per-series coverage, below the frozen 95% gate; the fetch failed closed | HELD-OUT — the gap is published, not papered over | ||
2025 was a moderate-gas, low-scarcity year: realised prices had a flat tail (P95/P50 ≈ 1.5) where the frozen stylised scarcity knee expects a steeper one (≈ 2.6–3.0). The knee is too convex for a calm year, so the model over-states the top of the distribution. The error is in the tail shape, not the level — the realised-2025 mean is the best of any backtest year (1.6%). The held-out 2026H1 window repeats the pattern: a hot P95 (55.6%, the third regime running) while the window mean transfers (1.4% — the first held-out mean pass in the record) and the P50 runs cold — realised P50 sat above the window mean, a top-heavy gas-cost distribution the frozen single-σ family cannot shape.
The body and the tail are coupled in the single-σ stylised curve family: you cannot flatten 2025's calm tail without regressing the 2022 energy-crisis tail it must also fit. With only one genuine scarcity year (2022) in the public record, there isn't enough tail data to fit a curve that's right for both regimes. This is a one-crisis-year data scarcity ceiling — more knobs would overfit, not fix it. 2026H1 graded exactly this diagnosis out of sample: the level machinery transfers (mean PASS), the shape does not (P50/P95 FAIL) — the fix is more crisis-regime data, not more shape degrees of freedom.
Every adoption rule was frozen in committed code before any held-out input was fetched — the git history proves the gap for both windows (2025; 2026H1 under protocol §14). True virgin hold-outs, graded once, published as-is.
Every dispatch is cross-checked by two independent solvers (HiGHS and CEMeridian) to a 1e-6 objective tolerance, so results aren't an artefact of one optimiser.
What sets central revenue holds up out of sample: the realised-2025 mean is the best of any year (1.6%) and the held-out 2026H1 window mean passes outright (1.4%). Where the P50 misses, it misses cold — the conservative direction — and the hot gap stays confined to the upside tail you should not be banking anyway.
Cells fit to the targets they're graded against are labelled fit (in-sample), never a pass — so you always know which numbers are out-of-sample evidence.
Size downside on the P90 (exceedance, low) band and the 1.10× P90-DSCR covenant — both conservative by construction and unaffected by an over-stated upside P95. Treat the model's central case (mean/P50) as the planning number, the P90 as the bankable floor, and do not lean on a single high percentile for downside protection. The one place the model is optimistic is the one place a prudent financing never relies on. On the held-out 2026H1 window the P50 miss is cold — model £74.0/MWh vs realised £96.1 — an under-statement of the central case, not an over-statement; the over-statement stays confined to the P95 upside tail. The full procedure for credit teams — P50 sizing, P90 covenant test, binding-year discipline — is written up in the underwriting note.