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Indicative. A national percentile — not a guaranteed connection date. How the grade is computed.

ATR85 curtailment estimator

How the site curtailment estimator works — the engine behind the Solar Workbench.

What the estimator does

For a postcode, a capacity (MW), and a technology (solar, battery, or DC load), the estimator computes the expected number of curtailment hours per year, with a P10/P50/P90 confidence band across the available years. The result feeds the IRR comparison and is shown on the Workbench as the headline figure.

How the estimate is built

  1. Match the postcode to the grid. The postcode is resolved to the high-voltage substation that feeds it (the highest-voltage feeding point wins).
  2. Pull the history. Every 15-minute interval of modelled loading for that substation, bucketed by calendar year (UTC).
  3. Count threshold crossings. Every interval where modelled loading meets or exceeds the curtailment threshold counts. Thresholds, drawn from ATR85 practice:
    • Solar / DC load: 95% (operators often curtail before a hard bind)
    • Battery: 100% (only at a true bind)
    Number of intervals × 0.25 h = hours.
  4. Percentile over years.P10 / P50 / P90 plus the worst year — linearly interpolated, with no Gaussian assumption.
  5. Wider band when history is short.With less than 12 months of data the P10/P90 band is widened linearly, up to ±50% at one month — so a few months of data is never presented as twelve years of certainty.

What the estimator explicitly does NOT do

  • N-1 contingency. Lines are treated at their published thermal rating, with no outages. Real-world curtailment is often N-1: a neighbouring line goes out for maintenance, your connection jumps from 60% to 110%, and you get curtailed. This is not captured.
  • Voltage / reactive power. The model ignores voltage and reactive power. In the Randstad, curtailment is often reactive-driven, so we under-detect there.
  • Sub-substation detail. We resolve to the substation, not to your specific connection cable. If that cable is the bottleneck, we miss it.
  • Operator policy. Operators may curtail ahead of the physics (precaution) or run past it (margin). We model the physics, not the policy.

Accuracy bands

Every estimate carries this band:

Classification-grade, not metering-grade. ±20–30 hours on a 100–300 h annual estimate in well-instrumented areas (FGU, Limburg); ±50–80 h in areas with heavy behind-the-meter solar (Brabant). Suitable for site screening and IRR comparison; not for bankable contract clauses.

The full accuracy-band table is on the main methodology page.