A small roof derives Sub-Ladder rungs below Google's smallest config 🟥

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/domain/modelling/test_solar_config_selection.py

@@ -219,3 +219,40 @@ def test_all_north_or_empty_yields_no_configs() -> None:
 
     # Assert
     assert configs == ()
+
+
+def test_small_roof_derives_sub_ladder_rungs_below_googles_smallest_config() -> None:
+    # ADR-0058 — the 824/1278 audit's property 750701 (UPRN 100010328594):
+    # max 5 panels → cap 3.5, Google's ladder starts at 4, so today the
+    # dwelling gets NO PV at all. Instead, derive Sub-Ladder Configurations at
+    # every whole rung from 2 (the install floor) to floor(cap), from the
+    # smallest rung, with per-segment yield scaled pro-rata (conservative —
+    # Google places panels best-first).
+
+    # Arrange — Google max 5, rungs at 4 (1278 kWh/yr) and 5 (1582), one
+    # south plane; cap = 0.70 × 5 = 3.5 < 4 → nothing fits the ladder.
+    potential = SolarPotential(
+        panel_capacity_watts=400.0,
+        max_array_panels_count=5,
+        configurations=(
+            SolarPanelConfiguration(
+                panels_count=4,
+                yearly_energy_dc_kwh=1278.0,
+                segments=(_segment(4, 180.0, 1278.0),),
+            ),
+            SolarPanelConfiguration(
+                panels_count=5,
+                yearly_energy_dc_kwh=1582.0,
+                segments=(_segment(5, 180.0, 1582.0),),
+            ),
+        ),
+    )
+
+    # Act
+    configs = select_conservative_configs(potential)
+
+    # Assert — rungs 2 and 3 (ascending), yields pro-rata off the 4-panel
+    # rung (1278 × 2/4 = 639, × 3/4 = 958.5), segments resized to the rung.
+    assert [c.panels_count for c in configs] == [2, 3]
+    assert [c.yearly_energy_dc_kwh for c in configs] == [639.0, 958.5]
+    assert [sum(s.panels_count for s in c.segments) for c in configs] == [2, 3]