diff --git a/domain/sap10_calculator/docs/BRIEF_APPENDIX_H_EN_15316_RESEARCH.md b/domain/sap10_calculator/docs/BRIEF_APPENDIX_H_EN_15316_RESEARCH.md
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+# Research brief — SAP 10.2 Appendix H solar HW vs BS EN 15316-4-3:2017
+
+## Goal
+
+Localise the bug that causes our SAP 10.2 Appendix H orchestrator
+([domain/sap10_calculator/worksheet/appendix_h_solar.py](../worksheet/appendix_h_solar.py))
+to compute monthly solar hot-water heat delivered **1.81× higher than
+the Elmhurst U985 worksheet** for cert 000565
+(`sap worksheets/extended test case/U985-0001-000565.pdf`). The
+discrepancy is the dominant remaining gap in cert 000565's HW pin
+(+272 kWh/yr cascade over worksheet).
+
+## What we already know
+
+### SAP 10.2 Appendix H spec text
+
+Located at
+[domain/sap10_calculator/docs/specs/sap-10-2-full-specification-2025-03-14.pdf](specs/sap-10-2-full-specification-2025-03-14.pdf),
+pages 74-78. The relevant equations are reproduced in this brief
+under "What we implemented" below.
+
+### S10TP-04 (BRE technical note)
+
+[domain/sap10_calculator/docs/specs/sap10 technical papers/S10TP-04 - Change to Appendix H to include solar space heating - V1_3.pdf](specs/sap10%20technical%20papers/S10TP-04%20-%20Change%20to%20Appendix%20H%20to%20include%20solar%20space%20heating%20-%20V1_3.pdf)
+confirms that **SAP 10.2 Appendix H implements Method 2 of BS EN
+15316-4-3:2017** (M3-8-3, M8-8-3, M11-8-3 modules). It states:
+> "The method itself is not reproduced in this technical note – it
+> is fully described in the Standard"
+
+So the authoritative formula lives in EN 15316-4-3:2017 Method 2,
+and the SAP spec text on p.76 is a (potentially abbreviated /
+typo-prone) restatement.
+
+### What we implemented
+
+Per SAP 10.2 spec p.76 verbatim:
+
+```
+(H7)m  = Appendix U §U3.3 tilted solar flux on collector aperture [W/m²]
+(H9)m  = (H1) × (H2) × (H7)m × (H8)                                [W]
+(H10)  = 5 + 0.5 × (H1)  OR  test-certificate value                [W/K]
+(H11)  = (H3) + 40·(H4) + (H10)/(H1)                               [W/m²K]
+(H14)  = (H12) [separate] OR (H12) + 0.3·((H13)-(H12)) [combined] [L]
+(H15)  = 75 × (H1)                                                 [L]
+(H16)  = ((H15)/(H14))^0.25                                        [-]
+(H17)m = (62)m − (63a)m                                            [kWh/month]
+(H18)m = 1 (HW-only) | 0 (SH-only) | (H17)/(H17+(98a)) (blended)  [-]
+(H20)m = 55 + 3.86·Tcold,m − 1.32·(96)m                            [°C]
+(H21)m = (H20)m − (96)m                                            [K]
+(H22)m = [(H18)·(H1)·(H11)·(H5)·(H21)·(H16)·((41)·24)] / [1000·(H17)]  [-]
+         clamped to [0, 18]
+(H23)m = [(H18)·(H6)·(H5)·(H9)·((41)·24)] / [1000·(H17)]               [-]
+         clamped to ≥ 0
+(H24)m = [Ca·Y + Cb·X + Cc·Y² + Cd·X² + Ce·Y³ + Cf·X³] × (H17)m    [kWh]
+         clamped to [0, (H17)m]
+```
+
+Where `X = (H22)m`, `Y = (H23)m`, `(41)m` is days-in-month per spec
+p.136, `(96)m` is external temp (Appendix U region 0 for SAP
+rating), `Tcold,m` is mains cold-water temp from Table J1.
+
+Coefficients per spec Table H3 (p.78):
+- Ca = 1.029
+- Cb = −0.065
+- Cc = −0.245
+- Cd = 0.0018
+- Ce = 0.0215
+- Cf = 0
+
+### Concrete diagnostic — cert 000565 (UK average climate, region 0)
+
+Inputs (all verified against worksheet):
+
+| Var | Value | Notes |
+|---|---|---|
+| H1 | 3.0 | aperture m² |
+| H2 | 0.8 | zero-loss efficiency |
+| H3 | 4.0 | linear heat loss coefficient |
+| H4 | 0.01 | second order heat loss coefficient |
+| H5 | 0.9 | loop efficiency (default; no test cert) |
+| H6 | 0.94 | incidence angle modifier (flat plate) |
+| H8 | 0.8 | overshading factor (Modest) |
+| H10 | 6.5 | overall heat loss (test-certificate value) |
+| H11 | 6.5667 | matches worksheet |
+| H12 | 53 L | dedicated solar storage |
+| H13 | 160 L | total cylinder volume |
+| H14 | 85.1 L | matches worksheet |
+| H15 | 225 L | matches worksheet |
+| H16 | 1.2752 | matches worksheet |
+
+Collector: West, 30° pitch. Climate: UK average (region 0) since
+Block 1 SAP rating.
+
+**Cascade vs worksheet per-month (H24)m kWh:**
+
+| Month | Cascade | Worksheet | Ratio |
+|---|---:|---:|---:|
+| Jan | 0 | 0 | – |
+| Feb | 0 | 0 | – |
+| Mar | 32.48 | 7.27 | **4.47×** |
+| Apr | 71.96 | 34.93 | 2.06× |
+| May | 106.53 | 66.05 | 1.61× |
+| Jun | 95.82 | 60.01 | 1.60× |
+| Jul | 90.52 | 58.25 | 1.55× |
+| Aug | 72.54 | 42.25 | 1.72× |
+| Sep | 39.93 | 12.58 | **3.17×** |
+| Oct | 0 | 0 | – |
+| Nov | 0 | 0 | – |
+| Dec | 0 | 0 | – |
+| **Σ** | **509.78** | **281.35** | **1.81×** |
+
+**Worksheet (H24)m values from
+`sap worksheets/extended test case/U985-0001-000565.pdf` page 4.**
+
+## Pattern clues
+
+The per-month ratio is **not constant**:
+- High-irradiation months (May-Aug): 1.55-1.72× over — looks like a
+  uniform ~1.7× scaling.
+- Edge months (Mar, Sep): 3-4× over — much worse than middle months.
+
+A uniform multiplicative bug would give the same ratio every month.
+The non-uniform pattern suggests one of:
+- A missing **threshold or clamp** that zeros out small contributions.
+- An additional **subtractive term** that's irradiation-dependent
+  (so it's significant when irradiation is low, negligible when high).
+- A different **polynomial form** that has a steeper rolloff at low Y
+  (Y is the irradiation-driven term).
+
+Specifically, if there's a term `−k·H17/X` or `−k·H17/Y²` somewhere,
+it would dominate at low Y / high X / large H17 — i.e., the
+shoulder-season months.
+
+## Constants we've ruled out
+
+The handover doc
+[HANDOVER_POST_S0380_69.md](HANDOVER_POST_S0380_69.md) records that
+prior agents tried these tweaks, none of which closed the gap:
+
+- Removing H8 from H9 (top-level Eqn H1 commentary uses
+  H1·H2·H6·η0·ηloop·Im, no H8 — inconsistent with line-ref (H23))
+- Keeping H8 in H9 (current)
+- Adding H5/H6 to H9 instead of having them in X/Y separately
+- Dividing by H8 inside X
+- Using horizontal solar flux instead of tilted
+
+Also verified by this brief author:
+- Polynomial coefficients match Table H3 verbatim.
+- (H7) tilted-flux conversion via Appendix U §U3.3 is correct.
+- (96)m external temps for region 0 match worksheet exactly.
+- (62)m HW demand monthly matches worksheet exactly.
+- All five "input" helpers (H10, H11, H14, H15, H16) match worksheet
+  to 4 decimal places.
+- (41)m × 24 = days × 24 = hours-in-month per spec p.136.
+- Im (Table U3) is the standard 24-hour-averaged W/m² (not daylight
+  only).
+
+## What we need from EN 15316-4-3:2017
+
+The standard is **108 pages**. Method 2 is the relevant slice (M3-8-3,
+M8-8-3, M11-8-3 modules per S10TP-04). The portion we need probably
+fits in 4-8 pages.
+
+### Specific questions
+
+1. **What is the exact Method 2 form of Equation H1 (Qs polynomial)?**
+   Does it have the same six coefficient terms as SAP Table H3, or
+   are there additional terms? Solar-thermal performance regressions
+   frequently include **mixed interaction terms** that SAP's
+   pure-power-of-X, pure-power-of-Y formulation omits:
+   - `Cg · X·Y`
+   - `Ch · X·Y²`
+   - `Ci · X²·Y`
+   - `Cj · Y/X` (or `X/Y`)
+   - A tank-loss term proportional to `(H17) × time`
+   - An irradiation-dependent subtractive term
+   The seasonal pattern of our over-count (uniform in summer,
+   much worse in shoulder months) is consistent with one or more
+   missing mixed terms — pure-X / pure-Y additions would shift the
+   ratio uniformly across months.
+
+2. **What is the exact Method 2 form of factor X (heat-loss factor)
+   and factor Y (irradiation factor)?** Does Method 2 multiply by the
+   same group of inputs as SAP (H22) / (H23)? In particular, does
+   Method 2 include a term that SAP's restatement on p.76 omits?
+
+3. **Are there any clamps, thresholds, validity ranges, or cutoffs
+   in Method 2 that the SAP spec didn't reproduce?** Specifically:
+   - A lower threshold on Y (or on Im) below which Qs = 0?
+   - A threshold on the storage tank correction H16?
+   - A "useful heat" filter that excludes months where solar
+     contribution < some % of demand?
+   - A "minimum collector temperature rise" filter (collector outlet
+     must exceed inlet by some ΔT before solar is credited)?
+   - A "minimum solar fraction" gate?
+
+4. **What are the validity / applicability ranges that Method 2
+   states for X and Y?** Regression-based correlation methods are
+   fit over a specific X / Y range and are explicitly invalid
+   outside that envelope. If the SAP spec doesn't reproduce the
+   range bounds, the cascade may be applying the polynomial in
+   shoulder months where Method 2 specifies a different rule
+   (zero, capped, interpolated). For cert 000565 our cascade
+   computes:
+   - X ranges from 3.98 (Jan) to 7.95 (Jul); always within the
+     SAP-stated [0, 18] clamp.
+   - Y ranges from 0.095 (Dec) to 1.34 (Jun); always > 0.
+   Does EN 15316 Method 2 state a Y_min below which the polynomial
+   doesn't apply? Does it state an X_max < 18?
+
+5. **Is the "hot water reference temperature" formula (SAP H20:
+   `55 + 3.86·Tcold − 1.32·Text`) Method 2's formula or a SAP-specific
+   substitute?** S10TP-04 mentions SAP uses a 41°C mixed-water
+   temperature for HW which differs from EN 15316. Are there other
+   SAP substitutions in this section that the spec didn't flag?
+
+6. **Does Method 2 use the same irradiation Im as a 24-hour-averaged
+   monthly W/m², or as a different averaging period (e.g. daylight
+   hours only)?** S10TP-04 says SAP retains Appendix U for irradiance
+   ("UK specific conditions"), but it's unclear whether the
+   downstream consumer of Im in Method 2 expects the same averaging
+   convention.
+
+7. **What is the relationship between (H21) "HW reference temperature
+   difference" and Method 2's ΔTm?** SAP p.76 defines
+   (H21)m = (H20)m − (96)m. Is this the same ΔT that EN 15316
+   Method 2 uses, or does Method 2 use a different reference (e.g.
+   collector outlet temperature, ambient + storage temperature
+   blend)?
+
+### Format we'd ideally get back
+
+A markdown table or short note that lists:
+
+| SAP 10.2 line | SAP 10.2 spec formula | EN 15316-4-3 Method 2 formula | Difference (if any) |
+|---|---|---|---|
+| (H22) | … | … | … |
+| (H23) | … | … | … |
+| (H24) polynomial | … | … | … |
+| … | … | … | … |
+
+Plus any clamps / thresholds the SAP spec elided.
+
+If the standard exposes intermediate values for a worked example
+(e.g. a reference cert), the per-month X / Y / Q numbers for that
+example would let us verify our orchestrator against EN-method ground
+truth directly.
+
+## Reference: where this matters
+
+Fixing this would close **~272 kWh/yr** on cert 000565's HW pin (3rd
+largest open residual on the wacky-stress-test cert). It would also
+make the Appendix H orchestrator (currently landed but **not
+integrated** into `water_heating_from_cert.solar_monthly_kwh` at
+[domain/sap10_calculator/worksheet/water_heating.py:943](../worksheet/water_heating.py#L943))
+safe to wire in — without the fix, integrating would *worsen* the
+residual (cert 000565 would go from +272 to −131 kWh/yr).
diff --git a/domain/sap10_calculator/docs/HANDOVER_POST_S0380_73_APPENDIX_H_BLOCKED.md b/domain/sap10_calculator/docs/HANDOVER_POST_S0380_73_APPENDIX_H_BLOCKED.md
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+# Handover — post S0380.70..73 + Appendix H investigation blocked on external standard
+
+Branch: `feature/per-cert-mapper-validation`. **HEAD `c63d6740`**.
+Predecessor: [`HANDOVER_POST_S0380_69.md`](HANDOVER_POST_S0380_69.md).
+
+## Slices committed this session (S0380.70..73)
+
+The Table 12d/12e header rule ("electricity → monthly cascade
+regardless of tariff") was applied consistently across every
+electric end-use:
+
+| Slice | Commit | What |
+|---|---|---|
+| **S0380.70** | `fc68fb21` | Secondary heating CO2/PE routed through lodged `secondary_fuel_type` (mirror of the cost-side fix). Closed cert 2102 (House coal secondary, +20.36 → +0.20 PE) + cohort-1 cert 0300-2747 (mains-gas secondary, +8.28 → +0.93 PE). |
+| **S0380.71** | `3d6cf5ea` | STANDARD-tariff electric main_heating PE/CO2 monthly cascade. New `_main_heating_primary_factor` helper mirroring `_main_heating_co2_factor_kg_per_kwh` from S0380.65. Dropped STANDARD-tariff annual-flat fallback in both helpers. |
+| **S0380.72** | `b0c4c6e0` | Hot water PE/CO2 monthly cascade. New `_hot_water_co2_factor_kg_per_kwh` + `_hot_water_primary_factor` helpers. Replaced 4 hardcoded `_STANDARD_ELECTRICITY_FUEL_CODE` and annual-flat factor call sites. |
+| **S0380.73** | `c63d6740` | Appendix M1 §3a D_PV cooking uses **L20 electricity** (138+28N) not **L18 heat gain** (35+7N watts × hours). 2.21× over-count fixed. Cohort cluster mean PE residual: −0.36 → −0.06 kWh/m² (cumulative S0380.71-.73: 48× compression). Surfaced 12 gas-combi PV certs at +0.5-1.6 PE (separate gas-fuel PE bug — re-pinned). |
+
+**Test baseline at HEAD `c63d6740`:** 547 pass + 9 expected
+`test_sap_result_pin[000565-*]` cascade-gap fails.
+Pyright net-zero on every touched file.
+
+## Cumulative ASHP cohort cluster closure (20 STANDARD-tariff certs)
+
+| Stage | Mean PE residual | Worst (cert 9796) |
+|---|---:|---:|
+| Pre-S0380.71 | −3.10 kWh/m² | −4.18 |
+| Post-S0380.71 (main heating) | −0.66 | −1.36 |
+| Post-S0380.72 (HW) | −0.36 | −1.08 |
+| Post-S0380.73 (cooking) | **−0.06** | **−0.53** |
+
+Compression: 48× on the mean, 8× on the worst cert. All 20 cluster
+certs now within ±0.53 kWh/m² of lodged values. Residuals scattered
+around zero (was overwhelmingly negative).
+
+## Open thread #1 — Cert 000565 Appendix H Solar HW (BLOCKED on EN 15316-4-3:2017)
+
+Cert 000565 has 9 expected `test_sap_result_pin[000565-*]` failing
+pins. The two biggest energy residuals are blocked on external data:
+
+| Pin | Δ | Status |
+|---|---:|---|
+| sap_score (int) | **0** ✓ EXACT | unchanged |
+| sap_score_continuous | +0.6334 | sub-spec |
+| ecf | −0.0643 | sub-spec |
+| total_fuel_cost | −56.08 | sub-spec |
+| co2 | −19.77 | sub-spec |
+| **space_heating** | **+266.11** | **BLOCKED — RR fold-in needs RdSAP §3.10 detailed-RR geometry** |
+| main_heating_fuel | +156.53 | follows space_heating |
+| **hot_water** | **+271.84** | **BLOCKED — Appendix H magnitude (see below)** |
+| lighting | +2.19 | sub-spec |
+| pumps_fans | +2.48 | blocked — PCDB MEV record not in repo |
+
+### Appendix H deep dive (NEW this session)
+
+Cert 000565 has solar HW lodged. Block 1 SAP rating expects
+H24=281.35 kWh/yr; our orchestrator gives **509.78 kWh/yr → 1.81×
+over-count**.
+
+**Verified by this session's investigation:**
+- All inputs (H1-H8, H10-H16) match worksheet to 4 decimal places.
+- All (H17)-(H23) formulas implement SAP 10.2 spec p.76 verbatim.
+- Polynomial coefficients (Ca-Cf) match spec Table H3 verbatim.
+- (H7) tilted-flux conversion via Appendix U §U3.3 is correct.
+- (96)m external temps for region 0 match worksheet exactly.
+- (62)m HW demand monthly matches worksheet exactly.
+
+**Per-month pattern (the strong clue):**
+
+| Month | Cascade | Worksheet | Ratio |
+|---|---:|---:|---:|
+| Mar | 32.48 | 7.27 | **4.47×** |
+| Apr | 71.96 | 34.93 | 2.06× |
+| May | 106.53 | 66.05 | 1.61× |
+| Jun | 95.82 | 60.01 | 1.60× |
+| Jul | 90.52 | 58.25 | 1.55× |
+| Aug | 72.54 | 42.25 | 1.72× |
+| Sep | 39.93 | 12.58 | **3.17×** |
+
+Non-uniform ratio (1.5-1.7× in summer, 3-4× in shoulder months)
+suggests a **missing clamp / validity envelope / useful-gain
+suppression** rather than a polynomial-coefficient error.
+
+**External research findings (ChatGPT-mediated, this session):**
+- A publicly visible draft of prEN 15316-4-3 shows Table B.1
+  coefficients matching SAP Table H3 exactly (Ca=1.029, Cb=−0.065,
+  …). So the polynomial isn't the bug.
+- The 6-term polynomial structure (no XY/X²Y/XY² interaction terms)
+  appears canonical in the f-chart method literature.
+- ChatGPT's verdict: the bug is likely in a Method 2 applicability
+  range, useful-gain suppression rule, or load-normalisation
+  definition that SAP didn't reproduce.
+
+**Research brief documenting the diagnostic:**
+[`BRIEF_APPENDIX_H_EN_15316_RESEARCH.md`](BRIEF_APPENDIX_H_EN_15316_RESEARCH.md).
+This is the document to hand to a research agent / human if BS EN
+15316-4-3:2017 access is available.
+
+**Current investigation path:** the user is generating 3 simple
+solar-HW cert worksheets (A baseline, B high-Y, C low-Y) to
+empirically test whether the 1.81× ratio is systematic across all
+solar HW shapes or specific to cert 000565. Across 3 certs:
+~36 month-data-points should let us empirically fit any missing
+correction term. See "Continuation instructions" below.
+
+## Open thread #2 — Elmhurst RdSAP solar HW collector defaults (RESOLVED)
+
+Cert 000565 worksheet uses H3=4.0, H4=0.01 for "Solar collector
+details known: No". These don't match SAP 10.2 Table H1 (flat plate
+a1=3.5, a2=0).
+
+**Source identified by sub-agent this session: RdSAP 10
+Specification §10.11 Table 29 "Heating and hot water parameters",
+row "Solar panel", page 58.** Verbatim:
+
+> "If solar panel present, the parameters for the calculation not
+> provided in the RdSAP data set are:
+> - panel aperture area 3 m²
+> - **flat panel, η₀ = 0.80, a₁ = 4.0, a₂ = 0.01**
+> - facing South, pitch 30°, modest overshading
+> - …
+> - pump for solar-heated water is electric (75 kWh/year)
+> - showers are both electric and non-electric"
+
+So RdSAP overrides the **input set** (a1, a2) but SAP 10.2's
+Appendix H is still the calculator. Our orchestrator uses the
+right Table 29 inputs (matching the worksheet), so this is **NOT**
+the source of the 1.81× over-count. The over-count is in the
+Appendix H formula itself.
+
+This resolves a long-standing default-source mystery but doesn't
+help with the H24 over-count.
+
+**Important:** changing H3/H4 to SAP Table H1 spec defaults makes
+the H24 over-count *worse* on cert 000565, not better.
+
+## Open thread #3 — 12 gas-combi PV certs at +0.5-1.6 PE
+
+S0380.73 cooking fix surfaced 12 gas-combi PV certs at residuals
++0.5 to +1.6 PE (cohort-1 cert 2130 + 11 cohort-2). Pre-S0380.73 a
+compensating bug (the cooking over-count) masked this. Now visible
+but **no worksheets available** for these certs — same "unanchored
+chase" situation as the 5 SAP-residual certs. Re-pinned at current
+residuals; investigation deferred until worksheets land.
+
+## Open thread #4 — 5 SAP-integer-residual certs
+
+Total 14 |Δsap| points outstanding across 5 API-only certs (notes
+in `test_golden_fixtures.py`):
+
+| Cert | Δsap | Shape |
+|---|---|---|
+| 0240 | −14 | Oil boiler + PV + RR |
+| 0390 | −7 | Oil boiler, 360m², age F masonry |
+| 6035 | −6 | Gas combi age A (pre-1900) |
+| 7536 | +1 | Multi-age extensions (D/L/F) |
+| 2130 | +1 | Shifted from 0 by S0380.73 cooking fix |
+
+All API-only (no worksheets). User has agreed not to chase these
+without worksheet ground truth (per session discussion).
+
+## How to run the baseline
+
+```bash
+PYTHONPATH=/workspaces/model python -m pytest \
+    backend/documents_parser/tests/test_summary_pdf_mapper_chain.py \
+    backend/documents_parser/tests/test_elmhurst_extractor.py \
+    backend/documents_parser/tests/test_elmhurst_end_to_end.py \
+    domain/sap10_calculator/worksheet/tests/test_e2e_elmhurst_sap_score.py \
+    domain/sap10_calculator/worksheet/tests/test_appendix_h_solar.py \
+    domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py \
+    domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py \
+    --no-cov -q
+```
+
+Expected: **547 pass + 9 expected `test_sap_result_pin[000565-*]`
+fails**.
+
+## Continuation instructions for next agent
+
+The user is generating 3 solar-HW cert worksheets (A baseline, B
+high-Y, C low-Y) per the spec at end of this session. They'll land
+in `sap worksheets/Solar HW tests/` or similar. Each cert directory
+contains:
+- A Summary_NNNNNN.pdf
+- A P960-0001-NNNNNN.pdf (worksheet equivalent of dr87/U985)
+
+### When the certs land
+
+1. **Run the orchestrator** for each cert with the worksheet's H1-H8
+   inputs:
+
+   ```python
+   from domain.sap10_calculator.worksheet.appendix_h_solar import (
+       solar_water_heating_input_monthly_kwh,
+   )
+   from domain.sap10_calculator.worksheet.water_heating import (
+       TABLE_J1_TCOLD_FROM_MAINS_C,
+   )
+   from domain.sap10_calculator.worksheet.solar_gains import Orientation
+   from domain.sap10_calculator.climate.appendix_u import external_temperature_c
+
+   # H1-H8 from worksheet's Appendix H section (page 4 in U985 format)
+   # (62)m monthly HW demand from worksheet
+   # te from external_temperature_c(region, m) for region 0 (Block 1
+   # SAP rating)
+   result = solar_water_heating_input_monthly_kwh(...)
+   ```
+
+2. **Extract worksheet (H24)m monthly values** from the cert's P960
+   PDF page 4 (or wherever Block 1 sits) using:
+
+   ```python
+   from pypdf import PdfReader
+   r = PdfReader(path_to_p960_pdf)
+   # Look for (63c) "Solar input" row in the §4 HW section
+   # Or (H24)m line directly in the Appendix H section
+   ```
+
+3. **Build a 36-point dataset** (3 certs × 12 months) of (cascade
+   H24, worksheet H24, X_cascade, Y_cascade, H17_cascade) and check:
+
+   - Does the per-month ratio show the same shape across all 3?
+     (Summer ~1.7×, shoulder 3-4×) → confirms systematic bug.
+   - Or does the ratio vary by cert? → suggests cert-specific input
+     differences.
+
+4. **Empirical fit attempt:** if the ratio pattern is systematic,
+   try fitting:
+
+   ```
+   Qs_corrected = Qs_cascade × g(X, Y, H17)
+   ```
+
+   for various g shapes (multiplicative, additive,
+   threshold-dependent). The fitted correction term + 3-cert
+   validation gives us a temporary closure even without the EN
+   standard.
+
+5. **Decision point:** if empirical fit closes all 3 cert + cert
+   000565 to <50 kWh/yr residual, ship as a spec-citation-pending
+   slice (note in the commit + memory that it's empirical pending
+   EN 15316-4-3 verification). Otherwise wait for the standard.
+
+6. **Integration:** if cert 000565 HW gap closes via this work,
+   wire the orchestrator into
+   [`domain/sap10_calculator/worksheet/water_heating.py:943`](../worksheet/water_heating.py#L943)
+   (currently hardcoded `solar_monthly_kwh=zero12`). This is the
+   step that lets cert 000565's HW pin go from +272 → ~0.
+
+### What NOT to do
+
+- Don't redo the work the prior agent already verified:
+  - Don't re-test H1-H8 input matching (verified)
+  - Don't re-test polynomial coefficients (matches Table H3)
+  - Don't re-test (H7) flux conversion (verified)
+  - Don't re-test SAP spec formula transcription (verbatim)
+
+- Don't chase the 12 gas-combi PV certs or the 5 SAP-residual certs
+  without worksheets — user has explicitly de-prioritised those.
+
+- Don't integrate the orchestrator into the cascade with the
+  current 1.81× over-estimate — that would WORSEN cert 000565's HW
+  residual from +272 → −131 per the handover prediction.
+
+## Key files touched this session
+
+| File | Touched in |
+|---|---|
+| `domain/sap10_calculator/rdsap/cert_to_inputs.py` | All 4 slices — new helpers + 4 rewires |
+| `domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py` | 5 new tests |
+| `domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py` | 33 cluster pin updates across S0380.71-.73 |
+| `domain/sap10_calculator/docs/BRIEF_APPENDIX_H_EN_15316_RESEARCH.md` | NEW — research brief |
+| `domain/sap10_calculator/docs/HANDOVER_POST_S0380_73_APPENDIX_H_BLOCKED.md` | NEW — this doc |
+
+## Spec source quick-reference
+
+- **SAP 10.2 full specification**: `domain/sap10_calculator/docs/specs/sap-10-2-full-specification-2025-03-14.pdf`
+  - Table 12d (monthly electric CO2 factors): p.195
+  - Table 12e (monthly electric PE factors): p.196
+  - Appendix H (solar thermal): p.74-78, Table H1 p.78, Table H3 p.78
+  - Appendix L (cooking electricity L20): p.91
+  - Appendix M1 §3a (D_PV definition): p.93-94
+- **S10TP-04** (BRE Appendix H change note): `domain/sap10_calculator/docs/specs/sap10 technical papers/S10TP-04 - Change to Appendix H to include solar space heating - V1_3.pdf`
+- **SAP 10.3** at `domain/sap10_calculator/docs/specs/sap-10-3-full-specification-2026-01-13.pdf`: **DO NOT reference** (project tracks 10.2 only per [[feedback-sap-10-2-only-never-10-3]]).
diff --git a/domain/sap10_calculator/docs/NEXT_AGENT_PROMPT_POST_S0380_73.md b/domain/sap10_calculator/docs/NEXT_AGENT_PROMPT_POST_S0380_73.md
new file mode 100644
index 00000000..55dd8f1b
--- /dev/null
+++ b/domain/sap10_calculator/docs/NEXT_AGENT_PROMPT_POST_S0380_73.md
@@ -0,0 +1,219 @@
+# Next-agent prompt — post S0380.73
+
+Branch: `feature/per-cert-mapper-validation`.
+HEAD: `c63d6740`.
+
+Read these in order before any tool call:
+
+1. [`HANDOVER_POST_S0380_73_APPENDIX_H_BLOCKED.md`](HANDOVER_POST_S0380_73_APPENDIX_H_BLOCKED.md) (full state)
+2. [`BRIEF_APPENDIX_H_EN_15316_RESEARCH.md`](BRIEF_APPENDIX_H_EN_15316_RESEARCH.md) (the unblock-this brief)
+
+Also load these memories before starting:
+
+- `project_cert_000565_recovery_state` — cert 000565 slice history
+- `project_golden_coverage_state` — cohort state + S0380.70-.73 closures
+- `feedback_sap_10_2_only_never_10_3` — **CRITICAL** — never reference SAP 10.3 spec
+- `feedback_verify_handover_claims` — verify spec citations before implementing
+- `feedback_spec_floor_skepticism` — "spec-precision floor" framing usually hides a real spec bug
+- `feedback_zero_error_strict` — pyright net-zero per touched file
+- `feedback_commit_per_slice` — one slice = one commit
+- `feedback_spec_citation_in_commits` — quote spec text + page in commit messages
+- `feedback_aaa_test_convention` — every new test uses `# Arrange / # Act / # Assert`
+- `feedback_e2e_validation_philosophy` — component pins at <1e-3; SAP integer delta=0; no adaptive ceilings
+
+## State summary
+
+**Cumulative session result:** test baseline went from 317 pass + 9
+expected fails → **547 pass + 9 expected fails**. The 9 expected
+fails are all `test_sap_result_pin[000565-*]` and reflect cert
+000565's residual gaps. Four cohort closure slices (S0380.70-.73)
+applied the SAP 10.2 Table 12d/12e header rule consistently to
+secondary heating, main heating, hot water, and the Appendix M1
+§3a D_PV cooking electricity formula.
+
+The ASHP cohort cluster of 20 STANDARD-tariff certs compressed
+from mean PE residual −3.10 → −0.06 kWh/m² across these 4 slices.
+That cluster is now closed.
+
+The remaining outstanding work is on cert 000565 (the wacky stress
+test). Two of cert 000565's biggest residuals (HW +272, space
+heating +266) are blocked on external data:
+
+- **Appendix H Solar HW magnitude** (HW +272 kWh/yr cascade over):
+  orchestrator at
+  [`domain/sap10_calculator/worksheet/appendix_h_solar.py`](../worksheet/appendix_h_solar.py)
+  produces 509.78 vs worksheet 281.35 = 1.81× over. All inputs
+  verified, all SAP 10.2 spec formulas implemented verbatim. The
+  bug is in a missing Method 2 clamp / validity envelope / useful-
+  gain suppression that SAP didn't reproduce from BS EN 15316-4-3.
+
+- **RR fold-in** (space_heating +266 kWh/yr): blocked on RdSAP §3.10
+  detailed-RR geometry / area formula not in repo.
+
+## Recommended next slice (the Appendix H empirical approach)
+
+The user is generating 3 simple solar-HW cert worksheets to
+empirically test the 1.81× over-count. These will land in
+`sap worksheets/Solar HW tests/` (or similar) as:
+
+- `A-baseline-south-modest/` (South, 30°, Modest overshading)
+- `B-highY-south-none/` (South, 30°, None/very-little overshading)
+- `C-lowY-north-significant/` (North or East, 60°, Significant
+  overshading)
+
+Each cert directory contains a `Summary_NNNNNN.pdf` and a
+`P960-0001-NNNNNN.pdf` (Elmhurst worksheet). All 3 certs share the
+same base envelope (28 Distillery Wharf, semi-detached, TFA 90 m²,
+age G, masonry cavity walls). Each cert has "Solar collector
+details known: No" so they all use RdSAP 10 Table 29 defaults
+(H3=4.0, H4=0.01 — verified in this session).
+
+### When the certs land
+
+1. **Run the orchestrator** for each cert. The probe pattern:
+
+   ```python
+   from domain.sap10_calculator.worksheet.appendix_h_solar import (
+       solar_water_heating_input_monthly_kwh,
+   )
+   from domain.sap10_calculator.worksheet.water_heating import (
+       TABLE_J1_TCOLD_FROM_MAINS_C,
+   )
+   from domain.sap10_calculator.worksheet.solar_gains import Orientation
+   from domain.sap10_calculator.climate.appendix_u import external_temperature_c
+
+   # H1-H8 from worksheet's Appendix H section
+   # (62)m monthly HW demand from worksheet (look for line ref (62))
+   # external temps for region 1 (Thames Valley) Block 1 SAP rating
+   te = tuple(external_temperature_c(1, m) for m in range(1, 13))
+
+   result = solar_water_heating_input_monthly_kwh(
+       collector_orientation=Orientation.S,  # or N/E per cert
+       collector_pitch_deg=30.0,             # or 60 per cert
+       region=1,
+       aperture_area_m2=3.0,                 # RdSAP Table 29 default
+       zero_loss_efficiency=0.8,
+       linear_heat_loss_a1=4.0,              # RdSAP Table 29 default
+       second_order_heat_loss_a2=0.01,       # RdSAP Table 29 default
+       loop_efficiency=0.9,
+       incidence_angle_modifier=0.94,
+       overshading_factor=0.8,               # or 1.0 / 0.65 per cert
+       overall_heat_loss_coefficient_from_test=6.5,
+       dedicated_solar_storage_volume_l=...,
+       combined_cylinder_total_volume_l=...,
+       hot_water_demand_monthly_kwh=(62)m,
+       wwhrs_monthly_kwh=(0.0,) * 12,
+       cold_water_temperatures_monthly_c=TABLE_J1_TCOLD_FROM_MAINS_C,
+       external_temperatures_monthly_c=te,
+       solar_hot_water_only=True,
+   )
+   ```
+
+2. **Extract worksheet (H24)m monthly** from each cert's P960 PDF.
+   For cert 000565 the (H24)m values are on page 4 (Block 1 SAP
+   rating). Look for line `(63c)` solar input row (12 monthly
+   values, negative sign convention) — its absolute value equals
+   (H24)m. Or find `Heat delivered to hot water` row with `(H24)`
+   label at the end.
+
+3. **Build a 36-point dataset** of `(cascade_H24, worksheet_H24,
+   X_cascade, Y_cascade, H17_cascade)` across the 3 certs.
+
+4. **Diagnostic analysis:**
+
+   - Does the per-month ratio show the same shape across all 3?
+     (Summer 1.5-1.7×, shoulder 3-4×) → confirms the bug is in the
+     formula, not in cert 000565's specific inputs.
+   - Does the ratio vary with Y? Plot ratio vs Y to look for a
+     threshold (sharp transition).
+   - Does the ratio vary with X? Plot ratio vs X to look for an
+     envelope.
+
+5. **Empirical fit attempt** (if ratio pattern is systematic):
+   try candidate corrections in this order:
+
+   - **Threshold on Y:** if Y < Y_min, set Qs = 0. Fit Y_min from
+     data (shoulder months that worksheet zeroes give the
+     threshold).
+   - **Useful gain factor:** Qs_corrected = Qs × max(0, 1 − k/Y).
+     Fit k.
+   - **X-validity clamp:** if X > X_max, apply a different rule.
+   - **Tank loss subtraction:** Qs_corrected = Qs − k·H17. Fit k.
+
+6. **Decision point:**
+
+   - **If empirical fit closes all 3 certs + cert 000565 to <50
+     kWh/yr residual:** ship as a spec-citation-pending slice with
+     `# TODO(EN-15316-verification)` comments + commit message
+     noting empirical-pending. Update
+     [`BRIEF_APPENDIX_H_EN_15316_RESEARCH.md`](BRIEF_APPENDIX_H_EN_15316_RESEARCH.md)
+     with the fitted-correction findings so a future research
+     trip can verify.
+   - **Otherwise:** hold and wait for BS EN 15316-4-3:2017 access.
+     Document the failed-fit attempts in the brief.
+
+### Cert 000565 HW integration (only after the formula bug closes)
+
+Wire the orchestrator into
+[`domain/sap10_calculator/worksheet/water_heating.py:943`](../worksheet/water_heating.py#L943)
+which currently hardcodes `solar_monthly_kwh=zero12`. This is the
+step that lets cert 000565's HW pin go from +272 → ~0.
+
+**DO NOT integrate the orchestrator at the current 1.81× over-
+estimate.** The handover predicts this would *worsen* cert 000565's
+HW residual from +272 → −131 (overshoot in the negative direction).
+
+## What NOT to do
+
+- Don't re-verify the work the prior agent already verified:
+  - H1-H8 input matching to worksheet (verified to 4 d.p.)
+  - Polynomial coefficients vs Table H3 (verbatim match)
+  - (H7) flux conversion vs Appendix U §U3.3 (verified)
+  - SAP spec formula transcription (verbatim)
+  - The H3=4.0, H4=0.01 default source (RdSAP 10 §10.11 Table 29
+    p.58 — found this session)
+  - ChatGPT-mediated research on EN 15316-4-3 already established
+    the polynomial coefficients match the prEN draft Table B.1
+    (so the polynomial isn't the bug)
+- Don't chase the 12 gas-combi PV certs or the 5 SAP-residual certs
+  without worksheets — user has explicitly de-prioritised those.
+- Don't reference SAP 10.3 ([[feedback-sap-10-2-only-never-10-3]]).
+
+## Standard workflow per slice
+
+1. Read SAP 10.2 spec page for the change — quote it in commit
+2. Probe current cascade output, identify exact spec-vs-cascade gap
+3. Write failing test FIRST (AAA structure)
+4. Implement helper / change
+5. Verify test passes
+6. Run full handover suite (command in handover doc)
+7. Check pyright on touched files — net-zero from baseline
+8. Commit with spec citation
+9. Update relevant memory if state changed
+
+## How to run the baseline
+
+```bash
+PYTHONPATH=/workspaces/model python -m pytest \
+    backend/documents_parser/tests/test_summary_pdf_mapper_chain.py \
+    backend/documents_parser/tests/test_elmhurst_extractor.py \
+    backend/documents_parser/tests/test_elmhurst_end_to_end.py \
+    domain/sap10_calculator/worksheet/tests/test_e2e_elmhurst_sap_score.py \
+    domain/sap10_calculator/worksheet/tests/test_appendix_h_solar.py \
+    domain/sap10_calculator/rdsap/tests/test_cert_to_inputs.py \
+    domain/sap10_calculator/rdsap/tests/test_golden_fixtures.py \
+    --no-cov -q
+```
+
+Expected: **547 pass + 9 expected `test_sap_result_pin[000565-*]`
+fails** (the 9 cert 000565 cascade-gap pins).
+
+## Memory hygiene
+
+After the next slice, update:
+
+- `project_cert_000565_recovery_state` — add Appendix H magnitude
+  outcome (empirical fit landed, or wait-for-EN documented).
+- `project_golden_coverage_state` — HEAD update.
+
+Good luck.