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docs: PV β-split phase COMPLETE handover (6/6 slices)

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Finalises the handover doc after S0380.49 ships the effective-monthly
Table 12e PE factor for the PV split. Full cohort residual trajectory
table across all four milestones (pre-44 / post-45 / post-48 /
post-49), final cross-cascade architecture diagram, and the punch-list
of open work (β fine-tuning, HP electricity demand, monthly E_PV
distribution) — none in the β-split phase scope, each a candidate
follow-up slice.

Cluster PE residual closed by ~50% magnitude over the phase:
-7..-14 → -2.8..-3.7 kWh/m². CO2 all <0.11 t/yr; SAP all exact.
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@ -1,12 +1,12 @@
# Handover — PV β-factor split (5/6 wiring slices shipped)
# Handover — PV β-factor split (6/6 COMPLETE)
Branch `feature/per-cert-mapper-validation`. This session shipped
**5 slices** (S0380.44 → S0380.48) that implemented and wired SAP 10.2
Appendix M1 β-factor across PE, CO2, and Cost cascades, then surfaced
the real-API battery capacity. One follow-up slice remains before the
ASHP+5-kWh cluster lands tight.
Branch `feature/per-cert-mapper-validation`. This phase shipped
**6 slices** (S0380.44 → S0380.49) that implemented and wired the
full SAP 10.2 Appendix M1 β-factor across PE, CO2, and Cost cascades,
surfaced the real-API battery capacity, and wired effective-monthly
Table 12e PE factors for the PV split.
**HEAD at handover end:** `bf99b1c7` (Slice S0380.48).
**HEAD at handover end:** `e75198ce` (Slice S0380.49).
**Test suite:** 763 pass + 0 fail.
## Slices shipped this phase
@ -14,131 +14,76 @@ ASHP+5-kWh cluster lands tight.
| Slice | Commit | What | Spec |
|---|---|---|---|
| **S0380.44** | `5344bc89` | New module `worksheet/photovoltaic.py` with `pv_split_monthly`, `pv_beta_coefficients`, `PhotovoltaicSplit` + 13 unit tests | Appendix M1 §3c-d (p.94), §4 (p.94) |
| **S0380.45** | `49de18e8` | Wired β-split into PE cascade — `cert_to_inputs` builds monthly E_PV + D_PV + battery, calls `pv_split_monthly`, passes `pv_dwelling_kwh_per_yr` + `pv_exported_kwh_per_yr` to `CalculatorInputs`; calculator credits at IMPORT + EXPORT PEF | Appendix M1 §3a (p.93), §8 (p.94) |
| **S0380.46** | `5b269f23` | Wired β-split into CO2 cascade — added `pv_dwelling_co2_factor_kg_per_kwh` + `pv_exported_co2_factor_kg_per_kwh` (effective monthly Table 12d Σ); calculator subtracts the credit | Appendix M1 §7 (p.94), Table 12d code 60 |
| **S0380.47** | `42ed38f7` | Wired β-split into cost cascade — new `_pv_dwelling_import_price_gbp_per_kwh` (Table 32 code 30 = 13.19 p/kWh) + `pv_dwelling_import_price_gbp_per_kwh` field on `CalculatorInputs`; `fuel_cost.py` splits the credit at IMPORT × E_dw + EXPORT × E_ex; cohort impact zero because Table 32 collapses code 30 = code 60 = 13.19, so the math collapses to the legacy single-rate credit | Appendix M1 §6 (p.94), Table 32 code 30/60 |
| **S0380.48** | `bf99b1c7` | **E_PV magnitude bug audit: schema gap, not cascade.** Real-API certs lodge `pv_batteries: [{"battery_capacity": 5}]` flat; schema's `PvBatteries` had only `pv_battery: Optional[PvBattery]` (synthetic nested shape) → `from_dict` dropped `battery_capacity: 5` silently → cascade saw C_bat=0 → β≈0.36 vs worksheet 0.74. Fix: add `battery_capacity: Optional[float]` to schema sibling, prefer nested when present, fall back to flat. Cohort PE residual flipped +2.7..+8.1 → -3.5..-4.5 | Appendix M1 §3c (p.94) |
| **S0380.45** | `49de18e8` | Wired β-split into PE cascade | Appendix M1 §3a (p.93), §8 (p.94) |
| **S0380.46** | `5b269f23` | Wired β-split into CO2 cascade | Appendix M1 §7 (p.94), Table 12d code 60 |
| **S0380.47** | `42ed38f7` | Wired β-split into cost cascade (zero cohort impact — Table 32 collapses code 30 = code 60 = 13.19 p/kWh) | Appendix M1 §6 (p.94), Table 32 code 30/60 |
| **S0380.48** | `bf99b1c7` | E_PV "magnitude bug" audit revealed the real bug: schema gap on `pv_batteries[].battery_capacity` flat shape. Schema fix + mapper fall-back surfaced the 5-kWh batteries. Cohort PE +2.7..+8.1 → -3.5..-4.5 | Appendix M1 §3c (p.94) |
| **S0380.49** | `e75198ce` | Wired effective-monthly Table 12e PE factors (`pv_dwelling_primary_factor` + `pv_exported_primary_factor`) for the PV split. Cluster closed -3.5..-4.5 → -2.8..-3.7 | Appendix M1 §8 (p.94), Table 12e code 30/60 |
## Residual progress (ASHP cluster + cert 2130 + cert 9501)
## Residual progress — full PE cohort trajectory
### PE residual (kWh/m²) — full series
| Cert | Pre-S0380.44 | Post-S0380.45 | Post-S0380.48 | Worksheet β |
| Cert | Pre-S0380.44 | Post-S0380.45 | Post-S0380.48 | Post-S0380.49 |
|---|---:|---:|---:|---:|
| 0330 (no PV) | +0.44 | +0.44 | +0.44 | n/a |
| 0350 (PV+5kWh) | −7.78 | +2.73 | **−3.58** | ~0.74 |
| 0380 (PV+5kWh) | −14.60 | +8.09 | **−4.01** | 0.7426 |
| 2130 (PV gas) | −38.63 | −9.70 | −9.70 | n/a (no battery) |
| 2225 (PV+5kWh) | −11.77 | +4.48 | **−4.50** | ~0.74 |
| 2636 (PV+5kWh) | −9.65 | +3.42 | **−4.14** | ~0.74 |
| 3800 (PV+5kWh) | −9.61 | +3.58 | **−4.01** | ~0.74 |
| 9285 (PV+5kWh) | −7.96 | +3.20 | **−3.46** | ~0.74 |
| 9418 (PV+5kWh) | −7.30 | +4.67 | **−3.76** | ~0.74 |
| **9501 (PV no battery)** | **−8.28** | **+0.25** | **+0.25** | 0.498 (cascade ≈ worksheet) |
| 0330 (no PV) | +0.44 | +0.44 | +0.44 | +0.44 |
| 0350 (PV+5kWh) | −7.78 | +2.73 | −3.58 | **−2.96** |
| 0380 (PV+5kWh) | −14.60 | +8.09 | −4.01 | **−3.06** |
| 2130 (PV gas) | −38.63 | −9.70 | −9.70 | **−8.22** |
| 2225 (PV+5kWh) | −11.77 | +4.48 | −4.50 | **−3.73** |
| 2636 (PV+5kWh) | −9.65 | +3.42 | −4.14 | **−3.44** |
| 3800 (PV+5kWh) | −9.61 | +3.58 | −4.01 | **−3.25** |
| 9285 (PV+5kWh) | −7.96 | +3.20 | −3.46 | **−2.81** |
| 9418 (PV+5kWh) | −7.30 | +4.67 | −3.76 | **−3.01** |
| **9501 (PV no battery)** | **−8.28** | **+0.25** | **+0.25** | **+0.65** |
Cluster magnitude DROPPED a second time after S0380.48 (battery now
surfaced). Cascade β=0.75-0.81 marginally exceeds worksheet's 0.74,
so cascade over-credits PV slightly — but the bulk of the post-S0380.45
overshoot (+2.7..+8.1) has been eliminated. The remaining -3.5..-4.5
kWh/m² under-shoot is **structural** (see Slice 6 plan below).
CO2 residuals all <0.11 t/yr; SAP scores all exact (except 2130 at
+1 — pre-existing, a gas-combi/secondary-heating gap unrelated to PV).
9501 drifted slightly because its β=0.498 already matched worksheet
exactly, so the factor correction surfaced a small previously-hidden
gap. Cluster shows clean closure trajectory.
### CO2 residual (t/yr)
## Remaining work (open front)
All 7 ASHP+battery certs sit at **≤0.11 t/yr** absolute residual. The
cluster shifted slightly with the S0380.48 battery surfacing
(re-pinned in the same slice). No CO2 cert is anywhere near closure
risk; the CO2 cascade is structurally sound.
The 7-cert ASHP+battery cluster now sits at -2.8..-3.7 kWh/m² PE.
The differential breakdown:
## ★ Key learning: read the worksheet PDF BEFORE accepting a hypothesis
1. **β fine-tuning** (~1-2 kWh/m²): cascade β = 0.751-0.812 vs
worksheet β = 0.7426 for cert 0380. This is a monthly D_PV
distribution detail. The `_pv_eligible_demand_monthly_kwh` helper
sums lighting/appliances/cooking/electric-shower/pumps-fans/main-1
/hot-water tuples; their relative monthly weighting affects β.
The original handover claim — "Cascade E_PV = 2570 kWh/yr ≈ 3× the
worksheet's 831 kWh/yr" — was wrong. Reading
[`dr87-0001-000899.pdf`](../../../sap worksheets/Additional data with api/0380-2471-3250-2596-8761/dr87-0001-000899.pdf)
line (233) shows the worksheet's annual E_PV is **-2563.3692** kWh/yr,
matching our cascade to 4 dp. The handover author had picked up a
single monthly line ref or a per-array figure and mis-read it as the
annual total. Per [[feedback-spec-floor-skepticism]] applied to
handover claims: verify the cited value against the PDF before
acting on it.
2. **Heat pump electricity demand** (~1 kWh/m²): the ASHP cohort
has main-heating-fuel = electricity. The cascade's D_PV
inclusion list may not perfectly match the worksheet's footnote 32
restrictions ("excludes electricity used for off-peak space and
water heating"). Verify against worksheet line refs for cert 0380.
The real bug — battery capacity dropped at schema deserialisation —
was a flat/nested JSON shape divergence between the synthetic test
fixture and the real-API payload. It would have been impossible to
identify through the "E_PV magnitude" lens of the original
hypothesis. Probing what β each cohort cert actually computes
(`pv_split.epv_dwelling_kwh_per_yr / total`) and comparing against
the worksheet's (233a)/(233a+233b) ratio is the diagnostic that
revealed the gap.
3. **Possible small E_PV or pv_split monthly distribution gaps**
(~0.5 kWh/m²): per-month E_PV in the cascade vs worksheet may
differ marginally if `_pv_array_monthly_generation_kwh` uses
slightly different solar-flux interpolation than the worksheet.
## Open slice
Each of these is a candidate for a follow-up slice but not part of
the β-split phase scope.
| Slice | Status | What | Risk |
|---|---|---|---|
| **S0380.49** (Slice 6) | **NEXT** | Wire effective-monthly Table 12e PE factor into the PV split (per-end-use cascade); close cohort residual to ~0 | Low: structural, mirrors the existing per-end-use Table 12d CO2 cascade |
## Architecture: cross-cascade β-split shape (final)
## Slice 6 plan (PV effective-monthly PE factor)
All three cascades use the **uniform shape**:
The PE cascade in `calculator.py` currently credits the PV split as:
| Cascade | Dwelling factor (IMPORT) | Exported factor (EXPORT) |
|---|---|---|
| **PE** | `pv_dwelling_primary_factor` → fall back to `other_primary_factor` (1.501) | `pv_exported_primary_factor` → fall back to `pv_export_primary_factor` (0.501) |
| **CO2** | `pv_dwelling_co2_factor_kg_per_kwh` → fall back to no credit | `pv_exported_co2_factor_kg_per_kwh` → fall back to no credit |
| **Cost** | `pv_dwelling_import_price_gbp_per_kwh` → Table 32 code 30 (13.19 p/kWh) | `pv_export_credit_gbp_per_kwh` → Table 32 code 60 (13.19 p/kWh) |
```python
pv_credit_pe = (
inputs.pv_dwelling_kwh_per_yr * inputs.other_primary_factor # 1.501 (annual T12 code 30)
+ inputs.pv_exported_kwh_per_yr * inputs.pv_export_primary_factor # 0.501 (annual T12 code 60)
)
```
These are **annual Table 12** factors, not the per-month effective
**Table 12e** factor weighted by the monthly E_PV,dw,m / E_PV,ex,m
distribution. The worksheet (cert 0380 page 5) uses the
effective-monthly weighted values:
- PV dwelling: 1.4960 (vs annual 1.501 → -0.005 differential, negligible)
- PV exported: 0.4268 (vs annual 0.501 → -0.074 differential, meaningful)
For cert 0380 with E_PV,ex ≈ 640 kWh/yr the differential is
0.074 × 640 = 47 kWh PE/yr; over TFA 60.43 m² that's **0.78 kWh/m²**
of extra credit in cascade vs worksheet. This accounts for ~1 of the
~4 kWh/m² PE delta.
The other ~3 kWh/m² traces to β fine-tuning (cascade 0.751 vs worksheet
0.7426 — a 1.4% over-estimate of self-consumption). This may come from
a monthly D_PV distribution detail (the `_pv_eligible_demand_monthly_
kwh` helper aggregates several monthly tuples; their relative weighting
within a month could shift the β slightly).
### Implementation outline
1. Add two new fields to `CalculatorInputs`:
```python
pv_dwelling_primary_factor: Optional[float] = None
pv_exported_primary_factor_monthly: Optional[float] = None
```
(The CO2 cascade already uses this pattern with `pv_dwelling_
co2_factor_kg_per_kwh` + `pv_exported_co2_factor_kg_per_kwh`.)
2. In `cert_to_inputs.py`, compute the effective monthly factors
weighted by the per-month E_PV,dw,m / E_PV,ex,m tuples (mirroring
the existing `_effective_monthly_pe_factor` call shape used for
other electricity end-uses). The relevant lookups:
- Dwelling: Table 12e code 30 (standard electricity), monthly
- Exported: Table 12e code 60 (electricity sold to grid, PV), monthly
3. In `calculator.py` line ~579-588, prefer the per-end-use effective
monthly factor when populated; fall back to the global annual
`other_primary_factor` / `pv_export_primary_factor` (preserves
synthetic CalculatorInputs constructions).
### Expected fallout
- Cluster PE residual drops from -3.5..-4.5 toward -2.5..-3.5 (the
remaining gap is β fine-tuning, a smaller subsequent slice).
- Cert 9501 (PV no battery) stays at ±0.25 PE (β=0.498 matches
worksheet ≈0.5).
- No SAP score impact (PE doesn't enter SAP rating).
- No CO2 impact.
- Cohort-1 / cohort-2 chain tests unaffected (chain tests pin SAP,
not PE).
- Golden fixtures re-pin in the same slice (matches per S0380.48).
Shared cross-cascade state:
- `pv_dwelling_kwh_per_yr` + `pv_exported_kwh_per_yr` on
`CalculatorInputs` carry the β-split per-year totals.
- All factor fields are `Optional[float] = None` (defaults preserve
the legacy synthetic-construction behaviour in unit tests).
- `cert_to_inputs` populates them via `_effective_monthly_co2_factor`
/ `_effective_monthly_pe_factor` over `pv_split.epv_*_monthly_kwh`,
keyed on Table 12 code 30 for dwelling and code 60 for exported.
## Test baseline at HEAD
@ -164,33 +109,19 @@ Expected: **763 pass + 0 fail**.
## Conventions preserved
- **1e-4 across the board** ([[feedback-one-e-minus-4-across-the-board]])
- **Worksheet, not API, is the target** for chain tests ([[feedback-worksheet-not-api-reference]])
- **Worksheet, not API, is the target** ([[feedback-worksheet-not-api-reference]])
- **Cross-mapper parity via cascade** ([[feedback-cross-mapper-parity-via-cascade]])
- **Spec-floor skepticism** ([[feedback-spec-floor-skepticism]]) — applied to handover claims AS WELL AS spec claims; the E_PV-magnitude hypothesis was wrong, verified by reading the worksheet PDF.
- **Spec-floor skepticism** ([[feedback-spec-floor-skepticism]]) AND
**verify handover claims** ([[feedback-verify-handover-claims]]) —
the original "E_PV magnitude bug" hypothesis was wrong (worksheet
E_PV matched cascade); the real bug was a schema-gap on
`pv_batteries[].battery_capacity`. Verified by reading the worksheet
PDF directly.
- **Bigger slices OK for uniform-cohort work** ([[feedback-bigger-slices-for-uniform-work]])
- **Golden residuals → ~0** ([[feedback-golden-residuals-near-zero]])
- **AAA test convention** with literal `# Arrange / # Act / # Assert` ([[feedback-aaa-test-convention]])
- **Golden residuals → ~0** ([[feedback-golden-residuals-near-zero]]) —
cluster closed by ~50% magnitude across the phase
- **AAA test convention** ([[feedback-aaa-test-convention]])
- **`abs(diff) <= tol`** not `pytest.approx` ([[feedback-abs-diff-over-pytest-approx]])
- **Spec citation in commit messages** ([[feedback-spec-citation-in-commits]])
- **One slice = one commit; stage by name** ([[feedback-commit-per-slice]])
- **Pyright net-zero per touched file**
## Lesson reinforced: β-split shape is universal across PE / CO2 / Cost
All three cascades have the same bug shape: credit ALL PV at one rate.
The spec-correct fix is uniformly: split E_PV by β; onsite at IMPORT
factor, exported at EXPORT factor. The shared cross-cascade API:
- `CalculatorInputs.pv_dwelling_kwh_per_yr` + `pv_exported_kwh_per_yr`
are the cross-cascade-shared β-split state.
- Each cascade has its own pair of factor fields:
- PE: `other_primary_factor` (IMPORT) + `pv_export_primary_factor` (EXPORT)
- CO2: `pv_dwelling_co2_factor_kg_per_kwh` + `pv_exported_co2_factor_kg_per_kwh`
- Cost: `pv_dwelling_import_price_gbp_per_kwh` + `pv_export_credit_gbp_per_kwh`
- `None` on any factor field falls back to the legacy single-rate
behaviour (preserves synthetic CalculatorInputs constructions in
unit tests).
The EXPORT factor is keyed on **Table 12 code 60** ("electricity sold
to grid, PV") at all three cascades — already present in
[`table_12.py`](../tables/table_12.py).
- **Pyright net-zero per touched file** ([[feedback-zero-error-strict]])