Model/docs/migrations/property-baseline-performance-table.md

# `property_baseline_performance` table — FE-owned migration

**Context:** Slice 6 (Hestia-Homes/Model#1135) of the `ara_first_run` rebuild. The
`PropertyBaselineOrchestrator` establishes a Property's **Baseline Performance** (ADR-0004) and persists it
via a new `PropertyBaselineRepository` port. This is a brand-new table — no predecessor.

Per ADR-0004's amendment, the lodged/effective pair does **not** land on `property_details_epc`
(which is being retired as too coupled to the legacy EPC-API schema). It lands here, as its own
aggregate's table.

The SQLModel row is defined in `infrastructure/postgres/` so the ephemeral-Postgres tests build it
via `SQLModel.metadata.create_all`. The **production migration is FE-owned (Drizzle ORM)** — a
straight lift-and-shift of the columns below.

## `property_baseline_performance` — one row per Property

| Column | Type | Notes |
|---|---|---|
| `id` | serial PK | |
| `property_id` | int, FK → `property.id`, **unique** | one Baseline Performance per Property |
| `lodged_sap_score` | int | Lodged Performance — gov register, off the Effective EPC |
| `lodged_epc_band` | text | the `Epc` enum, stored as its string value (e.g. `"C"`) |
| `lodged_co2_emissions_t_per_yr` | float | tonnes CO₂/yr (whole dwelling) |
| `lodged_primary_energy_intensity_kwh_per_m2_yr` | int | PEUI (kWh/m²/yr); **not** "heat demand" — see CONTEXT.md |
| `effective_sap_score` | int | Effective Performance — what modelling scored against |
| `effective_epc_band` | text | |
| `effective_co2_emissions_t_per_yr` | float | tonnes CO₂/yr (whole dwelling) |
| `effective_primary_energy_intensity_kwh_per_m2_yr` | int | kWh/m²/yr |
| `rebaseline_reason` | text | `none` \| `pre_sap10` \| `physical_state_changed` \| `both` |
| `space_heating_kwh` | float | EPC `renewable_heat_incentive` recorded demand. **Superseded** by `heating_kwh` (delivered) when the bill block populates; kept until then to avoid an empty-kWh gap, dropped in the population slice. |
| `water_heating_kwh` | float | EPC `renewable_heat_incentive`; **superseded** by `hot_water_kwh`. |

### Bill block (ADR-0014) — the energy bill, composed per section

Produced by **Bill Derivation**: the calculator's **delivered** kWh per end use priced at current
**Fuel Rates** (a committed snapshot, not SAP's standardised prices), per section + the total.
Per-section kWh is *delivered fuel* (demand ÷ efficiency — what the household pays for), distinct
from the recorded-demand `space_heating_kwh`/`water_heating_kwh` above which it supersedes.

All columns below are **nullable** (every one is `Optional[float]`, default `None`) and **FE-owned
(Drizzle)**. The `bill_` prefix is deliberate: it keeps the per-section columns from clashing with
the recorded-demand `space_heating_kwh` / `water_heating_kwh` above. The whole block is `None` for
one row together when no calculator ran (the stub path produced no `SapResult` to price); a section
absent from the bill leaves its two columns `None` (not `0` — it was not billed). `to_domain` uses
`bill_total_annual_bill_gbp IS NOT NULL` as the discriminator for "a bill was persisted".

| Column | Type | Notes |
|---|---|---|
| `bill_heating_kwh` | float, nullable | delivered fuel kWh (main + main-2 + secondary heating) |
| `bill_heating_cost_gbp` | float, nullable | priced at the heating fuel's current rate |
| `bill_hot_water_kwh` | float, nullable | |
| `bill_hot_water_cost_gbp` | float, nullable | |
| `bill_lighting_kwh` | float, nullable | |
| `bill_lighting_cost_gbp` | float, nullable | |
| `bill_appliances_kwh` | float, nullable | unregulated load — `None` until the appliances field lands on `SapResult` |
| `bill_appliances_cost_gbp` | float, nullable | |
| `bill_cooking_kwh` | float, nullable | unregulated load — `None` until `SapResult` carries it |
| `bill_cooking_cost_gbp` | float, nullable | |
| `bill_pumps_fans_kwh` | float, nullable | |
| `bill_pumps_fans_cost_gbp` | float, nullable | |
| `bill_cooling_kwh` | float, nullable | mostly absent in UK homes; carried for completeness as it affects the bill |
| `bill_cooling_cost_gbp` | float, nullable | |
| `bill_standing_charges_gbp` | float, nullable | daily standing charge × 365, once per distinct metered fuel (off-gas fuels have none) |
| `bill_seg_credit_gbp` | float, nullable | SEG export credit on PV (subtracted) |
| `bill_total_annual_bill_gbp` | float, nullable | Σ section costs + standing charges − SEG; the not-null discriminator for a persisted bill |

The calculator is **load-bearing** (ADR-0013 amendment): for `sap_version < 10.2` the `effective_*`
columns hold the calculator's output (so `effective_* != lodged_*` legitimately); at/above 10.2 they
mirror the lodged figures and divergence is logged. A cert the calculator cannot score aborts the
batch rather than persisting a wrong row.

### Population timing

The bill columns are now **populated**: the `PropertyBaselineOrchestrator` reads the current Fuel
Rates snapshot, builds a `BillDerivation`, and prices every scored property's `SapResult` →
`EnergyBreakdown` into a `Bill` that `from_domain` flattens onto these columns. They stay `None`
together only on the stub (no-calculator) path. The appliances / cooking sections remain `None`
until those fields land on `SapResult`. The Drizzle migration creates all `bill_*` columns nullable.