From dc55d3b8994de7d598e726fd565ecf6d71f00fc5 Mon Sep 17 00:00:00 2001
From: Khalim Conn-Kowlessar <kconnkowlessar@gmail.com>
Date: Wed, 24 Jun 2026 17:24:50 +0000
Subject: [PATCH] =?UTF-8?q?Price=20an=20off-peak=20meter=20line=20day/nigh?=
 =?UTF-8?q?t=20by=20its=20high-rate=20fraction=20=F0=9F=9F=A9?=
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Content-Type: text/plain; charset=UTF-8
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 domain/billing/bill.py                       |  8 +++++-
 domain/billing/bill_derivation.py            | 17 ++++++++++---
 tests/domain/billing/test_bill_derivation.py | 26 +++++++++++++++++++-
 3 files changed, 46 insertions(+), 5 deletions(-)

diff --git a/domain/billing/bill.py b/domain/billing/bill.py
index 5aff24cf..104bece2 100644
--- a/domain/billing/bill.py
+++ b/domain/billing/bill.py
@@ -28,11 +28,17 @@ class BillSection(Enum):
 @dataclass(frozen=True)
 class EnergyLine:
     """One section's delivered energy on one fuel. A section may have more than
-    one line (e.g. gas main heating + electric secondary heating)."""
+    one line (e.g. gas main heating + electric secondary heating).
+
+    ``high_rate_fraction`` is the calculator's High-Rate Fraction for this end
+    use — the share of its kWh billed at the day (high) rate on an Off-Peak
+    Meter — set only on ``ELECTRICITY_OFF_PEAK`` lines; ``None`` for single-rate
+    fuels, which bill at their flat unit rate."""
 
     section: BillSection
     fuel: Fuel
     kwh: float
+    high_rate_fraction: Optional[float] = None
 
 
 @dataclass(frozen=True)
diff --git a/domain/billing/bill_derivation.py b/domain/billing/bill_derivation.py
index c1a09c64..a73b7d4e 100644
--- a/domain/billing/bill_derivation.py
+++ b/domain/billing/bill_derivation.py
@@ -10,6 +10,7 @@ from domain.billing.bill import (
     BillSection,
     BillSectionCost,
     EnergyBreakdown,
+    EnergyLine,
 )
 
 _DAYS_PER_YEAR: Final[float] = 365.0
@@ -36,9 +37,7 @@ class BillDerivation:
         fuels_used: set[Fuel] = set()
         for line in breakdown.lines:
             section_kwh[line.section] += line.kwh
-            section_cost_p[line.section] += (
-                line.kwh * self._rates.unit_rate_p_per_kwh(line.fuel)
-            )
+            section_cost_p[line.section] += line.kwh * self._unit_rate_p_per_kwh(line)
             if line.kwh > 0:
                 fuels_used.add(line.fuel)
 
@@ -69,3 +68,15 @@ class BillDerivation:
             seg_credit_gbp=seg_credit_gbp,
             total_gbp=total_gbp,
         )
+
+    def _unit_rate_p_per_kwh(self, line: EnergyLine) -> float:
+        """Price one line's fuel (p/kWh). An Off-Peak Meter line blends day/night
+        by its High-Rate Fraction; every other fuel bills at its flat unit rate."""
+        if line.fuel is Fuel.ELECTRICITY_OFF_PEAK:
+            if line.high_rate_fraction is None:
+                raise ValueError(
+                    f"{line.section.value} bills on an off-peak meter but carries "
+                    "no high-rate fraction; cannot split day/night"
+                )
+            return self._rates.off_peak_blended_p_per_kwh(line.high_rate_fraction)
+        return self._rates.unit_rate_p_per_kwh(line.fuel)
diff --git a/tests/domain/billing/test_bill_derivation.py b/tests/domain/billing/test_bill_derivation.py
index cce045ee..14efea71 100644
--- a/tests/domain/billing/test_bill_derivation.py
+++ b/tests/domain/billing/test_bill_derivation.py
@@ -3,7 +3,7 @@ from __future__ import annotations
 import pytest
 
 from domain.fuel_rates.fuel import Fuel, UnpricedFuel
-from domain.fuel_rates.fuel_rates import FuelRate, FuelRates
+from domain.fuel_rates.fuel_rates import FuelRate, FuelRates, OffPeakRate
 from domain.billing.bill import BillSection, EnergyBreakdown, EnergyLine
 from domain.billing.bill_derivation import BillDerivation
 
@@ -17,9 +17,33 @@ def _rates() -> FuelRates:
             Fuel.ELECTRICITY: FuelRate(unit_rate_p_per_kwh=24.67, standing_charge_p_per_day=57.21),
             Fuel.OIL: FuelRate(unit_rate_p_per_kwh=9.16, standing_charge_p_per_day=0.0),
         },
+        off_peak=OffPeakRate(
+            day_p_per_kwh=29.73, night_p_per_kwh=13.89, standing_charge_p_per_day=56.99
+        ),
     )
 
 
+def test_an_all_night_off_peak_heating_line_bills_at_the_night_rate() -> None:
+    # Arrange — 10,000 kWh of electric storage heating on an Off-Peak Meter,
+    # charged wholly overnight (high-rate fraction 0.0).
+    breakdown = EnergyBreakdown(
+        lines=[
+            EnergyLine(
+                section=BillSection.HEATING,
+                fuel=Fuel.ELECTRICITY_OFF_PEAK,
+                kwh=10000.0,
+                high_rate_fraction=0.0,
+            )
+        ]
+    )
+
+    # Act
+    bill = BillDerivation(_rates()).derive(breakdown)
+
+    # Assert — every kWh at the night rate: 10000 × 13.89p = £1389.
+    assert bill.sections[BillSection.HEATING].cost_gbp == pytest.approx(1389.0)
+
+
 def test_derive_prices_a_single_gas_heating_line_with_its_standing_charge() -> None:
     # Arrange — 10,000 kWh of mains-gas heating.
     breakdown = EnergyBreakdown(