Blend an off-peak meter's day/night rate by high-rate fraction 🟥

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

domain/fuel_rates/fuel_rates.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 from collections.abc import Mapping
 from dataclasses import dataclass
+from typing import Optional
 
 from domain.fuel_rates.fuel import Fuel, UnpricedFuel
 
@@ -18,6 +19,25 @@ class FuelRate:
     standing_charge_p_per_day: float
 
 
+@dataclass(frozen=True)
+class OffPeakRate:
+    """An Off-Peak Meter's dual-rate tariff — a cheaper ``night`` (low) rate and
+    a dearer ``day`` (high) rate, plus the daily standing charge (ADR-0014).
+
+    Off-peak electricity has no single unit rate, so it does not live in the
+    single-rate ``FuelRates.rates`` map; each end use blends day/night by its own
+    High-Rate Fraction (a calculator output)."""
+
+    day_p_per_kwh: float
+    night_p_per_kwh: float
+    standing_charge_p_per_day: float
+
+    def blended_p_per_kwh(self, high_rate_fraction: float) -> float:
+        """Effective p/kWh for an end use billing ``high_rate_fraction`` of its
+        kWh at the day (high) rate and the remainder at the night (low) rate."""
+        raise NotImplementedError
+
+
 @dataclass(frozen=True)
 class FuelRates:
     """A current Fuel Rates snapshot — the rate per billing Fuel plus the SEG
@@ -32,6 +52,13 @@ class FuelRates:
     period: str
     seg_export_p_per_kwh: float
     rates: Mapping[Fuel, FuelRate]
+    off_peak: Optional[OffPeakRate] = None
+
+    def off_peak_blended_p_per_kwh(self, high_rate_fraction: float) -> float:
+        """Blended day/night p/kWh for an Off-Peak Meter end use at the given
+        High-Rate Fraction. Raises ``UnpricedFuel`` when the snapshot carries no
+        off-peak entry."""
+        raise NotImplementedError
 
     def unit_rate_p_per_kwh(self, fuel: Fuel) -> float:
         return self._rate(fuel).unit_rate_p_per_kwh

tests/domain/fuel_rates/test_fuel_rates.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
 
 
 def _rates() -> FuelRates:
@@ -14,6 +14,28 @@ def _rates() -> FuelRates:
     )
 
 
+def _off_peak_rates() -> FuelRates:
+    return FuelRates(
+        period="test",
+        seg_export_p_per_kwh=15.0,
+        rates={},
+        off_peak=OffPeakRate(
+            day_p_per_kwh=29.73, night_p_per_kwh=13.89, standing_charge_p_per_day=56.99
+        ),
+    )
+
+
+def test_off_peak_blended_rate_at_zero_high_fraction_is_the_night_rate() -> None:
+    # Arrange — an all-night load (storage heating, high-rate fraction 0.0).
+    rates = _off_peak_rates()
+
+    # Act
+    blended = rates.off_peak_blended_p_per_kwh(high_rate_fraction=0.0)
+
+    # Assert — every kWh bills at the cheap night rate.
+    assert blended == 13.89
+
+
 def test_unit_rate_and_standing_charge_read_back_for_a_priced_fuel() -> None:
     # Arrange
     rates = _rates()