From bda8a3f276e719a1917758c17e2121e05adbd5fc Mon Sep 17 00:00:00 2001
From: Khalim Conn-Kowlessar <kconnkowlessar@gmail.com>
Date: Wed, 24 Jun 2026 17:14:51 +0000
Subject: [PATCH] =?UTF-8?q?Expose=20an=20off-peak=20meter's=20standing=20c?=
 =?UTF-8?q?harge=20and=20full=20day/night=20blend=20range=20=F0=9F=9F=A9?=
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 domain/fuel_rates/fuel_rates.py            |  4 +++
 tests/domain/fuel_rates/test_fuel_rates.py | 34 ++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/domain/fuel_rates/fuel_rates.py b/domain/fuel_rates/fuel_rates.py
index 81f78ab0..2266b75a 100644
--- a/domain/fuel_rates/fuel_rates.py
+++ b/domain/fuel_rates/fuel_rates.py
@@ -69,6 +69,10 @@ class FuelRates:
         return self._rate(fuel).unit_rate_p_per_kwh
 
     def standing_charge_p_per_day(self, fuel: Fuel) -> float:
+        if fuel is Fuel.ELECTRICITY_OFF_PEAK:
+            if self.off_peak is None:
+                raise UnpricedFuel(fuel)
+            return self.off_peak.standing_charge_p_per_day
         return self._rate(fuel).standing_charge_p_per_day
 
     def _rate(self, fuel: Fuel) -> FuelRate:
diff --git a/tests/domain/fuel_rates/test_fuel_rates.py b/tests/domain/fuel_rates/test_fuel_rates.py
index 432fcb63..55472aeb 100644
--- a/tests/domain/fuel_rates/test_fuel_rates.py
+++ b/tests/domain/fuel_rates/test_fuel_rates.py
@@ -36,6 +36,40 @@ def test_off_peak_blended_rate_at_zero_high_fraction_is_the_night_rate() -> None
     assert blended == 13.89
 
 
+def test_off_peak_blended_rate_at_full_high_fraction_is_the_day_rate() -> None:
+    # Arrange — an all-day load (high-rate fraction 1.0).
+    rates = _off_peak_rates()
+
+    # Act / Assert — every kWh bills at the dearer day rate.
+    assert rates.off_peak_blended_p_per_kwh(high_rate_fraction=1.0) == 29.73
+
+
+def test_off_peak_blended_rate_blends_day_and_night_linearly() -> None:
+    # Arrange — 25% of the load at the day rate, 75% at night.
+    rates = _off_peak_rates()
+
+    # Act / Assert — 0.25 * 29.73 + 0.75 * 13.89 = 17.85
+    assert rates.off_peak_blended_p_per_kwh(high_rate_fraction=0.25) == pytest.approx(17.85)
+
+
+def test_off_peak_blend_without_a_snapshot_off_peak_entry_raises_unpriced_fuel() -> None:
+    # Arrange — a snapshot that carries no off-peak entry at all.
+    rates = _rates()
+
+    # Act / Assert
+    with pytest.raises(UnpricedFuel) as excinfo:
+        rates.off_peak_blended_p_per_kwh(high_rate_fraction=0.5)
+    assert excinfo.value.fuel is Fuel.ELECTRICITY_OFF_PEAK
+
+
+def test_off_peak_meter_standing_charge_reads_back() -> None:
+    # Arrange — the bill adds the off-peak meter's standing charge once per meter.
+    rates = _off_peak_rates()
+
+    # Act / Assert
+    assert rates.standing_charge_p_per_day(Fuel.ELECTRICITY_OFF_PEAK) == 56.99
+
+
 def test_unit_rate_and_standing_charge_read_back_for_a_priced_fuel() -> None:
     # Arrange
     rates = _rates()