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

@@ -35,7 +35,10 @@ class OffPeakRate:
     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
+        return (
+            high_rate_fraction * self.day_p_per_kwh
+            + (1.0 - high_rate_fraction) * self.night_p_per_kwh
+        )
 
 
 @dataclass(frozen=True)
@@ -58,7 +61,9 @@ class FuelRates:
         """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
+        if self.off_peak is None:
+            raise UnpricedFuel(Fuel.ELECTRICITY_OFF_PEAK)
+        return self.off_peak.blended_p_per_kwh(high_rate_fraction)
 
     def unit_rate_p_per_kwh(self, fuel: Fuel) -> float:
         return self._rate(fuel).unit_rate_p_per_kwh