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82 lines
3 KiB
Python
82 lines
3 KiB
Python
from __future__ import annotations
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from collections import defaultdict
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from typing import Final
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from domain.fuel_rates.fuel import Fuel
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from domain.fuel_rates.fuel_rates import FuelRates
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from domain.billing.bill import (
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Bill,
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BillSection,
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BillSectionCost,
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EnergyBreakdown,
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EnergyLine,
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)
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_DAYS_PER_YEAR: Final[float] = 365.0
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_PENCE_PER_POUND: Final[float] = 100.0
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class BillDerivation:
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"""Derives a Property's annual energy Bill by pricing a delivered-energy
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breakdown at current Fuel Rates (ADR-0014).
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Each end-use line is billed at its fuel's unit rate; **standing charges are
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added once per distinct fuel used** (a meter, not an end use — off-gas fuels
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carry a 0 standing charge so they contribute nothing); the SEG export credit
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is subtracted. Deterministic (ADR-0006). Raises ``UnpricedFuel`` (via
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``FuelRates``) on a fuel the snapshot does not price.
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"""
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def __init__(self, fuel_rates: FuelRates) -> None:
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self._rates = fuel_rates
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def derive(self, breakdown: EnergyBreakdown) -> Bill:
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section_kwh: defaultdict[BillSection, float] = defaultdict(float)
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section_cost_p: defaultdict[BillSection, float] = defaultdict(float)
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fuels_used: set[Fuel] = set()
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for line in breakdown.lines:
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section_kwh[line.section] += line.kwh
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section_cost_p[line.section] += line.kwh * self._unit_rate_p_per_kwh(line)
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if line.kwh > 0:
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fuels_used.add(line.fuel)
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sections = {
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section: BillSectionCost(
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kwh=section_kwh[section], cost_gbp=section_cost_p[section] / _PENCE_PER_POUND
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)
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for section in section_kwh
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}
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standing_charges_gbp = (
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sum(
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(self._rates.standing_charge_p_per_day(fuel) * _DAYS_PER_YEAR for fuel in fuels_used),
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0.0,
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)
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/ _PENCE_PER_POUND
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)
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seg_credit_gbp = (
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breakdown.exported_kwh * self._rates.seg_export_p_per_kwh / _PENCE_PER_POUND
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)
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total_gbp = (
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sum((section.cost_gbp for section in sections.values()), 0.0)
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+ standing_charges_gbp
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- seg_credit_gbp
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)
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return Bill(
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sections=sections,
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standing_charges_gbp=standing_charges_gbp,
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seg_credit_gbp=seg_credit_gbp,
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total_gbp=total_gbp,
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)
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def _unit_rate_p_per_kwh(self, line: EnergyLine) -> float:
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"""Price one line's fuel (p/kWh). An Off-Peak Meter line blends day/night
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by its High-Rate Fraction; every other fuel bills at its flat unit rate."""
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if line.fuel is Fuel.ELECTRICITY_OFF_PEAK:
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if line.high_rate_fraction is None:
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raise ValueError(
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f"{line.section.value} bills on an off-peak meter but carries "
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"no high-rate fraction; cannot split day/night"
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)
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return self._rates.off_peak_blended_p_per_kwh(line.high_rate_fraction)
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return self._rates.unit_rate_p_per_kwh(line.fuel)
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