energy.water-bill
Metered water and sewerage bill from two reads: volumetric charges, return to sewer, standing and surface water charges.
1.0.0 · published 2026-10-03 by charlie · Anterra
Pinned by 14 tests, run in TypeScript, Python and Rust.
What it does
A metered water and sewerage bill for England and Wales, from the reads at the start and end of the period and the water company's charges:
| line | how | |---|---| | water volume | m³ used x water rate per m³ | | sewerage volume | m³ used x return to sewer % x sewerage rate per m³ | | water standing charge | annual charge x days / 365 | | sewerage standing charge | annual charge x days / 365 | | surface water drainage | annual charge x days / 365, unless the property's surface water does not drain to the public sewer |
For example
water_bill(date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000…)→ cubic metres 123, days 182, water volume charge £255.05, sewerage volume charge £292.13, water standing charge £14.96, sewerage standing charge £19.95, surface water charge £27.42… six months, 123 m³: sewerage on 95% is exactly 29212.5p and rounds upwater_bill(date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000…)→ cubic metres 123, days 182, water volume charge £255.05, sewerage volume charge £292.13, water standing charge £14.96, sewerage standing charge £19.95, surface water charge £0.00,… surface water that does not reach the sewer is not chargedwater_bill(date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000…)→ cubic metres 123, days 182, water volume charge £255.05, sewerage volume charge £292.12, water standing charge £14.95, sewerage standing charge £19.94, surface water charge £27.42… rounding down every line
The function
The same function in TypeScript, Python and Rust, pinned by the same tests. Pick your language; the choice follows you around the registry.
def water_bill(opening_read: MeterRead, closing_read: MeterRead, meter_digits: int, tariff: WaterTariff, surface_water_drains: bool, currency: str, mode: RoundingMode) -> WaterBill| opening_read | MeterRead | the read at the start of the period, whole cubic metres |
| closing_read | MeterRead | the read at the end; lower means the meter rolled over |
| meter_digits | int | whole-m³ digits on the register, 1 to 15 |
| tariff | WaterTariff | the water company's charges for the period |
| surface_water_drains | bool | false when rainwater from the property does not reach the public sewer: no surface water charge |
| currency | string | the currency of the tariff |
| mode | RoundingMode | how each charge rounds to a whole minor unit |
| returns | WaterBill |
The types it declares, generated into your project
@dataclass(frozen=True)
class MeterRead:
"""One meter read."""
date: str
#: the register, whole cubic metres
value: int
@dataclass(frozen=True)
class WaterTariff:
"""A metered water and sewerage tariff, as a water company's charges scheme lists it."""
#: water supply per m³, in thousandths of a minor unit: £2.0736 is 207360
water_volume_rate: int
#: sewerage per m³ returned to the sewer, in thousandths of a minor unit
sewerage_volume_rate: int
#: water supply standing charge per year, in minor units
water_standing_charge: int
#: sewerage standing charge per year, in minor units
sewerage_standing_charge: int
#: surface water drainage charge per year, in minor units; 0 if not separately charged
surface_water_charge: int
#: share of water charged as returned to the sewer, basis points: 95% is 9500
return_to_sewer: int
@dataclass(frozen=True)
class WaterBill:
"""The bill, one charge per line."""
#: water used
cubic_metres: int
#: days between the reads
days: int
water_volume_charge: Money
#: on cubicMetres x returnToSewer
sewerage_volume_charge: Money
#: annual charge x days / 365
water_standing_charge: Money
#: annual charge x days / 365
sewerage_standing_charge: Money
#: annual charge x days / 365, or zero when surface water does not drain to the sewer
surface_water_charge: Money
#: the sum of the lines
total: MoneyYour code names it in one line, in the file that uses it
from fune.energy.water_bill import water_bill # energy.water-bill@^1Imports name this capability’s declared dependencies, which fune builds next to it in your project; each one links to its page.
from .dates_days_between import days_between ← from dates.days-between ^1.0.0 · built alongside by fune
from .energy_meter_advance import meter_advance ← from energy.meter-advance ^1.0.0 · built alongside by fune
from .energy_water_bill_types import MeterRead, WaterBill, WaterTariff
from .math_round_div import RoundingMode, round_div ← from math.round-div ^1.0.0 · built alongside by fune
from .money_amount import money ← from money.amount ^1.0.0 · built alongside by fune
MAX_SAFE = 9007199254740991
def _check_rate(what: str, value: object) -> None:
if not isinstance(value, int) or isinstance(value, bool) or value < 0:
raise ValueError("%s must be a whole number of 0 or more, received %s" % (what, value))
def _exact(product: int) -> int:
if product > MAX_SAFE:
raise ValueError("water charge too large to calculate exactly")
return product
def water_bill(
opening_read: MeterRead,
closing_read: MeterRead,
meter_digits: int,
tariff: WaterTariff,
surface_water_drains: bool,
currency: str,
mode: RoundingMode,
) -> WaterBill:
"""A metered water and sewerage bill: volumetric water, sewerage on the
share returned to the sewer, and standing and surface water charges
apportioned by days / 365, each line rounded once.
"""
cubic_metres = meter_advance(opening_read.value, closing_read.value, meter_digits)
days = days_between(opening_read.date, closing_read.date)
if days < 0:
raise ValueError(
"the closing read must not be dated before the opening read, received %s and %s"
% (opening_read.date, closing_read.date)
)
_check_rate("waterVolumeRate", tariff.water_volume_rate)
_check_rate("sewerageVolumeRate", tariff.sewerage_volume_rate)
_check_rate("waterStandingCharge", tariff.water_standing_charge)
_check_rate("sewerageStandingCharge", tariff.sewerage_standing_charge)
_check_rate("surfaceWaterCharge", tariff.surface_water_charge)
share = tariff.return_to_sewer
if not isinstance(share, int) or isinstance(share, bool) or share < 0 or share > 10000:
raise ValueError("returnToSewer must be basis points from 0 to 10000, received %s" % (share,))
water = round_div(_exact(cubic_metres * tariff.water_volume_rate), 1000, mode)
sewerage = round_div(_exact(cubic_metres * tariff.sewerage_volume_rate * share), 10_000_000, mode)
water_standing = round_div(_exact(tariff.water_standing_charge * days), 365, mode)
sewerage_standing = round_div(_exact(tariff.sewerage_standing_charge * days), 365, mode)
surface = round_div(_exact(tariff.surface_water_charge * days), 365, mode) if surface_water_drains else 0
return WaterBill(
cubic_metres=cubic_metres,
days=days,
water_volume_charge=money(water, currency),
sewerage_volume_charge=money(sewerage, currency),
water_standing_charge=money(water_standing, currency),
sewerage_standing_charge=money(sewerage_standing, currency),
surface_water_charge=money(surface, currency),
total=money(water + sewerage + water_standing + sewerage_standing + surface, currency),
)Install
fune buildWith that line in your source, in a Python project (language python in fune.project), fune build resolves it and its 4 dependencies, pins them in fune.lock, downloads only the Python package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:
fune add energy.water-billThe manifest, vectors and README with only the Python implementation. Install it without the registry with fune add ./energy.water-bill-1.0.0-python.fune, or fetch it from a terminal with fune pull energy.water-bill@1.0.0:python.
The whole function, every language, is one file too: energy.water-bill-1.0.0.fune, 28,608 bytes, sha256 79d9f71e0a111957c20dff84b0ca2a486863c3ca44ebdd8b2b69095d390ad594. It installs into a project of any language.
Customise it in your app
The seams this capability offers. Put a marker directly above a function of your own and fune build wires it into the built code; the package on the registry is not changed, the built file’s header lists it under CUSTOMISED, and fune hooks lists every hook in the project. How hooks work.
before — your function gets the arguments and returns them, changed or not, or throws to refuse the call.
# fune: before energy.water-billafter — your function gets the result and the arguments, and returns the final result.
# fune: after energy.water-billreplace — inside this capability’s code only, calls to a dependency go to your function, with the same signature. Other capabilities that use it are unaffected; write in * to replace it everywhere.
# fune: replace dates.days-between in energy.water-bill
# fune: replace energy.meter-advance in energy.water-bill
# fune: replace math.round-div in energy.water-bill
# fune: replace money.amount in energy.water-billstep — your function runs at a numbered point inside the function’s body, receives the in-scope values it names as parameters, and may return replacements. List the points with fune show energy.water-bill --steps.
# fune: step energy.water-bill after <n|label>Tests
A version published now needs at least 8 tests for every function, and one that expects the error for each function that throws; the registry refuses it otherwise. fune verify --all runs each case in TypeScript, Python and Rust, and a project runs them again with fune verify. This page lists the cases; it does not run them. The exact JSON is vectors.json.
| Case | Arguments | Expected | |
|---|---|---|---|
| six months, 123 m³: sewerage on 95% is exactly 29212.5p and rounds up | date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000… | → | cubic metres 123, days 182, water volume charge £255.05, sewerage volume charge £292.13, water standing charge £14.96, sewerage standing charge £19.95, surface water charge £27.42… |
| surface water that does not reach the sewer is not charged | date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000… | → | cubic metres 123, days 182, water volume charge £255.05, sewerage volume charge £292.13, water standing charge £14.96, sewerage standing charge £19.95, surface water charge £0.00,… |
| rounding down every line | date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000… | → | cubic metres 123, days 182, water volume charge £255.05, sewerage volume charge £292.12, water standing charge £14.95, sewerage standing charge £19.94, surface water charge £27.42… |
| a meter rolling over past 99999 | date 2026-01-01, value 99,990, date 2026-01-31, value 10, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000, … | → | cubic metres 20, days 30, water volume charge £41.47, sewerage volume charge £47.50, water standing charge £2.47, sewerage standing charge £3.29, surface water charge £4.52, total… |
| a leap year of standing charge is 366/365 of the annual charge | date 2024-01-01, value 500, date 2025-01-01, value 500, 5, water volume rate 0, sewerage volume rate 0, water standing charge 3,000, sewerage standing charge 0, surface water char… | → | cubic metres 0, days 366, water volume charge £0.00, sewerage volume charge £0.00, water standing charge £30.08, sewerage standing charge £0.00, surface water charge £0.00, total … |
| an ordinary year of standing charges is exactly the annual charges | date 2025-04-01, value 500, date 2026-04-01, value 500, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000, su… | → | cubic metres 0, days 365, water volume charge £0.00, sewerage volume charge £0.00, water standing charge £30.00, sewerage standing charge £40.00, surface water charge £55.00, tota… |
| a business with no water returned to the sewer pays no sewerage volume | date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000… | → | cubic metres 123, days 182, water volume charge £255.05, sewerage volume charge £0.00, water standing charge £14.96, sewerage standing charge £19.95, surface water charge £27.42, … |
| reads on the same day with no use cost nothing | date 2026-04-01, value 42, date 2026-04-01, value 42, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000, surf… | → | cubic metres 0, days 0, water volume charge £0.00, sewerage volume charge £0.00, water standing charge £0.00, sewerage standing charge £0.00, surface water charge £0.00, total £0.… |
| a closing read dated before the opening read is refused | date 2026-09-30, value 1,123, date 2026-04-01, value 1,000, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000… | → | error: the closing read must not be dated before the opening read |
| a return to sewer over 100% is refused | date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000… | → | error: returnToSewer must be basis points from 0 to 10000 |
Show the other 4 tests
| Case | Arguments | Expected | |
|---|---|---|---|
| a negative rate is refused | date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate -1, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000, sur… | → | error: waterVolumeRate must be a whole number of 0 or more |
| a fractional standing charge is refused | date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000.5, sewerage standing charge 4,0… | → | error: waterStandingCharge must be a whole number of 0 or more |
| a read that does not fit the register is refused | date 2026-04-01, value 1,000, date 2026-09-30, value 100,000, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,0… | → | error: reads must be whole numbers from 0 to 99999 |
| a lowercase currency is refused | date 2026-04-01, value 1,000, date 2026-09-30, value 1,123, 5, water volume rate 207,360, sewerage volume rate 250,000, water standing charge 3,000, sewerage standing charge 4,000… | → | error: is not an uppercase ISO 4217 currency code |
More from the author
The rates are the caller's: every company publishes its own charges scheme each April, and they differ by company and by customer type.
## Decisions
- **Return to sewer.** Sewerage is charged on the water assumed to go back to the sewer, usually 95% of what is supplied (90% for some, a site-specific figure for many businesses). It is a tariff field in basis points, and the sewerage volume charge is computed exactly on `m³ x rate x share` with one rounding, not on a rounded volume. - **Surface water drainage allowance.** Where no rainwater from the property drains to the public sewer (it goes to a soakaway, say), the company must not charge for surface water drainage. Pass `surfaceWaterDrains: false` and that line is zero. Companies that fold surface water into the sewerage standing charge publish a reduced sewerage charge for such properties instead; pass that tariff and a zero `surfaceWaterCharge`. - **Days.** The period is the days between the two read dates (`dates.days-between`): reads on 1 April and 30 September are 182 days. Annual charges are apportioned by `days / 365` in every year, including a leap year, which is the usual daily-apportionment rule in charges schemes; a company that apportions by 366 in a leap year will differ by a penny or so. - **Rollover** is handled by `energy.meter-advance`: a lower closing read is a meter that went round once. - **Each line is rounded once** by `mode` and the total is the sum of the lines, as the bill prints them. - **No VAT.** Water and sewerage services to households are zero-rated, and most business supplies are too; if yours are not, apply VAT to the total.
## Sources
- Ofwat, "Surface water and highway drainage", https://www.ofwat.gov.uk/households/your-water-bill/surfacewaterdrainage/ ("If no surface water from your property enters a public sewer then you may qualify for a reduction in your sewerage charge"). - Ofwat, "Metered customers and applying for metered charges", https://www.ofwat.gov.uk/households/your-water-bill/metering/ - The rates, standing charges, return-to-sewer percentage and apportionment rule are each company's own, in its annual charges scheme.
Files
| Path | Bytes |
|---|---|
| README.md | 2,758 |
| impl/python.py | 2,948 |
| impl/rust.rs | 5,152 |
| impl/typescript.ts | 2,871 |
| vectors.json | 8,328 |