energy.price-cap

priceCap("electricity", "direct-debit", "2026-05-01")
  -> unitRate 24670 (24.67p/kWh), standingCharge 57210 (57.21p/day), vatBasisPoints 500

export function priceCap(fuel: EnergyFuel, paymentMethod: PaymentMethod, onDate: string): PriceCap

export type EnergyFuel = "electricity" | "gas";

export type PaymentMethod = "direct-debit" | "standard-credit" | "prepayment";

/** The capped rates for one cap period, as Ofgem publishes them. */
export interface PriceCap {
  readonly fuel: string;
  readonly paymentMethod: string;
  /** GB: the England, Scotland and Wales average */
  readonly region: string;
  /** thousandths of a penny per kWh: 24.67p is 24670 */
  readonly unitRate: number;
  /** thousandths of a penny per day: 57.21p is 57210 */
  readonly standingCharge: number;
  /** VAT included in both figures: 500, or 0 where none applied */
  readonly vatBasisPoints: number;
  /** first day of the cap period */
  readonly validFrom: string;
  /** last day of the cap period */
  readonly validTo: string;
}

import { priceCap } from "#fune/energy.price-cap@^1";

import { PRICE_CAP, PRICE_CAP_HISTORY } from "./energy_price_cap_data.ts";  ← this capability’s own data, compiled from data/price-cap.json into the same file by fune build
import { type EnergyFuel, type PaymentMethod, type PriceCap } from "./energy_price_cap_types.ts";

const ISO_DATE = /^\d{4}-\d{2}-\d{2}$/;
const FUELS = ["electricity", "gas"];
const METHODS = ["direct-debit", "standard-credit", "prepayment"];

/** The Ofgem price cap unit rate and standing charge in force on `onDate`. */
export function priceCap(fuel: EnergyFuel, paymentMethod: PaymentMethod, onDate: string): PriceCap {
  if (!ISO_DATE.test(onDate)) {
    throw new RangeError(`onDate must be an ISO date (YYYY-MM-DD), received "${onDate}"`);
  }
  if (!FUELS.includes(fuel)) {
    throw new RangeError(`unknown fuel "${fuel}": expected electricity or gas`);
  }
  if (!METHODS.includes(paymentMethod)) {
    throw new RangeError(`unknown payment method "${paymentMethod}": expected direct-debit, standard-credit or prepayment`);
  }
  const rows = PRICE_CAP.filter((r) => r.fuel === fuel && r.paymentMethod === paymentMethod);
  if (rows.length === 0) {
    throw new RangeError(`no price cap figures for ${fuel} paid by ${paymentMethod}: this version carries GB-average Direct Debit figures only`);
  }
  const row = rows.find((r) => r.validFrom <= onDate && onDate <= r.validTo);
  if (row === undefined) {
    // A pruned build must not answer an old date with a newer cap.
    if (PRICE_CAP_HISTORY !== "full") {
      const earliest = rows.reduce((e, r) => (r.validFrom < e ? r.validFrom : e), rows[0].validFrom);
      if (onDate < earliest) {
        throw new RangeError(
          `no price cap for ${fuel}/${paymentMethod} on ${onDate}: this build was installed with history=${PRICE_CAP_HISTORY}, ` +
            `so it only carries periods from ${earliest}. Reinstall with history=full for older dates.`
        );
      }
    }
    throw new RangeError(`no price cap for ${fuel}/${paymentMethod} on ${onDate}`);
  }
  return {
    fuel,
    paymentMethod,
    region: row.region,
    unitRate: row.unitRate,
    standingCharge: row.standingCharge,
    vatBasisPoints: row.vatBasisPoints,
    validFrom: row.validFrom,
    validTo: row.validTo,
  };
}

Install

fune build

With that line in your source, in a TypeScript project (language typescript in fune.project), fune build resolves it and nothing else, pins them in fune.lock, downloads only the TypeScript 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.price-cap

The manifest, vectors and README with only the TypeScript implementation. Install it without the registry with fune add ./energy.price-cap-1.0.1-typescript.fune, or fetch it from a terminal with fune pull energy.price-cap@1.0.1:typescript.

The whole function, every language, is one file too: energy.price-cap-1.0.1.fune, 27,349 bytes, sha256 01528a344e3a739f896776b9ee4b867c47104d87ba50feba25906ff17ef70f52. It installs into a project of any language.

// fune: before energy.price-cap

// fune: after energy.price-cap

// fune: step energy.price-cap after <n|label>

More from the author

## Scope of the data

- **GB average, Direct Debit, single-rate electricity.** These are the figures Ofgem publishes as the headline cap for each period: the average across England, Scotland and Wales for a customer paying by Direct Debit. Nine periods, **1 October 2024 to 31 December 2026**. - **Not regional.** The cap differs by the 14 electricity regions. Ofgem publishes the regional rates only in its interactive tables and spreadsheets, which this version does not carry; a regional bill needs the supplier's own rates (which may be below the cap anyway). - **Not standard credit or prepayment.** The types list them because the cap is set for each, but Ofgem does not publish GB-average unit rates for them in a form that could be checked here, so asking for one is an error rather than a guess. They will be new rows, not new code. - **Not Economy 7 / multi-rate.** Ofgem caps a multi-register meter as a whole rather than publishing a GB-average day and night rate.

## Units and VAT

Rates are integers in **thousandths of a penny**, matching the rest of the `energy.*` capabilities (`energy.tariff-time-of-use`, `energy.bill-calculate`); Ofgem's figures are rounded to two decimal places, so the last digit is always 0.

The figures **include VAT**, and `vatBasisPoints` says how much: 5% on everything, except electricity from 1 October 2026, which the government zero-rated for domestic customers in Great Britain from 1 October 2026 to 31 March 2027 (Ofgem: "It does not include VAT from 1 October 2026 to 31 March 2027"). To get the pre-VAT rate for a bill, divide by `1 + vatBasisPoints / 10000`.

## Dates

A period runs `validFrom` to `validTo` inclusive (calendar quarters). A date outside every period this version carries is an error: the cap for a period not yet announced is not today's cap. A build installed with `history=current` keeps only periods still running and refuses older dates with an explanation.

## Sources

Ofgem news releases, "Changes to energy price cap between ..." (each gives the Direct Debit GB-average unit rate and standing charge for electricity and gas, and states the VAT they include):

- 1 January to 31 March 2026: https://www.ofgem.gov.uk/news/changes-energy-price-cap-between-1-january-and-31-march-2026 - 1 April to 30 June 2026: https://www.ofgem.gov.uk/news/changes-energy-price-cap-between-1-april-and-30-june-2026 - 1 July to 30 September 2026: https://www.ofgem.gov.uk/news/changes-energy-price-cap-between-1-july-and-30-september-2026 - 1 October to 31 December 2026: https://www.ofgem.gov.uk/news/changes-energy-price-cap-between-1-october-and-31-december-2026

Ofgem, "Energy price cap" consumer page, as archived by the Internet Archive (https://web.archive.org/web/2025*/https://www.ofgem.gov.uk/information-consumers/energy-advice-households/energy-price-cap) in February, May, August and November 2025, each showing the current and next period: October to December 2024, and each quarter of 2025.

Ofgem, "Energy price cap (default tariff) levels", https://www.ofgem.gov.uk/energy-regulation/domestic-and-non-domestic/energy-pricing-rules/energy-price-cap/energy-price-cap-default-tariff-levels (the regional and per-payment-method models this version does not carry).

1.0.1 fixes Python accepting a trailing newline or non-ASCII digits in onDate; adds tests.

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