energy-billing

Pure multi-product retail energy billing library for German markets.

energy-billing is the calculation core used by billingd — the Energy Billing Engine daemon (LF role). All commercial prices are user-defined in tarifbd; this crate handles only the arithmetic, regulatory rate application, and BO4E Rechnung JSON generation.

Design constraints

Constraint	Detail
No I/O	All inputs are passed as arguments. No database calls, no HTTP.
No async	Synchronous throughout.
No float money	All monetary amounts use `rust_decimal` with fixed precision.
Deterministic	Same inputs always produce the same output.
`#[must_use]`	All `calculate_*` functions are annotated `#[must_use]`.

Product categories

`calculate_*` function	Category	Key features
`calculate_strom`	`STROM`	Eintarif / Zweitarif HT/NT / RLM Leistungspreis; §14a Modul 1/3 Steuerungsrabatt; EEG-Umlage
`calculate_dynamic_strom`	`STROM` (§41a)	15-min Lastgang × hourly EPEX Spot; optional price floor (`dynamic_epex_floor_ct_kwh`); §14a compatible
`calculate_gas`	`GAS`	Brennwertkorrektur §10 GasGVV; Energiesteuer §2 EnergieStG; BEHG CO₂; H2-blend `gasqualitaet` audit annotation
`calculate_waerme`	`WAERME`	Fernwärme (Arbeitspreis + Grundpreis + Leistungspreis)
`calculate_solar`	`SOLAR`	Mieterstrom §42b EEG; Gemeinschaftliche Gebäudeversorgung §42a EEG; §12 Abs. 3 UStG Nullsteuersatz
`calculate_eeg`	`EEG`	EEG Gutschrift; §51 Negativpreisregel contractual feature via `kwh_during_negative_epex`
`calculate_einspeisung`	`EINSPEISUNG`	Feed-in credit note (Einspeisevergütung)
`calculate_waermepumpe`	`WAERMEPUMPE`	§14a Modul 1/3 Wärmepumpe reduced-rate
`calculate_wallbox`	`WALLBOX`	§14a Modul 1/3 Ladeeinrichtung reduced-rate
`calculate_hems`	`HEMS`	Home Energy Management System subscription + optional event-based charges
`calculate_emobility`	`EMOBILITY`	E-mobility charging (AC/DC) with optional kWh measurement
`calculate_energiedienstleistung`	`ENERGIEDIENSTLEISTUNG`	Custom energy service fee (flat or per-kWh)

§41a EPEX dynamic tariffs

calculate_dynamic_strom multiplies 15-minute meter intervals against hourly EPEX Spot prices fetched from tarifbd. An optional floor prevents credits during deeply negative hours:

let tariff = DynamicStromTariff {
    epex_markup_ct_kwh: dec!(2.50),          // markup above spot price
    network_costs_ct_kwh: dec!(8.20),        // NNE pass-through
    dynamic_epex_floor_ct_kwh: Some(dec!(5.00)), // floor: spot never below 5 ct/kWh
    ..Default::default()
};

Set dynamic_epex_floor_ct_kwh: Some(Decimal::ZERO) to allow zero but block negative credits. Set None to pass through all negative spot prices as credits.

§51 EEG Negativpreisregel (contractual LF feature)

For LF retail contracts that pass through EEG Gutschrift, the optional kwh_during_negative_epex field reduces the credited kWh. Unlike eeg-billing (which implements the mandatory §51 rule for NB/Einspeiser), this is a contractual feature only — the LF is not legally required to apply §51.

let input = EegMeterInput {
    kwh_total: dec!(320.0),
    kwh_during_negative_epex: Some(dec!(40.0)), // suspended hours per contract
    ..Default::default()
};

`BillingResult`

All calculate_* functions return Result<BillingResult, BillingError>. BillingResult is #[non_exhaustive] and provides:

let result = calculate_strom(&tariff, &meter)?;
result.assert_valid();                                     // panics on internal invariant breach
let nne_total = result.levy_total_eur();                   // sum of NNE/KA/Abgaben positions
let positions = result.positions_by_tag("14a").collect::<Vec<_>>(); // filter by tag
let base_total = result.position_total_by_tag("base");
let rechnung_json: serde_json::Value = result.rechnung_json(meta)?; // BO4E Rechnung

BO4E output

rechnung_json() returns a serde_json::Value shaped as rubo4e::current::Rechnung with rechnungsempfaenger, zahlungsziel, and typed Rechnungsposition entries. Pass this directly to billingd's persistence layer or the XRechnung/ZUGFeRD renderer.

Usage

[dependencies]
energy-billing = { path = "../crates/energy-billing" }

use energy_billing::{calculate_strom, StromTariff, StromMeterInput};
use rust_decimal_macros::dec;

let tariff = StromTariff {
    arbeitspreis_ht_ct_kwh: dec!(28.50),
    arbeitspreis_nt_ct_kwh: dec!(22.00),
    grundpreis_eur_month: dec!(9.95),
    ..Default::default()
};

let meter = StromMeterInput {
    kwh_ht: dec!(220.0),
    kwh_nt: dec!(80.0),
    billing_months: dec!(1),
    ..Default::default()
};

let result = calculate_strom(&tariff, &meter)?;
println!("Total: {} EUR", result.total_eur_gross());

Regulatory basis

§14a EnWG — Steuerung steuerbarer Verbrauchseinrichtungen (Modul 1/3 discounts, BK6-24-174)
§41a EnWG — Dynamic EPEX tariffs (mandatory offer from 2025)
§3 StromStG — Stromsteuer 2.050 ct/kWh (Regelsteuersatz)
§2 EnergieStG — Erdgassteuer 0.55 ct/kWh
BEHG — Brennstoffemissionshandel CO₂-Preis (Gas, update annually)
§10 GasGVV — Brennwertkorrektur (Zustandszahl, Brennwert)
§42b / §42a EEG 2023 — Mieterstrom / Gemeinschaftliche Gebäudeversorgung
§12 Abs. 3 UStG — PV Nullsteuersatz (since 01.01.2023)
§51 EEG 2023 — Negativpreisregel (contractual LF feature only)

Testing

cargo test -p energy-billing --all-features

44 tests covering all 12 product categories, §14a Modul 1/3, §41a floor, §51 suspension, BillingResult helper methods, rechnung_json, and XRechnung output.

energy-billing 0.10.0