eeg-billing

Pure EEG/KWKG feed-in settlement calculation for German energy markets.

eeg-billing is the settlement arithmetic core used by einsd — the Einspeiser Registry daemon. It covers the full EEG legal framework from EEG 2000 through EEG 2023 and KWKG, with all version-specific rule variants enforced automatically based on the plant's EegGesetz year.

zero I/O · zero async · zero unsafe · no float money (rust_decimal) · MSRV 1.94

Design constraints

Constraint	Detail
No I/O	All inputs are passed as arguments. No database calls, no HTTP.
No async	Synchronous — wraps cheaply in `tokio::task::spawn_blocking`.
No float money	Amounts are computed in `rust_decimal::Decimal`; every EUR result is rounded and range-checked through `EuroAmount` (i64 × 10⁻⁵ EUR).
Deterministic	Same inputs always produce the same output. Pure functions.
EEG-version-aware	`EegGesetz` enum drives all version-specific rule dispatch.
Domain-rich	Multiple domain modules covering settlement, degression, sanctions, repowering.

Architecture

SettleInput {
    scheme: SettlementScheme,        ← HOW remuneration is computed
    tariff_source: TariffSource,     ← WHERE the AW/rate comes from
    settlement_type: SettlementType, ← Initial / Correction / Reversal
    einspeisemenge_kwh, ...          ← measurement data
}
    │
    ▼
calculate_settlement(&SettleInput) → SettleOutput {
    settlement_eur,             ← total net payment in EUR
    eligible_kwh,               ← kWh used (may be reduced by §51)
    positions[],                ← itemized billing positions (Σ = settlement_eur)
    pflichtzahlung_eur,         ← §52 penalty (separate from Vergütung)
    verlaengerungsanspruch_qh,  ← §51a Förderdauer extension entitlement
    faelligkeitsdatum,          ← §26 Abs. 1 EEG 2023 — 15th of following month (when billing_date set)
    status,                     ← Calculated / NoData / PriceMissing / ...
}

The SettlementScheme + TariffSource split reflects the EEG structure directly:

SettlementScheme = which formula applies (FeedInTariff, MarketPremium, …)
TariffSource = where the AW comes from (Statutory, Auction(meta), Transitional(rule))

Every scheme is a struct variant carrying the figures its own formula needs — FeedInTariff { verguetungssatz_ct }, MarketPremium { direktverm_aw_ct, wind_korrekturfaktor, wind_standort } — so a rate cannot be set for a formula that does not read it. Only the measurement context (einspeisemenge_kwh, marktwert_ct_kwh, …) lives on SettleInput itself.

Ausschreibung is not a separate scheme. It is: scheme: MarketPremium + tariff_source: Auction(AusschreibungMetadata). It uses the MarketPremium calculation with an auction-determined anzulegender Wert. Award validity, award reductions, and revocation are the caller's responsibility — the library receives the already-resolved AW from the caller.

Settlement schemes

`SettlementScheme`	EEG basis	Formula
`FeedInTariff`	§21 EEG	`kwh × verguetungssatz_ct / 100`
`MarketPremium`	§23a EEG i.V.m. Anlage 1	`max(0, AW − MW) × kwh / 100` — no additive Managementprämie (see below)
`TenantElectricity`	§21 Abs. 3 EEG 2023	`kwh × (verguetung + zuschlag) / 100`
`PostEeg`	post-20yr	`kwh × EPEX / 100` (configurable `price_floor` on the variant)
`KwkSurcharge`	§§7, 8 KWKG	`eligible_kwh × Mischsatz / 100`, capped by the §8 Abs. 1–3 Vollbenutzungsstunden (`FoerderungBeendet`) and, separately, by the §8 Abs. 4 annual cap (`JahreskontingentErschoepft` — the year is used up, the Förderung is not)
`TemporaryFeedInTariff`	§21 Abs. 1 Satz 1 Nr. 3	Ausfallvergütung (temporary feed-in when the Direktvermarkter drops out)
`Eigenverbrauch`	§21 Abs. 3 EEG	No EEG feed-in remuneration is calculated.
`FlexibilityPremium`	§50b EEG 2023	`kwh × (verguetung + flex_praemie) / 100`
`FlexibilitySurcharge`	§50a EEG 2023	`kw × rate / 12` (monthly capacity payment)
`SonstigeDirektvermarktung`	§21a EEG	EUR 0 — direct third-party sale, no NB payment; period recorded in settlement history

§23a EEG 2023 i.V.m. Anlage 1 — Gleitende Marktprämie

Anlage 1 Nr. 3.1.2 (Monatsmarktwert) and Nr. 4.1.2 (Jahresmarktwert) give the whole formula:

MP = AW − MW          (Nr. 3.1.2)

with AW defined in Anlage 1 Nr. 1 as "der anzulegende Wert unter Berücksichtigung der §§ 19 bis 54" — so §36h, §51 and §§53b–54 all reach it — and floored at zero by Nr. 3.1.2 Satz 2.

There is no additive Managementprämie. §20 EEG 2023 has no Absätze at all; it lists the three conditions under which the Marktprämie is payable. Since EEG 2014 the marketing cost sits inside the anzulegender Wert, and its mirror image is the §53 Nr. 2 / Nr. 1 deduction of 0,4 ct (Solar, Wind) / 0,2 ct (everything else) that the Einspeisevergütung route takes off the same AW (rates::sect53_deduction).

Domain modules

eeg-billing/src/
├── formula.rs           Core settlement dispatcher — pure, all §§ rules applied
├── model.rs             SettleInput / SettleOutput / SettlePosition
├── scheme.rs            SettlementScheme, TariffSource, Paragraph100Rule
├── technology.rs        ErzeugungsArt (18 variants), InbetriebnahmeTyp, RepoweringScope
├── version.rs           EegGesetz (8 variants), §51 thresholds and kW-exemption tables
├── rates.rs             §§40–48 AW tables, date-keyed via the statutory Absenkungen
├── kwkg.rs              §§7–9 KWKG: Zuschlag je Leistungsanteil, Vollbenutzungsstunden
├── seed.rs              the §§40–49 net Einspeisevergütung series as flat rows
├── foerderdauer.rs      foerderendedatum_eeg(), §52 Pflichtzahlung, §51a extension
├── foerderungsende.rs   FoerderendeGrund enum, SanktionStatus lifecycle
│
├── degression.rs        §49 semi-annual solar AW degression — 1 % every 1 Feb / 1 Aug
├── direktverm.rs        §§20–22 — Direktvermarktungspflicht, Ausschreibungspflicht, §21b/§21c Wechsel
├── negativpreis.rs      §51 per-interval negative-price derivation (version-aware runs)
├── reductions.rs        §52 Pflichtzahlungen — §52 Abs. 6 netting (a euro-level offset)
├── aw_reductions.rs     §§53b–54 — cuts to the anzulegender Wert, before the formula
├── zusammenfassung.rs   §24 Abs. 1 — the full Zusammenfassung decision (Sätze 1–5)
├── settlement_state.rs  Monthly lifecycle state machine — Active/Reduced/Suspended/PostEeg
│
├── wind.rs              §36h Korrekturfaktor, WindStandort, Gütegrad/Standortklasse
├── biomasse.rs          §§42–44 fuel classes, Güllekleinanlage, §39i Abs. 1 Getreide/Mais
│
├── rounding.rs          kaufmännisches Runden (DIN 1333, half away from zero)
├── error.rs             SettlementError
│
├── tariff.rs            billing::PricingModel adapter — EegSettleTariff, VAT variants
├── bridge.rs            settlement_to_line_items() → billing::LineItem
├── gutschrift.rs        §14 UStG Gutschrift → rubo4e::current::Rechnung (feature `bo4e`)
└── ust.rs               §19 UStG Kleinunternehmer (E) / Regelbesteuerung (S)

§14 UStG Gutschrift (feature `bo4e`)

EEG feed-in is settled under the Gutschriftverfahren (§14 Abs. 2 Satz 2 UStG): the Netzbetreiber issues the settlement document to the Anlagenbetreiber. The settlement amount alone is not that document — VAT law requires a Gutschrift with the per-rate breakdown (EN 16931 BG-23).

gutschrift::settlement_to_gutschrift(output, vat, meta) produces it as a BO4E rubo4e::current::Rechnung: it assembles a billing::BillingDocument (positions + the VAT layers for the operator's declared tax status — Regelbesteuerung 19 % category S / §19 Kleinunternehmer 0 % category E) and renders it. The billing crate does the money and VAT (shared with energy-billing/grid-billing); the BO4E rendering lives here — the same per-crate bo4e pattern those crates follow, with no shared bridge crate.

Quick start

use eeg_billing::{SettleInput, SettlementScheme, SettlementStatus, calculate_settlement};
use rust_decimal::dec;

// §21 EEG 2023 — 500 kWh × 8.11 ct/kWh = 40.55 EUR
let out = calculate_settlement(&SettleInput {
    scheme: SettlementScheme::FeedInTariff { verguetungssatz_ct: dec!(8.11) },
    einspeisemenge_kwh: Some(dec!(500)),
    ..SettleInput::default()
});
assert_eq!(out.status, SettlementStatus::Calculated);
assert_eq!(out.settlement_eur, Some(dec!(40.55)));

// §23a EEG 2023 — Direktvermarktung
// AW = 6.28 ct; Monatsmarktwert = 4.50 ct
// Marktprämie = (6.28 − 4.50) × 100,000 / 100 = 1,780 EUR
use eeg_billing::{SettleInput, SettlementScheme, calculate_settlement};
use rust_decimal::dec;

let out = calculate_settlement(&SettleInput {
    scheme: SettlementScheme::MarketPremium {
        direktverm_aw_ct: dec!(6.28),      // statutory or tendered AW
        wind_korrekturfaktor: None,        // §36h, wind onshore only
        wind_standort: None,
    },
    einspeisemenge_kwh: Some(dec!(100_000)),
    marktwert_ct_kwh: Some(dec!(4.50)),    // Anlage 1 Nr. 3 Monatsmarktwert
    ..SettleInput::default()
});
// 1,780 EUR

§51 EEG — Negativpreisregel (keyed on the Inbetriebnahmedatum)

§51 is not a function of the law year. The Solarspitzengesetz rewrote it with effect from 25.02.2025, mid-year and inside the EEG 2023 range, so two "EEG 2023" plants are governed by different rules depending on the day they were commissioned. Derive the regime with NegativpreisRegime::fuer_inbetriebnahme — EegGesetz deliberately exposes no §51 rule.

Inbetriebnahme	Trigger	Exemption	§51a extension
≤ 2015-12-31	never (§100 Abs. 1 Satz 4 EEG 2017)	—	—
2016-01-01 – 2020-12-31	≥ 6 consecutive hours	Wind < 3 MW · others < 500 kW	none
2021-01-01 – 2022-12-31	≥ 4 consecutive hours	< 500 kW	ausschreibungspflichtige only
2023-01-01 – 2025-02-24	staged 4-3-2-1 h	< 400 kW	ausschreibungspflichtige only
≥ 2025-02-25	first negative ¼h	< 100 kW until iMSys · < 2 kW pending §85 Abs. 2 Nr. 12	all plants

Pilotwindenergieanlagen (§3 Nr. 37) are exempt under every version at any size (ist_pilotwindanlage).

derive_negativpreis applies the run-length threshold to a quarter-hour series; calculate_settlement then applies the size / iMSys / Pilotwind exemptions to the derived kWh.

§51 Abs. 3 — the Ausfallvergütung reporting duty

An operator on the Ausfallvergütung must report the feed-in during continuously negative prices with its §71 Abs. 1 Nr. 1 data. Unreported, the month's claim falls 5 % per calendar day such a period touched (sect51_abs3_unreported_days), floored at zero.

§100 — the Bestandsanlagen opt-in

A plant on an older vintage may declare in Textform that §§51/51a shall apply. The declaration runs from the end of the calendar year in which its iMSys goes in (optin_wirksam_ab), and its anzulegender Wert rises by SECT51_OPTIN_ZUSCHLAG_CT_KWH (0,6 ct/kWh) from then.

§51a — Verlängerungsanspruch

Non-solar plants extend by whole calendar days (96 QH/day, rounded up once over the total). Solar plants convert at factor 0,5 into Volllastviertelstunden and draw them down against the §51a Abs. 2 monthly table (73 in December, 508 in June). Returned in verlaengerungsanspruch_qh.

§51b EEG 2023 — Biogas Ausschreibung

§51b applies exclusively to biogas plants (fermentation, not biomethane) whose AW was set by BNetzA tender. Per §51b Satz 1 EEG 2023:

„Für Anlagen, die Biogas mit Ausnahme von Biomethan einsetzen und deren anzulegender Wert in einem Zuschlagsverfahren ermittelt worden ist, verringert sich der anzulegende Wert auf null für Zeiträume, in denen der Spotmarktpreis 2 Cent pro Kilowattstunde oder weniger beträgt.“

The statute explicitly reduces the AW to zero (not merely the Marktprämie). Since Marktprämie = max(0, AW − EPEX) × kwh/100, zeroing the AW makes the payment zero. The outcome is identical, but the legal mechanism matters for audit positions.

Two key differences from §51 (source: §51b Satz 2 EEG 2023):

§51 and §51a do not apply to §51b plants
No Verlängerungsanspruch accrues for §51b periods

use eeg_billing::{SettleInput, SettlementScheme, TariffSource, AusschreibungMetadata,
                  calculate_settlement};
use rust_decimal::dec;
use time::macros::date;

// Biogas auction plant: Marktwert 1.5 ct ≤ 2 ct → AW = 0, EUR 0
let out = calculate_settlement(&SettleInput {
    scheme: SettlementScheme::MarketPremium {
        direktverm_aw_ct: dec!(8.5),
        wind_korrekturfaktor: None,
        wind_standort: None,
    },
    tariff_source: TariffSource::Auction(AusschreibungMetadata {
        is_biogas_sect51b: true, // explicit biogas §51b flag
        ..AusschreibungMetadata::default()
    }),
    einspeisemenge_kwh: Some(dec!(10_000)),
    marktwert_ct_kwh: Some(dec!(1.5)), // ≤ 2 ct/kWh → §51b triggers
    // §101 Abs. 2 keys §51b on the **supply period**: without a billing_date
    // on or after 18.09.2025 the provision is not applied at all.
    billing_date: Some(date!(2026 - 01 - 31)),
    ..SettleInput::default()
});
assert_eq!(out.settlement_eur, Some(dec!(0.00000)));
assert!(out.positions[0].legal_basis.contains("51b"));

§100 EEG — Übergangsregelung auto-override

For old plants that fall under a specific §100 transition provision, supply tariff_source = Transitional(rule). The library automatically derives the correct EegGesetz for §51/§52 dispatch, preventing silent miscalculations:

`Paragraph100Rule`	Effective `EegGesetz`	§51 behaviour	Source
`Pre2016Bestandsschutz`	`Eeg2012`	Never applies	§100 Abs. 1 Satz 4 EEG 2017
`Eeg2017Negativpreis6h`	`Eeg2017`	≥6h; Wind <3 MW / other <500 kW	§100 Abs. 2 Nr. 13 EEG 2021
`BiomassOldFuelClassContinuation`	`Eeg2017`	≥6h; old §42–44 fuel rules	§100 Abs. 6 EEG 2023
`SmallBiomassBelow150kw`	`Eeg2017`	≥6h; small biomass FiT	§100 Abs. 11 EEG 2023
`OldPlantBeforeEeg2023`	`Eeg2021`	≥4h; all <500 kW	§100 Abs. 1 EEG 2023
all other variants	caller's `eeg_gesetz`	as per `eeg_gesetz`	—

use eeg_billing::{SettleInput, SettlementScheme, EegGesetz, calculate_settlement};
use eeg_billing::scheme::{TariffSource, Paragraph100Rule};
use rust_decimal::dec;

// Pre-2016 plant — §51 must NEVER apply, regardless of eeg_gesetz setting.
// TariffSource::Transitional auto-overrides to Eeg2012 → §51 exempt.
let out = calculate_settlement(&SettleInput {
    scheme: SettlementScheme::FeedInTariff { verguetungssatz_ct: dec!(8.11) },
    tariff_source: TariffSource::Transitional(Paragraph100Rule::Pre2016Bestandsschutz),
    eeg_gesetz: EegGesetz::Eeg2017,      // ← might be set wrong in DB; rule corrects it
    einspeisemenge_kwh: Some(dec!(1000)),
    kwh_during_negative_epex: Some(dec!(500)), // would trigger §51 under Eeg2017
    leistung_kwp: Some(dec!(1000)),             // 1 MW >> 500 kW threshold
    ..SettleInput::default()
});
// Pre2016Bestandsschutz → no §51 deduction → full 1000 kWh × 8.11 ct = 81.10 EUR
assert_eq!(out.settlement_eur, Some(dec!(81.10)));

Use SettleInput::effective_eeg_gesetz() directly when building settle logic outside the library.

§26 Abs. 1 EEG — Fälligkeitsdatum

SettleOutput.faelligkeitsdatum contains the 15th calendar day of the month following the billing month, computed automatically from billing_date:

§26 Abs. 1 EEG 2023: „monatlich jeweils zum 15. Kalendertag für den Vormonat"

Billing month	`faelligkeitsdatum`
June 2024	2024-07-15
December 2024	2025-01-15 (year rolls over)
February 2025	2025-03-15

None when billing_date is not set. The final Endabrechnung deadline (§26 Abs. 2, conditioned on §71 data submission) is outside the scope of this library.

§52 EEG — Sanctions

EEG 2023 (commissioned ≥2023): pflichtverstoss: Vec<Pflichtverstoss> → pflichtzahlung_eur. Vergütung continues. Abs. 5 caps each calendar month at 10 €/kW — „insgesamt auf 10 Euro pro Kilowatt installierter Leistung der Anlage und Kalendermonat begrenzt" — so every violation is spread over the months it runs in, each month is capped on its own, and the capped months are summed. A month under the ceiling does not release headroom for a month over it.

EEG ≤2021 (§100 Übergangsregelung): sanktion: Some(SanktionAlt::…). Three tiers: VerguetungAufNull / VerguetungAufMarktwert / VerguetungReduziert20Prozent.

§52 Abs. 6 Netting

use eeg_billing::reductions::apply_sect52_netting;
let r = apply_sect52_netting(settlement_eur, pflichtzahlung_eur);
// r.net_vergütung_eur              — disbursed to operator
// r.residual_pflichtzahlung_eur    — still owed to NB

§49 EEG 2023 — Semi-annual solar degression

The anzulegende Werte of §48 Abs. 1, 2 and 2a fall by a fixed 1 % every six months, on 1 February and 1 August, from 01.02.2024. Each step compounds on the unrounded predecessor (§49 Satz 2); the 2-dp rounding is presentation only. The GW-keyed "atmender Deckel" of §49 EEG 2021 is gone.

use eeg_billing::rates::solar_pv_ueberschuss_aw_ct;
use rust_decimal::dec;
use time::macros::date;

// 9 kWp roof, commissioned in the 1 Aug 2024 window:
// §48 Abs. 2 Nr. 1 base 8.60 ct × 0.99² = 8.42886 → 8.43 ct gross AW.
assert_eq!(solar_pv_ueberschuss_aw_ct(dec!(9), date!(2024-09-01)), Some(dec!(8.43)));

§§20–22 EEG 2023 — Veräußerungsformen, Direktvermarktungspflicht, Ausschreibung

The EEG has no section imposing a duty to market directly. The duty is the shadow of §21 Abs. 1 Satz 1 Nr. 1: the Einspeisevergütung mit gesetzlich bestimmtem anzulegenden Wert exists only „für Strom aus Anlagen mit einer installierten Leistung von bis zu 100 Kilowatt", so anything larger has to take the Marktprämie — which §20 pays only for months the Strom is direkt vermarktet.

use eeg_billing::direktverm::{SolarSegment, direktvermarktungspflicht, requires_ausschreibung};
use eeg_billing::ErzeugungsArt;
use rust_decimal::dec;
use time::macros::date;

// §21 Abs. 1 Satz 1 Nr. 1 — the ceiling, keyed on the Inbetriebnahmedatum.
assert_eq!(direktvermarktungspflicht(dec!(150), date!(2024-05-01)), Some(true));
// A pre-2016 plant is governed by a text outside mako's corpus: unanswered, not "no".
assert_eq!(direktvermarktungspflicht(dec!(600), date!(2013-05-01)), None);

// §22 Abs. 3 Satz 2 — Solar has two thresholds, keyed on the Segment.
assert!( requires_ausschreibung(dec!(900), ErzeugungsArt::SolarAufdach,      SolarSegment::Zweites));
assert!(!requires_ausschreibung(dec!(900), ErzeugungsArt::SolarFreiflaeche, SolarSegment::Erstes));
// §22 Abs. 5 Satz 2 — Wasserkraft and Geothermie are never tendered, at any size.
assert!(!requires_ausschreibung(dec!(5000), ErzeugungsArt::Wasserkraft, SolarSegment::Erstes));

§21b Abs. 1 Satz 2 (a change takes effect only on the first of a month) and §21c Abs. 1 Satz 1 (the Mitteilung is due before the preceding month begins) are validate_wechsel. There is no „one switch per calendar month" rule — Satz 2 already makes a second change within one month impossible.

ErzeugungsArt

Variant	Technology	Notes
`SolarAufdach`	Rooftop PV (Gebäude/Lärmschutzwand)	§48 Abs. 2 rates; zweites Segment (§3 Nr. 41b) → tender > 750 kW
`SolarFreiflaeche`	Ground-mounted PV	§48 Abs. 1/1a; erstes Segment (§3 Nr. 41a) → tender > 1 MW
`SolarAgriPv`	Agri-PV	§48 Abs. 1 S. 1 Nr. 5 lit. a, uplift Abs. 1b; erstes Segment — there is no 6-MW Agri-PV exemption
`SolarMieterstrom`	§21 Abs. 3 building solar	—
`SolarStecker`	Steckersolargerät, ≤2 kW and ≤800 VA inverter	§3 Nr. 43, §8 Abs. 5a, §9, §10a Abs. 2
`WindOnshore`	Wind onshore	No fixed AW — §46 Abs. 1 computes it from §36h Abs. 1 with the Vorvorjahr auction average; a Zuschlag is required only above 1 MW (§22 Abs. 2 S. 2 Nr. 1)
`WindOffshore`	Wind offshore	Zuschlag and AW come from the WindSeeG, to which §22 Abs. 1 refers
`Biomasse`	Biomasse	§42 — 12,67 ct ≤150 kW Bemessungsleistung; above that, tender
`BiomassHolz`	Feste Biomasse	No separate AW; tendered plants meet the §39i Abs. 2 Höchstbemessungsleistung
`Biogas`	Fermentation biogas	§43 Bioabfälle / §44 Gülle where they qualify
`Biomethan`	Upgraded biomethane	Excluded from the §42 statutory value by Satz 2
`Klaergas` / `Grubengas` / `Deponiegas`	Special gases	§41 — one ladder each, Abs. 1/2/3
`Wasserkraft`	Hydro	§40 — seven tiers by Bemessungsleistung
`Geothermie`	Geothermal	§45 — flat 25,20 ct
`Gezeiten`	Tidal, wave, salinity gradient, current	§40 — these are Wasserkraft (§3 Nr. 21 lit. a); there is no §41a EEG
`Kwk`	CHP/BHKW	KWKG, not EEG — priced by `kwkg`, not by a rate table

The statutory rates

rates carries the anzulegende Werte — the ct/kWh a Netzbetreiber owes an Anlagenbetreiber, each a figure fixed by statute:

Erzeugungsart	§ EEG 2023	Shape
Wasserkraft (incl. Gezeiten, § 3 Nr. 21 lit. a)	§ 40 Abs. 1	seven tiers, `12,03` → `3,37`
Deponiegas	§ 41 Abs. 1	`7,46` ≤ 500 kW, `5,17` ≤ 5 MW
Klärgas	§ 41 Abs. 2	`5,93` ≤ 500 kW, `5,17` ≤ 5 MW
Grubengas	§ 41 Abs. 3	`5,98` ≤ 1 MW, `3,81` ≤ 5 MW, `3,37` above
Biomasse	§ 42 Satz 1	one tier, `12,67` ≤ 150 kW; above that, tender
Bioabfallvergärung	§ 43 Abs. 1	`14,16` ≤ 500 kW, `12,41` ≤ 20 MW
Güllevergärung	§ 44 Abs. 1	`22,00` ≤ 75 kW, `19,00` ≤ 150 kW
Geothermie	§ 45 Abs. 1	flat `25,20`
Solar	§ 48	see below (§ 101 Abs. 1 Satz 2)
Wind an Land	§ 46 Abs. 1	no fixed figure: the AW is § 36h Abs. 1 with the Zuschlagswert replaced by the Vorvorjahr auction average (§ 46 Abs. 2). A Zuschlag is needed only above 1 MW (§ 22 Abs. 2 Satz 2 Nr. 1)

Every figure is asserted against the statute by rates::statutory_rate_tests, which walks each ladder tier by tier and pins the two non-tables — § 42 answering Err above 150 kW rather than inventing a rate, and wind_onshore_lookup answering None.

§ 101 — a provision under a Genehmigungsvorbehalt has a start date

§ 101 EEG 2023 lists provisions that "erst nach der beihilferechtlichen Genehmigung durch die Europäische Kommission … angewandt werden" dürfen, and for some names the version that applies meanwhile. Two of them are load-bearing here:

§ 48 Abs. 2 (Satz 2 fallback) — why the in-force solar base values are 8,60 / 7,50 / 6,20 and not the consolidated 8,51 / 7,43 / 7,64.
§ 51b — the biogas AW → 0 at a spot price ≤ 2 ct/kWh. It names no fallback version, so it applies only from the Commission's approval on 18 September 2025 (version::SECT51B_GENEHMIGT_AB), keyed on the settled supply period. An undated settlement does not apply it: a provision under a Genehmigungsvorbehalt is not applied on the strength of not knowing when the supply happened.

Every table is the Startwert as enacted. The statutory annual Absenkung is separate, and each Erzeugungsart carries its own rate and cadence — biomass steps on 1 July, the rest on 1 January — via degression::JaehrlicheAbsenkung.

Repowering

⚠ Repowering law is nuanced. foerderendedatum_repowering() computes a hypothetical funding end (Dec 31 of year+20) if and only if the applicable repowering provisions actually result in a new Förderzeitraum. Whether that is the case depends on the type and extent of the repowering (RepoweringScope::Full vs. partial), the applicable §22 EEG provisions for the specific plant, and current BNetzA guidance. RepoweringScope::resets_foerderdauer_definitely() returns true only for full replacement; partial replacements are legally contested. Always obtain qualified legal or regulatory advice for the specific plant situation before relying on this function.

use eeg_billing::{foerderendedatum_repowering, RepoweringScope};
use time::macros::date;

// Full repowering — uses statutory Dec 31 rule (§25 Abs. 1 Satz 2 EEG 2023)
// Whether Förderdauer actually resets depends on the type of repowering.
let new_end = foerderendedatum_repowering(date!(2025-06-01)).unwrap();
assert_eq!(new_end, date!(2045-12-31));

// Partial replacement: Förderdauer does NOT reset
assert!(!RepoweringScope::RotorOnly.resets_foerderdauer_definitely());

Settlement calculation pipeline

The typical pipeline for a single billing period:

1. Einspeisemenge input (from metering / edmd)
         │
         ▼
2. Eligibility check (FoerderungBeendet? foerderendedatum > billing_date?)
         │     ↳ also: §35a award_expired check for Ausschreibungsanlagen
         ▼
3. Scheme dispatch (FeedInTariff / MarketPremium / KwkSurcharge / …)
         │     ↳ §51b: AW = 0 when EPEX ≤ 2 ct/kWh (biogas Ausschreibung)
         ▼
4. §51 Negativpreisregel (version-aware kWh reduction)
         │     ↳ §51a: Verlängerungsanspruch accrued (solar: 0.5×, others: 1×)
         ▼
5. §25 Abs. 1 Satz 3 billing_days_fraction (partial-month commissioning/decommissioning)
         │
         ▼
6. §52 Pflichtzahlungen (separate penalty, Vergütung unchanged)
         │
         ▼
7. §52 Abs. 6 netting (optional: NB deducts penalty from disbursement)
         │
         ▼
8. SettleOutput { settlement_eur, eligible_kwh, positions, pflichtzahlung_eur, faelligkeitsdatum }

(§§53b–54 do not appear as a step: they cut the anzulegender Wert at step 3,
 before the scheme formula, because the Marktprämie floors at zero.)

VAT is applied by the caller via EegSettleTariff + ust::ust_tax_layers() — not inside calculate_settlement. A feed-in Gutschrift has exactly two treatments, and each yields exactly one tax layer (the exempt one included):

`VatStatus`	Rate	EN 16931 category	Basis
`Regelbesteuerung`	19 %	`S` — Standard	§12 Abs. 1 UStG
`Kleinunternehmer`	0 %	`E` — Exempt	§19 UStG (tax not levied)

The status is a declared property of the operator (masterdata), not something plant size decides — VatStatus::default_for_plant only suggests the value a new plant would usually carry. §12 Abs. 3 UStG is deliberately absent: its 0 % Nullsteuersatz taxes the supply of the PV hardware, not the feed-in of electricity. Its only bearing here is indirect — because a ≤30 kWp operator buys the plant at 0 %, they have no input tax to reclaim and stay §19 Kleinunternehmer.

An exempt supply is still a taxable supply, so it belongs in the EN 16931 BG-23 VAT breakdown under its own UNTDID 5305 category with a zero tax amount. Omitting the layer would drop that turnover from the breakdown entirely and understate the taxable base. §19 UStG does not levy the tax at all and maps to E, which carries the exemption reason EN 16931 requires (BT-120).

A document mixing treatments — a 0 % PV feed-in credit beside 19 % NNE grid charges — cannot use a single status. Build the layers directly and scope each to its own positions with FixedRateTax::with_tag, so each contributes its own breakdown entry.

Scope

Explicitly in scope — tested and production-oriented:

§21 EEG Einspeisevergütung (all EEG versions 2000–2023)
§20 EEG Marktprämie + §§22/22a/28 Ausschreibung
§21 Abs. 3 Mieterstrom, §50a/b Flexibilitätsprämie, §7 KWKG
§51/§51a/§51b Negativpreisregel, §52 sanctions and Abs. 6 netting
§53 Einspeisevergütungsabzug, and the AW-level cuts of §53b (Regionalnachweise, 0,1 ct/kWh, statutory-AW plants only), §53c (Stromsteuerbefreiung, capped at the §3 StromStG rate) and §54 (solar first-segment auctions, four Absätze)
§19 EInsMan curtailment compensation (separate position, §51 exempt)
§49 semi-annual solar degression, §36h wind Korrekturfaktor
§24 Zusammenfassung: sind_eine_anlage decides the whole of Abs. 1 — the four cumulative conditions of Satz 1 and the Sätze 2–5 carve-outs (same Biogaserzeugungsanlage, Freifläche vs. building solar, differing Netzverknüpfungspunkte, disregarded Steckersolargeräte) — and returns the rule that decided. Ownership is deliberately not an input: Satz 1 says "unabhängig von den Eigentumsverhältnissen".
§24 multi-block allocation: CapacityBlock allocates the metered Einspeisemenge across the blocks of an already-fused plant group by installed capacity, by largest remainder (billing::proportional_split) so the blocks add back up to the metered quantity exactly. A per-block share rounded on its own does not: three equal blocks of a 1000 kWh month take 333.333 kWh each and settle 999.999. Expired blocks are allocated and then dropped — their capacity still counts towards the plant, and the energy falling to them is simply no longer eligible.
§42b EnWG GGV / §21 Abs. 3 multi-meter split is not modelled here — the metering topology, Eigenverbrauch/Überschuss split and GGV tenant allocation live in the external metering crate (AggregationRule, compute_virtual_meter) + edmd; this crate settles the resulting Einspeisemenge
SettlementType: Initial, Correction (with original_id), Reversal
§25 billing_days_fraction (partial billing periods per §25 Abs. 1 Satz 3)
§26 Abs. 1 Fälligkeitsdatum (15th of following month, auto-computed)
TariffSource::Transitional(Paragraph100Rule) → effective_eeg_gesetz() auto-override

Intentionally out of scope (caller's responsibility):

The §21b/§21c Wechsel act — direktverm::validate_wechsel decides it here, but einsd owns the plant record it decides against and issues the §21c notification
§§53b–54 fact lookups — einsd reads the triggering facts and passes an AwReductionContext; the amounts themselves are statutory and live here
§55 Pönalen — outside this domain entirely: a bidder↔regelverantwortlicher-ÜNB obligation from the tender process, not operator↔NB settlement. einsd tracks commissioning deadlines for the Erlöschen of a Zuschlag (§36e/§37e/§39e)
§52 cumulative months tracking — einsd computes from violation_start dates
§ 147 AO / GoBD receipt archival — einsd manages settlement_receipt_history
Redispatch 2.0 compensation (§13a/§14 EnWG) — see crates/mako-redispatch
SEPA CT payment dispatch — handled by accountingd
EPEX Spot price import — handled by einsd

Regulatory basis

Topic	Source
EEG 2023	BGBl. I Nr. 28, 10.01.2023
§48 Abs. 2 / 2a anzulegende Werte	Fassung vom 15.05.2024, kept in force by §101 Abs. 1 Satz 2 pending EU state-aid approval; cross-checked against the BNetzA "Anzulegende Werte für Solaranlagen" tables
§49 Solardegression	fixed 1 % every six months from 01.02.2024, compounded unrounded (§49 Satz 2)
§36h Korrekturfaktor	§36h Abs. 1 Satz 2 Stützwerte, linear interpolation (Satz 3), Satz 4 out-of-range rules
KWKG 2023	BGBl. I Nr. 59, 28.12.2023
§23a Marktprämie	`MP = AW − MW`, floored at zero (Anlage 1 Nr. 3.1.2) — no additive Managementprämie
§51 Negativpreisregel	reduces the AW, so it reaches the Marktprämie; size test aggregated per §24 (§51 Abs. 2 Satz 2)
§51 Bestandsschutz	§100 Abs. 1 Satz 4 EEG 2017 — boundary 2016-01-01
§51b mechanism	`verringert sich der anzulegende Wert auf null` — AW = 0 (§51b Satz 1 EEG 2023)
§52 Pflichtzahlungen	€10/kW/month; §52 Abs. 3 retroactive €2/kW
§53 Vergütungsabzug	Solar/Wind: −0.4 ct; Biomasse/Wasser/Gas: −0.2 ct
§100 Übergangsregelung	Settlement rules resolved per applicable §100 transition provisions

Legal disclaimer

This library implements a deterministic computation of EEG/KWKG settlement rules based on the cited statutory provisions.

Certain provisions — particularly those subject to evolving case law, BNetzA guidance, or Clearingstelle EEG|KWKG interpretations — may admit multiple legally defensible readings. Where applicable, this library documents its chosen interpretation and notes where alternatives exist.

Users remain responsible for:

Validating the chosen interpretation against the EEG/KWKG version applicable to their specific settlement scenario
Confirming correctness against current BNetzA guidance and publications
Consulting Clearingstelle EEG|KWKG rulings where relevant
Obtaining qualified legal advice before using this library in contested settlements

The library has not been validated against official DSO settlement examples, BNetzA benchmark calculations, or Clearingstelle decisions. It is production-oriented but not independently legally certified.

Source: EEG 2023 Clearingstelle EEG|KWKG working text (23.12.2025). Cite as: Clearingstelle EEG|KWKG, Arbeitsausgabe EEG 2023.

Testing

cargo test -p eeg-billing --all-features
# lib tests + proptest integration + regulatory showcase + doctests

The regulatory showcase (tests/regulatory_showcase.rs) is executable documentation for every §§ rule, including the Anlage 1 Marktprämie formula, §51 version-specific thresholds, §52 Abs. 6 netting, §100 Übergangsregelung, and all settlement scheme edge cases.

Related crates

Crate	Role
`eeg-billing` ← this crate	EEG/KWKG feed-in settlement — schemes, degression, sanctions, repowering
`mako-markt`	Marktstammdaten — Marktlokation, Messlokation, Marktpartner, Rollenzuordnung — the model the plant and Marktlokation inputs are keyed on
`energy-billing`	The consumption leg — the customer's retail invoice
`einsd`	Production daemon — the Einspeiser Registry

Part of mako, an open-source Rust platform for German energy market communication (Marktkommunikation). Full documentation: https://hupe1980.github.io/mako/

eeg-billing 0.20.0