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 (Solarpaket I) and KWKG 2023, with all version-specific rule variants enforced automatically based on the plant's EegGesetz year.

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

Design constraints

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))

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

§20 Abs. 3 EEG 2023 — Managementprämie

This library's implementation computes the Managementprämie by incorporating it into the AW before the spread calculation. The implementation is based on this reading of §20 Abs. 3 EEG 2023:

„Bei der Berechnung der Marktprämie ist der anzulegende Wert um 0,4 Cent pro Kilowattstunde zu erhöhen.“

Under this reading:

eff_AW = direktverm_aw_ct + managementpraemie_ct
Marktprämie = max(0, eff_AW − EPEX) × kwh / 100

When EPEX > eff_AW, the total payment is zero — no guaranteed floor.

⚠ This is one defensible interpretation. Verify before production use. The Managementprämie treatment has evolved across EEG versions and is subject to evolving BNetzA guidance and bilateral contract terms. This implementation must be independently verified against:

• the EEG version applicable to the specific plant (EEG 2017/2021/2023 differ),
• current BNetzA published guidance on §20 Abs. 3 EEG 2023, and
• the contractual framework between Netzbetreiber, Direktvermarkter, and operator. Do not rely on this formula for settlement disputes without such verification.

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 (19 variants), InbetriebnahmeTyp, RepoweringScope
├── version.rs           EegGesetz (8 variants), §51 thresholds and kW-exemption tables
├── rates.rs             Static AW tables: solar PV (Solarpaket I), wind, biomasse, KWKG
├── foerderdauer.rs      foerderendedatum_eeg(), §52 Pflichtzahlung, §51a extension
├── foerderungsende.rs   FoerderendeGrund enum, SanktionStatus lifecycle
│
├── degression.rs        §23a quarterly solar PV degression — Quarter, DegressionTier
├── direktverm.rs        §§20–22 — mandatory threshold, Ausschreibungspflicht, period model
├── metering.rs          Multi-meter Messkonzept — §42b GGV, §14a HT/NT
├── reductions.rs        §§52–54 reduction pipeline — §52 Abs. 6 netting, §53c, §54
├── settlement_state.rs  Monthly lifecycle state machine — Active/Reduced/Suspended/PostEeg
│
├── solar.rs             §48 PV subtypes, §12 Abs. 3 UStG, Agri-PV bonus
├── wind.rs              §36k Korrekturfaktor, WindStandort, Gütegrad/Standortklasse
├── biomasse.rs          §43/§44 fuel classes, Güllekleinanlage (≤75 kW, ≥80% Gülle)
│
├── tariff.rs            billing::Tariff adapter — EegSettleTariff, VAT variants
├── bridge.rs            settlement_to_line_items() → billing::LineItem
└── ust.rs               §12 Abs. 3 UStG, §19 UStG Kleinunternehmer

Quick start

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

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

// §20 EEG 2023 — Direktvermarktung
// eff_AW = 6.28 + 0.4 = 6.68 ct; EPEX = 4.50 ct
// Marktprämie = (6.68 − 4.50) × 100,000 / 100 = 2,180 EUR
use eeg_billing::{SettleInput, SettlementScheme, calculate_settlement};
use rust_decimal_macros::dec;

let out = calculate_settlement(&SettleInput {
    scheme: SettlementScheme::MarketPremium,
    einspeisemenge_kwh: Some(dec!(100_000)),
    direktverm_aw_ct: Some(dec!(6.28)),    // statutory AW (before Managementprämie)
    epex_avg_ct_kwh: Some(dec!(4.50)),
    managementpraemie_ct: Some(dec!(0.4)), // §20 Abs. 3: incorporated into AW
    ..SettleInput::default()
});
// 2,180 EUR; see Managementprämie section for treatment caveats

§51 EEG — Negativpreisregel (version-aware)

Pass kwh_during_negative_epex — the engine applies the correct threshold automatically.

§51a — Verlängerungsanspruch

Solar PV: ceil(lost_qh / 2) · Others: lost_qh (1:1). 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_macros::dec;

// Biogas auction plant: EPEX 1.5 ct ≤ 2 ct → AW = 0, EUR 0
let out = calculate_settlement(&SettleInput {
    scheme: SettlementScheme::MarketPremium,
    tariff_source: TariffSource::Auction(AusschreibungMetadata {
        is_biogas_sect51b: true, // explicit biogas §51b flag
        ..AusschreibungMetadata::default()
    }),
    einspeisemenge_kwh: Some(dec!(10_000)),
    direktverm_aw_ct: Some(dec!(8.5)),
    epex_avg_ct_kwh: Some(dec!(1.5)), // ≤ 2 ct/kWh → §51b triggers
    managementpraemie_ct: Some(dec!(0.4)),
    ..SettleInput::default()
});
assert_eq!(out.settlement_eur, Some(dec!(0)));
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:

use eeg_billing::{SettleInput, SettlementScheme, EegGesetz, calculate_settlement};
use eeg_billing::scheme::{TariffSource, Paragraph100Rule};
use rust_decimal_macros::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,
    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
    verguetungssatz_ct: dec!(8.11),
    ..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"

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. Multiple violations summed, capped at §52 Abs. 5.

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

§23a EEG 2023 — Quarterly solar PV degression

use eeg_billing::degression::{solar_ueberschuss_rate_for_quarter, DegressionTier, Quarter};
use rust_decimal_macros::dec;

// 9 kWp, Q4 2024 (2 quarters after Solarpaket I), 1% tier → 8.51 × 0.99² = 8.34 ct
let rate = solar_ueberschuss_rate_for_quarter(
    Quarter { year: 2024, quarter: 4 },
    dec!(9),
    DegressionTier::Standard,
);
assert_eq!(rate, Some(dec!(8.34)));

§§20–22 EEG 2023 — Direktvermarktung rules

use eeg_billing::direktverm::{is_direktvermarktung_mandatory, requires_ausschreibung};
use eeg_billing::{EegGesetz, ErzeugungsArt};
use rust_decimal_macros::dec;

assert!(is_direktvermarktung_mandatory(dec!(150), EegGesetz::Eeg2023)); // >100 kW: mandatory
assert!(requires_ausschreibung(dec!(1500), ErzeugungsArt::SolarAufdach)); // >1 MWp: tender

ErzeugungsArt

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. §53b regional reduction / §54 Ausschreibungsreduzierung
         │
         ▼
8. §52 Abs. 6 netting (optional: NB deducts penalty from disbursement)
         │
         ▼
9. SettleOutput { settlement_eur, eligible_kwh, positions, pflichtzahlung_eur, faelligkeitsdatum }

VAT (§12 Abs. 3 UStG / §19 UStG Kleinunternehmer / Regelbesteuerung) is applied by the caller via EegSettleTariff + ust::ust_tax_layers() — not inside calculate_settlement.

Scope

Explicitly in scope — tested and production-oriented:

• §21 EEG Einspeisevergütung (all EEG versions 2000–2023)
• §20 EEG Gleitende Marktprämie + §§22a/28 Ausschreibung
• §38a Mieterstrom, §50a/b Flexibilitätsprämie, §7 KWKG
• §51/§51a/§51b Negativpreisregel, §52 sanctions, §53/§53b/§54 reductions
• §19 EInsMan curtailment compensation (separate position, §51 exempt)
• §23a quarterly degression, §36k wind Korrekturfaktor
• §24 multi-block Anlagenzusammenfassung: proportional allocation for pre-aggregated plant groups

  The library computes settlement after §24 aggregation has been determined by the caller. The legal aggregation analysis itself (operator identity, location, commissioning window, technology criteria) is not performed here — that is the caller's responsibility.

• §42b GGV / §38a multi-meter Messkonzept
• 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):

• §21b monthly switch enforcement — enforced by einsd (validate_switch_to_vergütung)
• §53b/§54 DB lookups — resolved by einsd before calling calculate_settlement
• §55 Pönalen computation — einsd tracks commissioning deadlines
• §52 cumulative months tracking — einsd computes from violation_start dates
• §22 MessZV 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

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
# 82 lib tests + 159 regulatory showcase + 60 doctests = 301 total

The regulatory showcase (tests/regulatory_showcase.rs) is executable documentation for every §§ rule, including the correct EEG 2023 Managementprämie formula, §51 version-specific thresholds, §52 Abs. 6 netting, §100 Übergangsregelung, and all settlement scheme edge cases.