docs product EV_FINANCIAL_METHOD

EV energy and cost method — exact cross-language contracts

Date: 2026-10-20. EE-11 is LOCAL_DONE / INTEGRATION_PENDING; other runtime methods remain unimplemented until their owning acceptance. Owner: EV rules, EE-02/11/15/16/23. Extends, never reinterprets, financial-v1. Freeze machine vectors before code; numeric input ceilings/rounding precision follow a recorded need and existing bounds, not arbitrary guesses or provider float examples.

Independent review, 2026-10-21: audited EE-11 checkpoint repairs false Rust overflow, fractional/calendar decoding and typed-input validation.53 shared vectors now execute in Go/Kotlin/Rust without new dependencies. Large rational intermediates remain exact; only the rounded money result is subject to the existing centavo ceiling. No provider/source, session persistence, public API or production acceptance follows from this.

EE-02 checkpoint, 2026-10-21: exact policy/data contract and24 shared CSV arithmetic vectors are frozen and validated in Go/Kotlin/Rust. This is contract arithmetic, not acceptance of the EE-11 runtime tariff engine, measurement/window method, provider billing or SQL/report implementation.

EE-11 checkpoint, 2026-10-21: the pure tariff scenario engine is implemented in Go/Kotlin with a Rust parity oracle over 42 shared synthetic rows (evidence). Only INCLUDED tax with complete mandatory components, no undeclared minimum/maximum stage and the supported final half-up rounding produce KNOWN centavos; absent energy, incomplete/unknown fees, eligibility mismatch, partial validity windows and money overflow are explicit refusals/UNKNOWN. This remains a local pure engine: provider billing terms, SQL/report persistence, monthly timezone allocation and public ranking are not accepted by this checkpoint.

Basis and valid windows

Persist actual paid BRL in centavos; energy in an exact declared Wh-compatible scale, power W, time seconds and odometer metres. Raw rate decimal, unit, billing step, eligibility, minimum/maximum, tax basis, validity and timezone are preserved and converted using checked integer/rational operations. Freeze final display rounding and provider billing rounding separately; no rounding each intermediate unless the actual tariff specifies it. Cross-language Rust/Go/Kotlin vectors must agree. Unknown fees, amount, energy or basis are nullable, never zero defaults.

Keep four methods distinguishable: reference/modelled vehicle efficiency; meter/charger-basis purchased-energy interval; actually measured vehicle energy; and monetary observed-window cost. Compare only the same method/basis/unit and matching eligibility. No phone GPS distance or nominal battery inferred energy silently completes an odometer/energy window.

An electric matched interval requires a declared source/measurement basis, positive complete distance and included energy with accepted start/end charge state/anchor/gap policy. Different battery state at endpoints affects bought energy versus consumption; disclose that limitation or return UNKNOWN under the frozen method. Never call arbitrary sum of purchases divided by driving distance exact battery efficiency. For a PHEV do not infer distance split between motors.

Monthly money totals use event occurrence in the selected IANA timezone. A session can preserve start, end and payment timestamps; the frozen event-time rule selects the monthly expense basis. Crossing midnight/month does not permit inventing per-minute energy/cost without interval evidence. A monthly total does not establish a complete monthly distance denominator. Retain separately labelled valid observed-window metrics. A backdated correction invalidates dependent output.

Worked synthetic acceptance vectors

1. Unit conversion: 150 Wh/km = 0.15 kWh/km = 15 kWh/100 km = 0.54 MJ/km. A PBE field labelled equivalent km/L is retained under its actual method; no implicit conversion to physical litres or guessed kWh based on that label.
2. Actual energy charge: 30 kWh at BRL 1.80/kWh = BRL 54.00; plus explicit BRL 2.00 session fee = BRL 56.00. At 160 km over a valid same-basis matched interval, bought energy is 18.75 kWh/100 km and cost is BRL 0.35/km, labelled purchased-energy/session-cost basis, not battery laboratory efficiency.
3. Separate parking: add BRL 8.00 parking to the previous same-window case: fuel/energy-session cost stays BRL 56.00, all included cost becomes BRL 64.00 and BRL 0.40/km. Do not include parking twice or label BRL 0.40 energy-only.
4. Multi-component tariff: 12 kWh at BRL 2.00/kWh plus 600 charging seconds at BRL 0.50/minute plus BRL 2.00 fixed = BRL 31.00 before idle charge. Six idle minutes with five-minute grace, one-minute billing steps and BRL 1.00 per charged minute add BRL 1.00: total BRL 32.00. Freeze inclusive boundary and step semantics with additional zero/exact-boundary/partial-step cases.
5. Free is explicit: known zero energy/session price plus BRL 10.00 parking is not a free whole visit. Missing energy price cannot be treated as zero; unknown mandatory fees prevent ranking by complete estimated total.
6. PHEV monetary aggregation: 25 L at BRL 6.00 = BRL 150.00 plus 20 kWh at BRL 2.00 = BRL 40.00; total BRL 190.00 over a valid same 500 km window is BRL 0.38/km. Do not derive liquid efficiency as 500/25 or electric efficiency as 500/20 without proven propulsion-specific distance/energy method.
7. Measurement distinction: 18 kWh measured at supply and 16 kWh measured at battery over verified corresponding boundaries differ by 2 kWh. Preserve both measurement bases and provenance; do not assert a universal loss rate, degradation or every future session's efficiency from this observation.
8. SOC estimate: nominal 60 kWh and 50 percentage-point SOC change does not establish 30 metered kWh. Usable/gross capacity, display estimation and losses are unknown unless supplied. Retain a labelled model estimate separately; do not overwrite the reported meter energy or actual paid total.
9. Home tariff: separate TE/TUSD references are incomplete as a final home rate without the applicable other components and basis. A user-supplied known effective BRL/kWh can drive a labelled estimate; actual paid session allocation remains a separate asserted amount. Solar/free/employer-paid is explicit data, not inferred from a location label or a vehicle model.
10. Wallet is not a second session: adding BRL 100.00 prepaid credit and later paying BRL 40.00 for one session does not create BRL 140.00 of consumed-energy expense. Wallet/payment processing is outside this scope; do not fabricate ledger energy or double count imported session totals and tariff components.
11. Unknown or incompatible windows: missing distance, mismatched SOC/gaps, meter-basis change, partial-month coverage and reset segments produce the relevant UNKNOWN reason; raw monetary/energy totals remain readable. A cost correction/deletion recomputes affected revisions, not an unrelated vehicle.
12. Comparability and bounds: unequal-distance fleet/vehicle summaries use eligible summed energy/cost/distance, never average individual ratios. Keep L, m3 and kWh totals separate. Refuse negative/overflow/noncanonical decimal, unknown currency conversion, unsupported precision and zero denominators.

These are test targets, not observed customer results, active billing terms or implemented measurements. Add timezone/daylight-change, member/nonmember, effective-tariff switch, provider rounding, capped/minimum charge and incomplete components to the frozen test matrix before ranking a real offer.