The fuel-cost adjustment is settled before it is announced.
Every Japanese electricity tariff passes fuel costs through by a formula fixed in the supplier's 約款. Its inputs are customs import prices for crude, LNG and steam coal, averaged over a window three to five months back. Once the last month of that window prints, the rate is arithmetic — 8 to 13 days before the supplier publishes it, and about two months before the month it applies to. This page computes that interval.
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The calculation
average fuel price = round100( α·crude + β·LNG + γ·steam coal ) unit rate = round0.01( (average − base fuel price) × base unit rate ÷ 1,000 ) − government rebate
α, β and γ are heat shares divided by calorific values; they, the base fuel price and the base unit rate are all published in each supplier's tariff schedule. The only remaining input is the customs release.
The implementation reproduces 21 announced months from 関西電力 and 東京電力エナジーパートナー to the last 銭. Coefficients and the verification procedure are in methodology; endpoints are in the API reference.
The cap
Regulated tariffs cap the average fuel price at 1.5× the base. All ten incumbents removed that cap from their liberalised menus between October 2022 and May 2023.
関西電力's regulated low-voltage tariff is against its cap now, so the same supplier charges two different rates in the same month depending on which menu a customer is on. A capped tariff is a call on fuel prices held by the customer, which is why comparing headline rates alone does not settle which menu is cheaper.
Data
Ministry of Finance customs releases (crude 30301, LNG 3050103, steam coal 3010105) and the published parameters in each tariff schedule. Revisions are stored as additional vintages rather than overwriting, so a month can be reproduced from what was visible on the day its supplier announced. Coverage begins January 2024.