Methodology & validation

gen_t = the area's actual 30-minute output on each day, scaled so Σ energy = capacity × capacity-factor × hours; price_t = the area day-ahead price. Rate <80% signals cannibalisation. For a plant using the area's own profile the monthly capture price equals that month's FIP reference M.

base′ = base price + 発電側課金相当額 add-on. A = previous fiscal-year average market price; M = output-weighted monthly area market price; M′ = same month of the previous fiscal year; NFC = volume-weighted price of the last four 非FIT非化石証書（再エネ指定）auctions; BC = balancing-cost allowance by delivery FY and FIP-start FY. Computed each month and paid on metered output. Legal basis: 再エネ特措法施行規則 第3条の4 (基準価格・参照価格) and 第3条の5 (プレミアム), with the 調達価格等算定委員会 opinions.

Perfect-foresight day-ahead optimisation (upper bound): the n cheapest half-hours charge and the n dearest discharge, charging before discharging. Energy convention: 'delivered' → e_dis = power·0.5, e_ch = e_dis/η; 'charge' → e_ch = power·0.5, e_dis = e_ch·η. A 'forecast-based capture %' scales the result to reflect imperfect dispatch.

Deviation volume = mean absolute error (not 1σ). Per slot the adverse spread is the direction that hurts (shortage − day-ahead when short, day-ahead − surplus when long) and the favourable spread the direction that helps. The default error % is the EPRX 三次② area aggregate (day-ahead→gate-closure revision), scaled by a plant-level multiplier. Imbalance unit prices use the official 不足/余剰 series (equal in every published slot).

Prices/volumes per area × product from EPRX 取引結果（速報値）. The cap in force steps 19.51 → 15 (14 Mar 2026) → 10 (1 Sep 2026) for 一次/一次オフライン/二次①/複合; 7.21 for 二次②/三次①; no cap for 三次②. Headroom is measured against the cap in force on each day.

15–20 year nominal project model: revenue schedule (FIP/merchant/PPA) net of O&M, degradation, augmentation and other costs (land lease, insurance, 発電側課金). Debt is sized by the gearing input (not a target DSCR), with a tenor and interest rate. Tax applies only when a corporate tax rate is entered: tax depreciation on the statutory tables (耐用年数省令 別表第十 200% declining balance with the 償却保証額 switch to 改定償却率, or 別表第八 straight line; both leave ¥1), 固定資産税 on a declining assessed value floored at 5% of acquisition cost (地方税法; first-year value = cost × (1 − 減価率 ÷ 2)), loss carry-forward (losses usable 10 years, oldest first, deducting up to the 50%/100% limit input — 100% for 中小法人等, 国税庁 No.5762/No.5800), and the 解体等積立金 decommissioning reserve for FIT/FIP solar ≥10 kW and FIT/FIP wind (withheld in the last 10 support years, released at decommissioning to pay the decommissioning cost which is deductible when incurred; reserve itself not deducted when withheld nor taxed when released — assumption, no 国税庁 ruling). FIT reverts to merchant revenue after the purchase period (20 yrs solar ≥10 kW and wind, 10 yrs solar <10 kW). The equity (levered) case computes tax on income after interest with the same loss rules, so the interest tax shield (= unlevered tax − levered tax) is only credited when tax is actually saved; it lifts the equity IRR and the DSCR's CFADS uses the levered tax. FIP revenue uses the dynamic 20-year projection (market escalation, BC schedule, grid add-ons). LCOE excludes corporate tax.