Compare ownership costs, site yield and usable tariff benefits, verify dated incentives and inspect full-horizon NPV, payback reversals and sensitivities.
Identify whether each proposal is a cash purchase, financed ownership, lease or power-purchase agreement and record who receives energy value, incentives and service obligations. Collect complete prices, financing schedules, roof/site work, warranties and exit terms. Use one currency and a consistent real or nominal basis.
Obtain a site-specific AC production model and comparable load/tariff data. Value self-use and exports independently, including credit usability and incremental charges. Keep an explicit degradation scenario separate from weather variability, availability and tariff changes so the same effect is not counted twice.
Check current programs for the owner, equipment, installation date and jurisdiction. For U.S. residential section 25D, IRS guidance says expenditures after December 31, 2025 are ineligible; completion of original installation generally determines that expenditure timing. Do not apply the former through-2032 assumption to a new 2026 installation. Record only supported receipts in the actual year.
Enter every analysis year with avoided imports, usable export receipts, verified other receipts, operations, replacements and other owner cash costs. Include financing payments or contract obligations on the chosen ownership basis. Use zero rows for empty years and a discount rate consistent with the currency and inflation treatment.
Save lower-output, lower-credit, higher-service-cost and replacement-timing cases with stated reasons. Use the explicit scenario mode when simple annual changes fit the proposal, or the ledger for irregular flows. Inspect the complete horizon because a replacement can reverse an earlier payback crossing.
Export the accepted proposal, source dates, assumptions, full ledger and sensitivity cases. Record why the chosen option fits the owner’s budget and service needs. After commissioning, update actual bills, realized incentives and costs while preserving the original decision case for a fair comparison.
Retain actual proposal terms, ownership responsibilities and excluded costs. · Retain the official program source, eligibility evidence and expected payment or tax-use timing.
Compare two documented whole-system costs and modeled energy cases on equivalent assumptions.
Calculate bill benefit using actual self-use, exports, rates and realizable credits. · Reconcile measured self-use/export quantities and realized tariff benefit.
Project an entered annual energy-decline scenario or review normalized measured change. · Vary the stated energy-decline scenario without assuming a warranted lifetime.
Enter verified upfront receipts separately from later annual receipts. · Calculate full-horizon NPV and first versus retained cumulative recovery. · Compare dated and scenario cash flows, discounting and retained recovery. · Save actual annual receipts and costs alongside the original forecast.
Provide a clearly labeled constant-benefit simple-payback comparison.
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As reviewed September 9, 2026, IRS FAQ 1 and 7 state the December 31, 2025 expenditure cutoff and explain that paying before that date does not qualify an installation completed afterward.
A complete horizon includes recurring and irregular costs; first recovery alone can hide later reversals.
Review the actual proposal, ownership arrangement and local utility context.
Project costs, owner equity and debt payments must be arranged so financing is not double counted.
An attractive long-horizon model can still require cash the owner needs sooner or allocate risks they do not accept.