An incentive changes the cash flow—it does not change the physics
The same PV array produces roughly the same energy whether a tax credit exists or not. What changes is the upfront cost, financing requirement and payback period. Keep the energy model separate from the incentive model so an expiring programme does not invalidate the whole calculation.
Use the project date that the programme actually uses
Some programmes use installation/commissioning date, some use expenditure timing, some require application approval before work starts, and some depend on available funding. Never move a deadline from a marketing page into a financial model without reading the official eligibility rule.
Recalculate export value separately
Export tariffs, net billing and feed-in payments affect the value of electricity you do not use on site. A battery can raise self-consumption but adds cost and conversion losses. Compare a solar-only case, solar-plus-battery case and backup-resilience case separately.
Three current examples show why dates matter
- United States: the IRS says the residential clean-energy credit is not available for expenditures made after December 31, 2025.
- Australia: the federal battery STC calculation changed on 1 May 2026 and now tapers by usable capacity.
- Germany: published EEG remuneration bands change with commissioning periods and system size.
Build a transparent worksheet
Record equipment cost, installation cost, financing, expected annual generation, self-consumption, import tariff avoided, export tariff received, battery losses, maintenance assumptions and any incentive with its exact eligibility date. Then run sensitivity cases instead of publishing one “years to payback” number.
Use the Melnti calculators for the energy side and the appropriate location guide for current market rules.