Calculation approach
Every calculator on this site uses transparent, deterministic formulas — no machine learning, no proprietary models, no black boxes. The math behind each tool is explained in a 'How this calculator works' section on the calculator page.
Solar production
We calculate annual solar production as: system size (kW) × peak sun hours (per day) × 365 days × (1 − system loss). Standard NREL system loss is 14%, accounting for inverter efficiency, wiring losses, soiling, and shading. Annual production is then degraded by 0.5% per year — a conservative figure for modern Tier 1 panels.
Solar savings
Savings equal the value of electricity offset: kWh produced × your electricity rate. We compound utility rates by your specified annual increase (default 3%, in line with the long-run U.S. average) to project savings over 25 years.
Federal tax credit
The federal Investment Tax Credit (ITC) under the Inflation Reduction Act of 2022 is 30% of qualified expenditures through 2032, stepping down to 26% in 2033, 22% in 2034, and expiring for residential systems in 2035. We apply the rate corresponding to your install year.
Financial calculations
Net Present Value (NPV) discounts future cash flows back to today's dollars. Loan payments use standard amortization. Lease costs apply an annual escalator to the monthly payment.
Home energy
Appliance costs use standard DOE/Energy Star wattage and usage data. Heat pump comparisons use heating degree days × home square footage × a building factor to estimate heating energy demand, then divide by each system's efficiency rating.
Data sources
- **Electricity rates:** U.S. Energy Information Administration (EIA), updated annually - **Solar costs:** EnergySage, NREL, SEIA — updated annually - **Sun hours:** NREL solar resource maps — essentially static - **Tax credits:** Internal Revenue Code §48, as amended by the Inflation Reduction Act of 2022 - **Appliance data:** U.S. Department of Energy, Energy Star
Limitations
Our calculators use state-average data. They cannot account for: roof orientation and pitch, shading from trees and buildings, your specific utility's rate structure (especially time-of-use and demand charges), local permitting costs, installer-specific pricing, or state and local incentives beyond the federal credit.
For any major investment decision, get multiple quotes from certified installers in your area. Use our calculators as a starting point — they give you the rough shape of the answer, but the final decision should be informed by site-specific quotes and your personal financial situation.