Methodology
Every number on the site is a planning estimate. This is exactly how it's calculated.
Our calculators share one model. Here are its assumptions.
From your monthly bill and local electricity rate we derive annual kWh, then size the system to your chosen offset (default 100%) using local peak-sun hours and a system derate of 0.84 (soiling, inverter losses, wiring, temperature). Size is capped between 1 kW and 30 kW, and to your roof if you provide its area (~18 W per usable sq ft).
Gross cost = system watts × a typical local price per watt. A battery, if selected, is added at about $1,000/kWh. These are regional averages, not installer quotes.
The federal residential tax credit is applied by install year: 30% through 2025, 0% for 2026 and later residential systems. State credits, upfront rebates and production incentives are applied from a representative per-state table where we have modeled data (about 22 states); the rest use a generic no-incentive fallback. Sales- and property-tax exemptions are noted but not added into dollar figures.
Year-one production = size × peak-sun hours × 365 × derate. We apply 0.5%/yr panel degradation and a 3%/yr utility rate escalation over a 25-year horizon. Payback is cumulative: the year in which running savings (plus any production incentives) equal the net cost.
The financed payment uses a standard amortization on the net cost at the APR and term you enter (defaults: 6.9%, 20 years).
The model can't see your roof orientation, shading, main panel capacity or a specific installer's pricing. Use it for planning and comparison, then get real quotes and check them with the Quote Checker. See data sources.
This page is the summary. The complete methodology covers the decision engine, the three-scenario comparison, the sensitivity analysis and the installer match-score weights, function by function.