Methodology
This is a financial-decision product, so nothing here is a black box. Every calculator, the decision engine and the Solar Advisor all call the same functions, documented below. Figures are planning estimates, never guarantees.
Data vintage: the reference data (sun hours, electricity rates, installed price ranges, incentive records) was last reviewed August 2026. See data sources for where each figure comes from.
From your monthly bill and the local residential electricity rate we derive annual kWh (or you enter it directly). We size the array to your chosen bill offset (default 100%):
size (kW) = annual kWh ÷ (peak-sun-hours × 365 × 0.84 derate)
The 0.84 derate covers inverter losses, wiring, soiling and temperature. Roof orientation and shading apply a production multiplier (south & unshaded = 1.00; east/west ≈ 0.86; north ≈ 0.70; "some" shade ≈ 0.88; "heavy" ≈ 0.72). Size is clamped to 1–30 kW, and to your roof area if supplied (~18 W of panel per usable sq ft).
Gross cost = system watts × a typical installed price per watt for your state (a low–high range from our state table; the midpoint is used unless you override it). A battery, if selected, is added at about $1,000/kWh installed plus a base install cost. These are regional averages — not an installer quote.
The federal residential clean-energy credit (Section 25D) is applied by the year the system is placed in service: 30% through 2025, 0% for 2026 and later (the credit was terminated for post-2025 residential installs by the 2025 budget-reconciliation law). State income-tax credits, upfront rebates and production/SREC payments are applied from a per-state table where we have modeled data; other states use a no-incentive fallback. Sales- and property-tax exemptions are described but not added into dollar totals. The full structured incentive records — with source and last-verified date — are what getSolarIncentives() returns and what the state incentive pages display.
Year-1 production = size × peak-sun-hours × 365 × derate × roof factor. Over a 25-year horizon we apply 0.5%/yr panel degradation and a 3%/yr utility-rate escalation (adjustable). Payback is cumulative: the first year in which running bill savings (plus any production incentives) equal the net cost. "Estimated return" is 25-year savings ÷ net cost − 1.
Cash uses net cost up front. Loan amortizes the net cost at the APR/term you set (default 6.9%, 20 yr) and reports the monthly payment, the net monthly impact vs. your savings, and the total financing cost — note that dealer/origination fees baked into solar-loan APRs (often 10–30% of the amount financed) are not modeled. Solar + battery adds the battery cost and reports backup runtime for essential loads; a battery normally lengthens payback.
We re-run the whole model at electricity-price escalation of 2%, 4% and 6%/yr, and across three combined cases: optimistic (lower install price, 4.5% escalation), base (midpoint price, 3%) and conservative (higher install price, 2%). This brackets the plausible range rather than giving one false-precision number.
Each installer is scored 0–100 on transparent, fixed weights:
Because our installer records are publicly sourced and not yet independently verified, the reputation, warranty and verification components currently use a neutral baseline for every company. The ranking is algorithmic and may include companies that have a commercial relationship with SolarInstallers.io — that is disclosed on every page that shows a ranking. See how we verify installers.
Your exact roof planes, main-panel capacity, tree growth, a specific installer's pricing and equipment, your tax situation, and your utility's exact net-metering successor tariff. Use this for planning and comparison, then get real quotes, compare them side by side, and sanity-check the one you like.