Methodology
This site lives or dies on trust, so every number has a provenance. This page documents how each class of data is produced.
Calculator engine version: 2026.08.1.
1 · The data-class taxonomy
Every figure on the site carries one of these labels, visually and in the data:
| Class | Meaning | Example |
|---|---|---|
| Verified current | Confirmed against an authoritative source at last check | EIA state electricity rates |
| Market estimate | Compiled from multiple public sources; a range, not a guarantee | State median $/W |
| Your input | Values you enter into a calculator | Your annual kWh |
| Projection / scenario | Modeled future values under visible assumptions | Year-10 savings at 2% escalation |
| Expired / ended | No longer available — rendered as expired, never as current | §25D residential credit |
| Recheck needed | We could not verify current status at last sweep | Some state program records |
| Data unavailable | No reliable public source found | ZIP-level price precision (we refuse to invent it) |
2 · Cost per watt ($/W) — state estimates
State median / low / high cash $/W figures are market estimates compiled 2026-Q1/Q2 from: EnergySage marketplace quote data (national median ≈ $2.95/W), NREL residential system cost benchmarks (≈ $2.62/W), installer price surveys, and state-level market reports. Ranges are set to span typical cash quotes for a straightforward rooftop install, before incentives. We do not publish ZIP-level or city-level exact prices — there is not enough public data to do that honestly. Outliers (very low “panel-only” deals, very high “whole-home+EV+battery” projects) are excluded from medians and documented where material.
3 · Electricity rates
State residential rates and monthly usage come from the U.S. Energy Information Administration (EIA), Electric Power Monthly, May 2026 reporting (published with ~3-month lag). Rates are statewide averages — a household’s actual utility, tariff, and usage tier can differ substantially; we say so on every page and provide utility records where we have them.
4 · Solar resource / production factors
Production factors (kWh per installed kW per year) are latitude-derived estimates based on NREL GHI/PVWatts-style annual insolation, then reduced ~14% for system losses (inverter, soiling, mismatch, wiring). They are planning values, not roof-level engineering. For site-specific numbers, use PVWatts or ask the installer to show their modeling.
5 · Incentives
Every incentive record carries: name, jurisdiction, type, amount (quoted from program materials), eligibility, status, source, and lastChecked. Status rules: shown as current only when verified at last sweep; expired when the program ended (e.g., §25D 2025-12-31, NJ SRECs 2022, CO income-tax credit 2021); recheck when unverifiable. We never present an expired incentive as available, and we never invent amounts. Primary references: IRS guidance, state energy offices, DSIRE, utility tariffs.
6 · Savings, payback & LCOE calculations
- Savings: year-n savings = (self-consumed kWh × retail rate) + (exported kWh × export rate), escalated annually at a visible, editable rate (default 2%; options 0/2/4%). Production degrades 0.5%/yr after year 1.
- Payback: simple payback = net cost ÷ year-1 savings, plus a cumulative model that finds the crossing year. Ranges, not guarantees.
- LCOE (simplified): (net cost + 1%/yr O&M allowance) ÷ lifetime production, undiscounted. Presented as a comparison to expected grid cost, with assumptions visible — not as an investment metric.
- Escalation honesty: the escalation assumption is always visible and never hidden. Try 0% and 4% to see how much the result moves.
7 · Financing
Loan payments use standard amortization. Dealer-fee math: financed principal = cash price ÷ (1 − dealer fee); we display the principal, the fee in dollars, monthly payment, and total paid over the term. Lease/PPA totals accumulate payments with the stated escalator over the term, undiscounted, and are labeled as such. We do not assert that any path is universally better.
8 · Quote analysis
The quote analyzer normalizes a quote to $/W, compares it to the state’s low/high range, and classifies it (below / typical / above / well-above). Red flags are rule-based on what the user enters (missing models, no production estimate, an implied federal credit in a 2026 price, undisclosed dealer fee, escalators >4%). We do not accuse specific companies without evidence, and legitimate add-ons are explained, not penalized.
9 · Data freshness
Anchors: rates 2026-05 · cost data 2026-Q1 · incentives 2026-08-21. Records show their own dates. When data goes stale or policy changes, status flips to “recheck needed” or “expired” — it never silently stays “current.”
10 · What we will not do
- We will not publish ZIP-level or city-level exact prices without real underlying data.
- We will not present the (expired) federal credit as available.
- We will not guarantee savings, payback, ROI, or home-value increases.
- We will not rank installers by lead commission, and paid installers can never alter pricing benchmarks.
- We will not collect contact information before showing calculator results.
Full source list: sources page · corrections: contact.