Methodology & data sources
Transparency is the core of our E-E-A-T: this page documents where our data comes from, how often it is refreshed, and the formulas behind every calculation.
Data sources
| Source | Refresh cadence | License |
|---|---|---|
| U.S. EIA — Electric Power Monthly (residential prices) | monthly | U.S. Government public domain |
| NREL PVWatts Calculator (production estimates) | none | U.S. Government public domain |
How the solar payback calculation works
The payback calculator runs entirely in your browser. For each year over a 25-year horizon it:
- Computes annual production = system size (kW) × production factor (kWh per kW per year).
- Applies panel degradation of 0.5%/year (NREL median for modern modules).
- Values production at your electricity rate, grown each year by the escalation you enter (default 3%).
- Subtracts the up-front net cost = size × 1,000 × cost-per-watt, less any tax credit you enter.
Simple payback is the year your cumulative savings first exceed the net cost. The model deliberately excludes financing interest, inverter replacement, maintenance, and the time-value of money, so it is a conservative planning estimate rather than a financial projection.
Where the per-state numbers come from
- Electricity rates: average residential price by state from the EIA Electric Power Monthly (data period March 2026, released May 2026), in cents/kWh.
- Production factor: derived from each state's average daily peak sun hours (NREL/National Solar Radiation Database) using production ≈ peak sun hours × 365 × ~0.80 system derate, then cross-checked against NREL PVWatts city runs. Actual output varies ±10% or more with roof orientation, shading and weather.
Federal tax credit note (2026)
The 30% federal residential clean energy credit (Internal Revenue Code Section 25D) was repealed for systems placed in service after December 31, 2025 under the 2025 budget law. Our defaults therefore use a 0% federal credit for 2026 installs. State and utility incentives still apply and vary widely; we mention the main ones on each state page but do not model them in the headline number.
Limitations
Figures are estimates for general information and may lag the underlying source or contain errors. They are not financial, tax or engineering advice. Always get a written quote and verify incentives against the primary source before relying on them. See our disclaimer.