How we check facts and build our estimates

Plug-in solar rules are new and changing fast, and secondary sources already disagree on dates and caps. This page explains which sources we trust, how we handle conflicts, and exactly how our numbers are calculated.

State law facts

Primary sources first. We read the statute, the enacted or enrolled bill, or the regulator's own page. News and trackers are used to find laws and to fill gaps, and are labeled as reporting.
Every fact has a link. Each state page lists its sources with their type: statute, bill text, government, reporting or tracker.
Conflicts are shown, not hidden. When sources disagree, for example on Colorado's wattage cap or Maryland's effective date, the page says what we could not confirm.
Dates. All state pages show the date the facts were last checked. A full re-check runs at least monthly and whenever a governor signs or vetoes a bill.
No legal advice. We summarize laws for general understanding. Your utility, your lease and a licensed electrician have the final word for your home.

The calculator formula

Yearly production in kWh is:

panel kW × peak sun hours × 365 × orientation factor × (1 − shade loss) × 0.82

Peak sun hours: daily sunlight on a 20° south-facing surface at each state's largest city, from NREL PVWatts v8 and the National Solar Radiation Database, as compiled by TheGreenWatt.
Orientation factor: our own model of how tilt and facing change yearly sunlight, relative to that 20° south reference. Data and method.
Mounting options: 90°, 60°, 35°, 10°.
Shade loss: 0%, 10%, 25%, 50% for the four shade choices. These are rounded rules of thumb, not measurements.
0.82: a system performance ratio covering wiring, heat, dust, mismatch and microinverter losses, comparable to PVWatts' default 14% losses plus inverter efficiency.
Clipping: if panel watts exceed 1.25 times the inverter limit, we reduce output to approximate lost midday peaks.

Savings count only the energy your home uses as it is produced, from a self-use curve modeled hourly with typical-year weather. Exported power is valued at zero, matching the plug-in laws in Vermont and Maine, which exclude these devices from net metering.

Savings = used kWh × electricity price. The default price is your state's average residential price for January through August 2026 from the EIA. Payback = kit price ÷ yearly savings. The 10-year figure applies 0.5% yearly panel degradation and holds the electricity price flat.

How it compares with PVWatts

PVWatts models a specific address hour by hour and is more precise for one location. Our approach trades that precision for state-level speed and for balcony angles PVWatts users rarely enter. For a final decision, run your exact address and angles in NREL's PVWatts, then multiply its yearly kWh by your price and the self-use share our calculator shows.

Content and AI

We use software, including AI tools, to help gather sources, draft and check calculations. Every published fact is checked against its cited source, and every number comes from the data and code described here. We do not publish product reviews of equipment we have not tested, and we do not invent experiences.

Corrections

Found an error? Email hiruben@gmail.com. We fix confirmed errors promptly and log substantive changes on the corrections page.