Methodology and sources
Last updated: 2026-10-11
1. Solar production
For a reference city in each of the 51 states and D.C., and for 98 additional cities, we queried PVWatts v8 from the U.S. Department of Energy's National Laboratory of the Rockies (formerly NREL) for a 1 kW fixed roof-mounted system with standard modules and 14% system losses, using typical-year (TMY) weather. For each state we store monthly AC output for five orientations (south, southeast, southwest, east, west) at 20° and 30° pitch. City pages use the city's own south-facing output and apply its state's orientation ratios. Data retrieved 2026-10-11; nothing is fetched while you use the site.
2. Electricity prices
State averages come from the U.S. EIA Electric Power Monthly, Table 5.6.A (residential sector), currently July 2026. Recent months are preliminary. Your own all-in rate from your bill is always better.
3. Self-consumption and export credits
- Monthly use = annual use split by days in the month.
- Used on the spot = the smaller of the month's production or the month's use × your daytime share.
- Exports = production − used on the spot. Grid purchases = use − used on the spot.
- Export credit = exports × credit rate (full retail, half retail or about a quarter of retail, depending on the option you pick). Credits for the year are capped at the value of a year of grid purchases, which models the common annual true-up where excess credits are not paid out.
- Savings = used on the spot × your price + export credits.
4. Payback and return
We project 25 years with 0.5% annual panel degradation and the price change you choose. Payback is the year cumulative savings minus maintenance reach the installed cost minus incentives. IRR uses the same cash flows, without financing or taxes.
5. Default values
The preset system size (7 kW), usage (10,500 kWh/yr) and cost ($3,000/kW) are round example values so the tool shows a result, not quotes. The federal 30% residential credit does not apply to systems placed in service after December 31, 2025 (IRS), so incentives default to $0.
6. Known limits
- Monthly, not hourly, matching can overstate on-the-spot use. Use a conservative daytime share.
- No batteries, time-of-use rates, demand charges, fixed fees, minimum bills or taxes on credits.
- Reference-city weather: your neighborhood, elevation and shading (trees, chimneys, nearby buildings) can change output.
- Net metering rules vary by utility and change over time; check yours before deciding.
7. Corrections
Spotted an error? Tell us through the contact page. Material corrections are noted on the affected page with a date. Who runs the site and how it's funded is on the about page.