How We Calculate Solar Data
Last updated: April 2026 · Data refreshed monthly
Every number on Sunfinder — yield, payback, savings — comes from two public, authoritative sources: PVGIS satellite data from the EU Joint Research Centre, and official US electricity rates from the Energy Information Administration. Here's exactly how we use them.
Data Sources
PVGIS — EU Joint Research Centre ↗
Satellite-derived solar irradiance data
PVGIS (Photovoltaic Geographical Information System) is a free public tool from the EU Joint Research Centre that calculates solar electricity production for any location on Earth using 10+ years of satellite radiation data. We query the PVGIS API at the exact latitude/longitude of each city using standard parameters: 1 kWp system, 14% losses, fixed mounting at the optimal tilt angle.
U.S. Energy Information Administration (EIA) ↗
Residential electricity rates by state
We use the EIA's Electric Power Monthly (Table 5.6.A) for average residential retail electricity prices by state. This is the official US government source for electricity pricing. Rates are updated monthly — we refresh our data on the 1st of each month.
Calculation Formulas
All calculations are open and reproducible. The scripts that generate our data are available on our GitHub repository.
Directly from PVGIS API. Already accounts for panel losses (14%), temperature, and shading typical of residential installs.
The national median residential system in 2026 is 7–9 kW. We use 8 kW as a representative middle value.
$2.70/W is the 2026 national average installed cost per the Lawrence Berkeley National Laboratory "Tracking the Sun" report. Regional prices range from ~$2.40/W (TX, AZ) to ~$3.20/W (NE, HI).
the expired federal tax credit, valid for systems installed through December 31, 2032. Applies dollar-for-dollar against federal income tax liability.
We assume 30% self-consumption at full retail rate, and 70% exported at retail rate (net metering). For states with reduced export rates (e.g., California NEM 3.0), we use the actual export rate from state policy data.
Net cost = gross cost − federal ITC − state credit (if any). Annual savings as calculated above. Rounded to one decimal place.
The 0.95 factor accounts for 0.5%/year panel degradation (industry standard) over 25 years, compounded. Does not include electricity price inflation — actual savings will likely be higher.
Assumptions & Limitations
System size: We model an 8 kW system because it represents the national median in 2026. Your actual system size depends on your roof area, energy usage, and budget. Use our Solar Calculator to model your specific situation.
Self-consumption ratio: We assume 30% of solar production is consumed directly and 70% is exported. This is a conservative average for a home that is occupied during the day. Households with high daytime usage (remote workers, EVs, pool pumps) will have higher self-consumption and better economics.
Net metering: We assume full retail-rate net metering for all states except California (NEM 3.0, avoided-cost rate of ~5¢/kWh for exports). If your utility does not offer net metering or uses a lower export rate, your actual savings will be lower.
Electricity price inflation: Our payback and savings calculations use today's electricity rates. The US EIA forecasts average residential rates to rise 2–3% per year. If rates increase as projected, your actual savings will be higher and payback shorter than shown.
Panel degradation: We apply a 0.5%/year degradation factor for 25-year savings estimates, consistent with Tier 1 panel warranties.
What we don't include: Shading, tree cover, roof condition, utility interconnection fees, insurance, or maintenance costs. These vary widely and require a site-specific assessment from a licensed installer.
Update Schedule
PVGIS solar yield data is re-queried on the 1st of each month for all 250 cities. EIA electricity rates are updated monthly when new data is released (typically the 2nd week of each month). Incentive data (state tax credits, net metering policies) is reviewed quarterly and updated when changes are announced.