About the data

Where the numbers come from

Every irradiance figure on this site comes from the NASA POWER project — a reanalysis of satellite and meteorological observations, served through a public API that needs no key. We request the daily global horizontal irradiance (ALLSKY_SFC_SW_DWN) for each city's exact coordinates over the full calendar year 2024.

Peak sun hours are reported as annual irradiance divided by 365, which is the standard definition and matches how PVWatts-style tools express it.

The full dataset is embedded in the page as JSON, so every calculation you run happens in your browser. We do not send your usage figures to a server.

How the panel calculation works

• Daily load = monthly kWh ÷ 30.
• Array kW = daily load ÷ (peak sun hours × system efficiency × roof factor).
• System efficiency = 0.86 × 0.96, the NREL PVWatts default for system losses and inverter efficiency.
• Panels = array kW ÷ panel wattage, rounded up.
• Annual output is summed month by month using each month's measured irradiance.

What this tool is not

It is a sizing estimate, not a design. Shading from trees and buildings, roof age, panel brand, snow and soiling all move the real number. Always have a licensed installer check the design and confirm the panel and inverter datasheets before you buy anything.

Accuracy check

Spot-checking the dataset against known values: Phoenix averages 5.84 peak sun hours and Seattle 3.48. That ordering, and the ~40% spread between them, matches the real-world gap between the Sonoran Desert and the Pacific Northwest.