Solar Irradiance
The power of sunlight reaching a surface, measured in watts per square meter (W/m²) — the fundamental input that determines how much electricity a solar panel can generate.
Solar irradiance measures the instantaneous power of solar radiation per unit area (W/m²). Standard Test Conditions (STC) for panel ratings use 1,000 W/m² — roughly equivalent to full noon sun at sea level on a clear day. Real-world irradiance is almost always lower, varying with time of day, season, cloud cover, atmospheric haze, and altitude.
Global Horizontal Irradiance (GHI) is the total solar radiation on a horizontal surface — the metric used in most solar resource maps. Direct Normal Irradiance (DNI) measures radiation from the direct solar disk, used for concentrated solar applications. Diffuse Horizontal Irradiance (DHI) is scattered skylight — relevant because panels produce some electricity even in diffuse conditions, which is why Germany (low irradiance) has significant solar capacity.
Irradiance data for any US location is available from NREL's National Solar Radiation Database (NSRDB) and the PVWatts tool. Installers use historical irradiance data averaged over 10–30 years to estimate production, which is why production estimates come with ±10–15% uncertainty in any given year.
Real-World Example
The installer's production estimate used 30-year average irradiance data showing 4.7 kWh/m²/day at the site location; the homeowner understood actual first-year production could be 10% higher or lower depending on weather.