Irradiance
The power of solar radiation received per unit area, measured in W/m² (watts per square meter). Standard peak irradiance at earth's surface is ~1,000 W/m² under clear skies at solar noon.
Why It Matters
Your location's average irradiance (available through NREL's NSRDB) determines how much electricity solar panels will produce. The Southwest U.S. has 30-50% more irradiance than the Northeast.
Related Terms
Peak Sun Hours
The equivalent number of hours per day when solar irradiance averages 1,000 watts per square meter. Varies by location.
Standard Test Conditions (STC)
The standardized laboratory conditions under which solar panels are rated: 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum. Panel wattage ratings are always at STC.
Capacity Factor
The ratio of actual energy produced by a system to its maximum potential output. Solar typically has a 15-25% capacity factor.