Solar Irradiance
Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation, measured in watts per square meter (W/m²). It is the fundamental measure of the solar energy available at a location and varies based on the Sun's position in the sky, atmospheric conditions, and geographic location.
At the top of Earth's atmosphere, the solar irradiance is approximately 1,361 W/m², a value known as the solar constant. By the time sunlight reaches the Earth's surface, absorption and scattering by the atmosphere reduce this to approximately 1,000 W/m² under clear-sky, sea-level conditions — the Standard Test Condition (STC) used for solar panel ratings.
In practice, actual surface irradiance varies continuously throughout the day and year, and averages significantly below the peak value. Cloudy conditions can reduce irradiance to 100–300 W/m² or less.
Why Irradiance Matters for Solar
The power output of a solar photovoltaic (PV) panel is directly proportional to the irradiance it receives. A panel rated at 400W under STC (1,000 W/m²) produces approximately 200W when irradiance is 500 W/m², all else equal.
Annual solar energy production is calculated by integrating irradiance over time, converted to kWh/m²/year. SolarScope uses NASA POWER and NREL data to provide accurate irradiance estimates for any location.