Solar Irradiance Calculator

Solar irradiance is the power of solar radiation per unit area, measured in watts per square metre (W/m2). The solar irradiance at the top of the atmosphere (solar constant) is approximately 1,361 W/m2 per NASA measurements. At the Earth's surface, atmospheric absorption and scattering reduce irradiance to 800 to 1,050 W/m2 on a clear day at sea level. The annual average global horizontal irradiance (GHI) at most US locations ranges from 3 to 6 kWh/m2/day.

The Beer-Lambert law describes how solar radiation is attenuated as it passes through the atmosphere: I = I0 x T^(1/cos(z)), where I0 is the extraterrestrial irradiance (solar constant), T is the atmospheric transmittance coefficient (approximately 0.7 to 0.75 for a clear standard atmosphere), and z is the solar zenith angle in degrees. This is the basis of the NOAA and NASA simplified clear-sky irradiance models.

The effective irradiance on a tilted panel is: I_tilted = I x cos(angle of incidence), where the angle of incidence depends on the tilt angle relative to the sun's position. At the optimal tilt (equal to the site latitude for fixed panels), annual energy collection is maximised. A panel perpendicular to the incoming radiation captures maximum irradiance at that instant. NREL's PVWatts calculator uses more detailed models that account for diffuse and reflected radiation.

The solar zenith angle (SZA) is the angle between the sun and the vertical (zenith). When the sun is directly overhead, SZA = 0 deg. At sunrise and sunset, SZA = 90 deg. SZA depends on latitude, time of day and day of year. Solar irradiance decreases as SZA increases because the same solar energy is spread over a larger atmospheric path length and ground area. NOAA's Solar Position Calculator and NASA's POWER Data Access Viewer provide SZA for any location and time.

The solar constant (I0) is the total solar irradiance received per unit area at the mean Sun-Earth distance, measured perpendicular to the solar beam at the top of the atmosphere. Its value is 1,361 W/m2 per the 2014 NASA/GCOS total solar irradiance composite. It varies by about 0.1% over the 11-year solar cycle. This value is used as the reference irradiance in all solar energy and atmospheric physics calculations.