Insolation
The solar electromagnetic radiation power reaching a specified surface area per unit time.
Governing formula
S = S₀ (R₀ / R)² cos Z
SI unit
W/m² or kWh/m²/day
In depth
Short for 'Incoming Solar Radiation'. Total solar irradiance at the top of Earth's atmosphere is S₀ ≈ 1361 W/m². Surface insolation depends on solar zenith angle Z, latitude, seasonal Earth-Sun distance, and cloud cover attenuation.
Examples in the real world
Equatorial regions receiving higher annual mean insolation than polar regions due to direct solar zenith angles.