Solar Irradiance on a Tilted Surface

Calculate the sunlight power hitting a tilted solar panel or window from the sun's position and the surface orientation, free online.

Direct normal irradiance (W/m²)

Sun elevation above the horizon (°)

Surface tilt from horizontal (°)

Azimuth difference: sun vs. surface facing (°)

Surface area (m²)

Panel efficiency (%) — optional (optional)

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The cosine of the angle of incidence is cos θ = sin(elevation)·cos(tilt) + cos(elevation)·sin(tilt)·cos(azimuth difference). Direct irradiance on the surface is DNI × cos θ. A surface facing the sun straight-on receives the full DNI (about 900–1000 W/m² at noon on a clear day).