pvlib.irradiance.isotropic(surface_tilt, dhi)[source]

Determine diffuse irradiance from the sky on a tilted surface using the isotropic sky model.

\[I_{d} = DHI \frac{1 + \cos\beta}{2}\]

Hottel and Woertz’s model treats the sky as a uniform source of diffuse irradiance. Thus the diffuse irradiance from the sky (ground reflected irradiance is not included in this algorithm) on a tilted surface can be found from the diffuse horizontal irradiance and the tilt angle of the surface.

  • surface_tilt (numeric) – Surface tilt angle in decimal degrees. Tilt must be >=0 and <=180. The tilt angle is defined as degrees from horizontal (e.g. surface facing up = 0, surface facing horizon = 90)

  • dhi (numeric) – Diffuse horizontal irradiance in W/m^2. DHI must be >=0.


diffuse (numeric) – The sky diffuse component of the solar radiation.



Loutzenhiser P.G. et. al. “Empirical validation of models to compute solar irradiance on inclined surfaces for building energy simulation” 2007, Solar Energy vol. 81. pp. 254-267


Hottel, H.C., Woertz, B.B., 1942. Evaluation of flat-plate solar heat collector. Trans. ASME 64, 91.

Examples using pvlib.irradiance.isotropic