The isotropic sky diffuse model is the simplest of the POA sky diffuse models and forms the foundation upon which more accurate models build.  The isotropic sky diffuse model assumes that the diffuse radiation from the sky dome is uniform across the sky.  The POA sky diffuse irradiance ($E_{d,iso}$) is calculated as a fraction of the measured diffuse horizontal irradiance ($DHI$) as:

$E_{d,&space;iso}=DHI\times&space;\frac{1+\cos&space;\left&space;(&space;\theta_{T}&space;\right&space;)}{2}$ ,

where $\theta_{T}$ is the tilt angle of the array.

References:

•  [1] 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
• [2] Hottel, H.C., Woertz, B.B., 1942. Evaluation of flat-plate solar heat collector. Trans. ASME 64, 91.

Content contributed by Sandia National Laboratories