Isotropic

Method

The isotropic model assumes the diffuse light comes evenly from every part of the sky. The tilted plane then sees the fraction of the sky dome it faces. The model is simple and tends to underestimate the diffuse light on planes that face the sun on clear days.

The math

$$D_{plane} = D\,\frac{1+\cos\beta}{2} \tag{1}$$

where

Symbol	Meaning	Unit
$D_{plane}$	the diffuse light on the module plane	W/m²
$D$	the diffuse horizontal irradiance of the weather file	W/m²
$\beta$	the tilt of the module plane from the horizontal	degrees

References

1. Liu & Jordan 1963, The long-term average performance of flat-plate solar-energy collectors, Solar Energy 7(2) 53–74

API

array.transposition_model = isotropic

Symbols follow the symbol registry.