Isotropic diffuse

Method

In a timestep during which the sun never rises above the horizon, the file's diffuse light is placed on the plane as if it came uniformly from the whole sky, together with the ground-reflected part. There is no direct or circumsolar light. The low-light threshold, the masks and the reflection factors then apply as usual, and the inverter starts when the light is enough.

The math

$$G_{plane} = D\,\frac{1+\cos\beta}{2} + \text{albedo}\,G_h\,\frac{1-\cos\beta}{2} \tag{1}$$

where

Symbol	Meaning	Unit
$G_{plane}$	the 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
$\text{albedo}$	the ground albedo of the month, a fraction	—
$G_h$	the global horizontal irradiance	W/m²

References

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

API

site.night_irradiance = isotropic_diffuse

Symbols follow the symbol registry.