IAM factor on global

What this step does

The row measures the light the module glass reflects away beyond what it reflects at normal incidence, taken from the plane irradiance. The incidence-angle modifier (IAM) curve at the timestep's incidence angle reduces the direct sunlight. Sky and ground light arrive from every direction, so they are reduced by an average of the curve over the directions the modules see. The Model tab's Incidence-angle modifier, beam step sets the curve, and its Incidence-angle modifier, diffuse and albedo step sets the two averages.

The math

1. Running total
2. Loss at a timestep

$$E_{after} = E_{before} - E_{row},\qquad E_{row} = \frac{\Delta t}{1000}\sum_i L_i \tag{1}$$ $$L_i = (1-F(\theta_i))(B_i + C_i) + (1-F_{IAM,sky})\,D_i + (1-F_{IAM,gnd})\,G_{gnd,i} \tag{2}$$

where

Symbol	Meaning	Unit
$E_{before}$	the running total of the loss diagram before this row	kWh/m²
$E_{after}$	the running total after this row	kWh/m²
$E_{row}$	the row taken as a loss	kWh/m²
$\Delta t$	the length of one timestep	hours
$L_i$	the irradiance lost at timestep $i$	W/m²
$F(\theta)$	the incidence-angle modifier: the share of the light the glass accepts at incidence angle $\theta$, relative to normal incidence	—
$\theta_i$	the angle between the sun and the perpendicular to the module plane at timestep $i$	degrees
$B_i$	the direct sunlight on the plane	W/m²
$C_i$	the circumsolar part of the diffuse light on the plane	W/m²
$F_{IAM,sky}$	the modifier averaged over the sky the modules see	—
$D_i$	the sky diffuse light on the plane	W/m²
$F_{IAM,gnd}$	the modifier averaged over the ground the modules see	—
$G_{gnd,i}$	the ground-reflected light on the plane	W/m²

Reading the number

A loss, negative. Modules with anti-reflective glass lose about 2 % over a year and plain glass about 3 %. Trackers keep the incidence angle small and lose less; steep or east-west planes lose more.

API

loss.iam

Symbols follow the symbol registry.