PVWatts 8 fixed layout

Method

The rear model runs on the geometry PVWatts version 8 fixes for every system. The slope length is the length of a 300 W module of the module type's efficiency with a length 1.7 times its width (1), one module up on a tracker and two up on a fixed rack (2). The module type follows the cell technology as in the representative module: thin film (18 %) for CdTe, CIGS and amorphous silicon, standard (19 %) otherwise. The slope length is 1.638 m on a tracker and 3.277 m on a fixed rack for the standard type, 1.683 m and 3.367 m for thin film. The height above ground is 1 m, the lower edge on a fixed rack and the axis on a tracker. The transparent fraction is 0.013 and the structure blocks nothing (3). The ground cover ratio, the tilt and the tracking stay the array's. The collector width, the height above ground, the transparent fraction and the structure shading factor of the request are not used by the rear model.

The math

1. Module length
2. Slope length
3. Clearance, transmission and structure shade

$$A = \frac{300\ \text{W}}{\eta \cdot 1000\ \text{W/m}^2},\qquad l = 1.7\,\sqrt{A / 1.7} \tag{1}$$ $$L = n_y\,l \tag{2}$$ $$h = 1\ \text{m},\qquad \tau = 0.013,\qquad f_s = 0 \tag{3}$$

where

Symbol	Meaning	Unit
$A$	the area of the 300 W module	m²
$\eta$	the efficiency of the module type: 0.19 standard, 0.18 thin film	—
$l$	the module length	m
$L$	the slope length of a row	m
$n_y$	the modules up a row: 1 on a tracker, 2 on a fixed rack	—
$h$	the height above ground: the lower edge on a fixed rack, the axis on a tracker	—
$\tau$	the transparent fraction of the module plane	—
$f_s$	the structure shading factor of the rear	—

References

1. NREL SAM Simulation Core (SSC) source code, release 2026.7.3, github.com/NatLabRockies/ssc: ssc/cmod_pvwattsv8.cpp, the representative module's geometry and the rear irradiance in exec() (calc_rear_side(0.013, 1, module.length · nmody), passed to the module model without a mismatch)

API

bifacial.geometry = pvwatts_layout

Symbols follow the symbol registry.