PV loss due to irradiance level — loss row

What this step does

The row measures the loss from the module's lower efficiency at irradiances below 1,000 W/m², taken from the array energy. For every timestep it records the difference between the module's nameplate power scaled by the irradiance and its maximum power at that irradiance and 25 °C. The series and shunt resistances of the module set its size. The Model tab's Module model step sets the module's maximum power. The PVWatts DC model has no low-light loss, so the row is 0 with it.

The math

Running total
Loss at a timestep

E_{after} = E_{before} - E_{row},\qquad E_{row} = \frac{\Delta t}{1000\,P_{DC}}\sum_i L_i \tag{1}

L_i = N\left[P_{nom}\frac{G_i}{G_{ref}} - P_{mpp}(G_i, T_{ref})\right] \tag{2}

where

Symbol	Meaning	Unit
$E_{before}$	the running total of the loss diagram before this row	kWh/kWp
$E_{after}$	the running total after this row	kWh/kWp
$E_{row}$	the row taken as a loss	kWh/kWp
$\Delta t$	the length of one timestep	hours
$P_{DC}$	the DC nameplate of the system	kWp
$L_i$	the power lost at timestep $i$ over the whole system	W
$N$	the number of modules in the system	—
$P_{nom}$	the module nameplate power at standard test conditions	W
$G_i$	the irradiance the module converts at timestep $i$ : the effective irradiance on the front plus the bifaciality factor times the rear irradiance	W/m²
$G_{ref}$	the reference irradiance of the standard test conditions (1,000 W/m²)	W/m²
$P_{mpp}(G, T)$	the maximum power of one module at irradiance $G$ and cell temperature $T$ , from the module model	W
$T_{ref}$	the reference cell temperature (25 °C)	°C

Reading the number

A loss, negative: typically 0.5 to 1.5 % of the nominal energy on utility and rooftop systems alike. A low shunt resistance and a cloudy climate make it larger; a higher series resistance makes it smaller.

References

De Soto, Klein & Beckman 2006, Improvement and validation of a model for photovoltaic array performance, Solar Energy 80(1) 78–88

API

loss.pv_irradiance

Symbols follow the symbol registry.