Light Intensity & Solar PV Module Performance
Updated: Aug 8
From n-type to p-type and multi-crystalline to mono-crystalline Silicon, there are many different kinds of solar panels and each type of solar panel responds differently to various amounts of light intensity.
While solar panels are often tested using a standardized level of irradiation, the outdoor application of solar panels never involves a consistent light level.
In fact, solar panels also known as PV modules may be installed in geographies that typically experience low-light conditions, and thus, there is a variation in the output of the photovoltaic system.
Light Intensity and Low Light
Light intensity is a way to measure light's energy. It refers to the total number of photons received per unit area in a given time for a particular wavelength of light.
The intensity of sunlight varies from 0 to 1kw /sq.mt.
Low light conditions may be witnessed during the day e.g. early mornings and late evenings; due to geography, places located closer to the north and south poles have very low light for 6 months each year; factors like diffused shadows in the vicinity of the installations, high air pollution levels, etc.
Current from a solar panel decreases linearly with decreasing irradiance, while the voltage drops logarithmically. However, there is significant variation among solar panels with respect to these declines.
The low-light functionality of a solar cell is primarily reliant on the shunt resistance and series resistance of the cells, which are the resistances related to contacts at the top and the bottom of the cell, and the resistance related to the current that is circulated by the emitter respectively.
At low light levels, the impact of shunt resistance becomes increasingly relevant. As intensity decreases, the bias point and current also decrease, with the equivalent resistance of the solar cell starting to approach the shunt resistance. When these two resistances are nearly equal, the fraction of the overall current flowing through the shunt resistance rises, in doing so, it increases the fractional power loss as a result of shunt resistance. Therefore under cloudy conditions, a solar cell with a high shunt resistance keeps a greater portion of its original power compared to a solar cell with a low shunt resistance.
Manufacturers typically rate PV modules at standard test conditions (STC). The STC rating involves only one temperature (25 degrees), one irradiance (1000 W/m2 ), and one sunlight spectrum (Air Mass) of 1.5G (global). However, the actual energy production of field-installed PV modules is a result of a range of operating conditions in all but the most extreme locations.
Part one of the IEC 61853 energy rating standards require that module power be measured and reported at different combinations of irradiance and temperature covering nearly the full range of operating conditions all but the most extreme locations.
Irradiance Spectrum Module Tempreature
W/m2 15⁰C 25⁰C NOCT ⁰C 50⁰C 75⁰C
1100 AM 1.5 NA NA
1000 AM 1.5 NA
800 AM 1.5
600 AM 1.5 NA
500 AM 1.5 NA NA NA NA
400 AM 1.5 NA NA
200 AM 1.5 NA NA NA
100 AM 1.5 NA NA NA
Why is it better in some Modules?
As described earlier, under low light conditions, a solar cell with a high shunt resistance keeps a greater portion of its original power compared to a solar cell with a low shunt resistance.
The effect of concentration on the IV characteristics of a solar cell. The series resistance has a greater effect on performance at high intensity and the shunt resistance has a greater effect on cell performance at low light intensity.
Below are the plots showing the performance of a solar cell at low light (-0.2 of SUN) conditions varying with respect to the shunt resistance.