Factors affecting PV Module performance
When PV is made and sold it is rated at STC (Standard Test Conditions)
STC: 1000W/m2 (brightness of light about like noon), 25 ̊C (77 ̊F), 1.5AM (Atmosphere Mass spectrum of light)
However, in the real world, PV modules operate under different conditions.
The best energy production can be observed on a bright cold day.
Temperature and Voltage Conditions
BRIGHTness = MORE CURRENT
Brightness means more photons knocking electrons lose, which translates to more current (amps). Bright light can be referred to as increased irradiance and since irradiance starts with the letter I, then it is easy to remember that it increases I (symbol for current).
COLDness = MORE VOLTAGE
Cold temperatures increase voltage. We can easily remember it as T in volt.
Silicon semiconductors just work better in the cold. That is exactly why your computer has a fan that cools the silicon computer chip.
The problem with cold temperatures increasing voltage is that sunlight, which is good for solar cells, causes heat, which is bad for power production.
Displayed below on the right is a graph signifying the behaviour of IV curves with different amounts of irradiance (light) at a constant temperature. It is observed how more the irradiance, more the I (current).
For the graph on the left, we can see that when the temperature is changed voltage changes. Colder temperatures cause higher voltages. That is why we like to mount our PV so that there is airflow, to blow the heat away. PV can get very hot in the sun. But it is better to have a hot sunny day than a cold dark day, because with darkness, you lose more power than with heat.
Testing Parameters Of PV Modules
STC, Standard Test Conditions is measured in watts and is the standard way modules are tested and sold. STC is what is always indicated on the back of the module.
NOCT, Nominal Operating Cell Temperature is a testing standard geared to the operational conditions of solar cells, defined as the temperature reached by open-circuited cells in a module assuming 800W/ m² irradiance, 20°C ambient temperature and wind speed of 1m/ s with the PV module at a tilt angle of 45°
AM (Atmospheric Mass) 1.5
All of the tests are carried out with the spectrum of light equivalent to 1.5 atmospheres. By definition 1 atmosphere is when the sun is directly overhead at sea level. This test was standardized when the atmosphere was measured at Cape Canaveral Florida on Equinox.