There are various factors that can influence the performance of solar PV modules, including temperature and irradiance.
The open circuit voltage of a PV module varies with cell temperature. As the temperature increases, due to environmental changes or heat generated by internal power dissipation during energy production, the open circuit voltage (Voc) decreases. This in turn reduces the power output. The design of a solar PV system must take into account the PV module temperature coefficient, comparing the expected average cell temperature in its operational environment, against the STC data used to calculate the module output.
In the same way, irradiance will also affect module performance, with a reduction of sunlight resulting primarily in a reduction in current and consequentially a reduced power output.
When comparing I-V curves measured in the field with predicted profiles, full consideration of these factors needs to be taken into account if the comparison is to produce meaningful results.