Ideally, I-V curve measurements should be made when there is a clear sky and little wind. Changes in irradiance will produce variations in PV module temperature that can affect the accuracy of I-V curve measurements. If the irradiance has increased significantly immediately before measurements are made, the PV module temperature may not have stabilised. Changes in irradiance during an I-V curve measurement can influence the shape of the I-V curve.
However, the reality is that time and contractual constraints limit the periods in which it is possible to perform a test. I-V curve measurements should be made under the most possible stable conditions and variations in irradiance and module temperature should be recorded. If the changes in irradiance alter the shape of the I-V curve the measurement should be repeated.