The shape of the I-V curve varies with irradiance and below a critical level of irradiance the curve shape changes dramatically. This can result in errors in evaluating the performance of the system. Poor results may be expected where measurements are taken in low irradiance.
PV string and array performance measurements should be performed at stable irradiance conditions of at least 400 W/m2 as measured in the plane of the array. For measurements to be converted to STC, IEC 61829:2015 Photovoltaic (PV) array —On-site measurement of current-voltage characteristics recommends that the total in-plane irradiance shall be at least 700 W/m2.