Power is a function of current and voltage and so anything that reduces the current or voltage generated by a PV system will reduce the power that is produced. The shape or profile of the I-V curve therefore provides a highly effective visual indication of the performance of a PV module or string.
Real-time power monitoring systems provide an indication of the actual power generated however, if the actual yield is less that the design figure, they provide no information other measurement tools are required to identify the root cause of an under-performing PV system.
If when tested, the I-V curve for the installed string shows a distinct change in the angle of slope, has 'humps' or shifts in the shape of the curve, then these will indicate that something is wrong with one or more of the individual modules.
An assessment of the I-V curve during the installation or commissioning or as part of a period inspection and test of a system can therefore help to verify that all the modules are healthy and performing at a consistent level in line with their specified parameters.