The performance index is the ratio of measured energy from a PV system to the predicted energy using a PV performance model. Unlike with the performance ratio, the performance index very close to 1 for a well functioning PV system and should not vary by season due to temperature variations.
There are several formal definitions of performance indices. They are discussed in detail in PV Performance Assessment, from SunSpec and San José University.
- Energy Performance Index (EPI) – this form uses energy (kWh) amounts in each of the time intervals being considered.
- Power Performance Index (PPI) – this for uses instantaneous power (kW) measurements and predictions for the assessment and is reported as a time series.