They are defined in IEC 61724-1:2021 and are used by asset owners, operators, and independent engineers to detect underperformance, benchmark plants, and fulfill contractual obligations. Two primary indicators are in common use: Performance ratio (PR) and Energy performance index.
6V) means you've got the probes reversed. Pro tip: Test under sunlight or strong artificial light – shaded panels might not generate enough voltage for accurate readings. For systems using industry-standard MC4 connectors, polarity is built into the design.