Performance Ratio (PR)

Performance ratio (PR) is a solar plant’s actual energy output divided by the output expected from the measured irradiance at nameplate rating, a dimensionless measure of how well the plant converts sunlight.

IEC 61724-1 defines it. PR removes the weather, so two plants in different climates can be compared and one plant can be compared with itself month to month. A falling PR with steady irradiance points to soiling, shading, inverter faults, or degradation. The figure is only as good as the irradiance sensor and the data acquisition system that logs it, which is why IEC 61724 specifies sensor class and sampling.