A low performance ratio means the plant delivers less energy than its irradiance and rated power would predict. It is the main indicator of how efficient and reliable a PV plant is overall. Many things lower PR: module temperature and ventilation, shading or soiling of the modules or the irradiance sensor, cable and inverter losses, mismatch, grid unavailability and ageing. Before treating a low PR as an equipment fault, check for grid curtailment. Curtailment shows up as a flat, controlled output level below what the irradiance supports, while inverters and trackers look fully available. To confirm curtailment, compare plant setpoints, inverter active power limits, dispatch signals, meter output and curtailment flags with the available power estimated from irradiance. On plants that are curtailed often, use curtailment-aware metrics. Otherwise crews may chase equipment problems that are not there. If the plant is not curtailed, trend PR over time and against module temperature. Compare each inverter's energy or power with modeled values to find the underperforming unit. A decline much faster than the usual 0 to 1 percent per year points to module faults such as potential induced degradation. Check for module discoloration, or run low-light voltage tests on site.
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observed_asc1 drafted · lead Flat output below available power
During grid curtailment the low PR reflects export limits rather than equipment health; inverter and tracker availability can look normal.
sec=h4 h=576525a653a1sec=h5 h=d55aa663f3f5c2 verified · fact Declining performance ratio
PR plotted over time and against module temperature shows small shifts in system performance and long-term degradation.
sec=1.1 h=6ef65608e9f7sec=4.1.1 h=d5ee0a10b7d9verified by: review-workflowverified on: 2026-10-20review note: support check: partial - Passages mention PR over time and long-term degradation, but not explicitly a declining PR from AC energy vs POA irradiation as a signature.
c3 drafted · lead Energy ratio out of range
Daily energy from each inverter or meter is compared with modeled expected energy; the test fails when the ratio falls outside its pass band.
tbl=13.r4 h=729bb0c18bacreview note: support check: partial - Energy ratio test compares daily energy to expected and passes between 70-110%, but the passage doesn't link it to low PR or say 'below expected'; pass band is stated.
c4 verified · fact Performance index below unity
Performance Index alerts trigger on a low ratio of measured to modeled power for meters and inverters.
sec=h11 h=52c6aca287c4sec=h1 h=ad50c4b37958verified by: review-workflowverified on: 2026-10-20review note: support check: partial - PI alerts trigger on low measured-to-modeled power ratio, but 'below unity' and 'sustained period' (only time thresholds implied) are not exactly stated.
degradation_ratec5 drafted · lead 0–1 PERCENT-PER-YR
Conditions: Normal expected power-output degradation over time; rates well above this suggest module faults such as PID
sec=2.4.6 h=454242c5993bexplains c3presents_as c8