Low energy yield is a symptom, not a diagnosis
A system producing less than expected can be affected by weather, new shading, dirt, inverter clipping, outages, export curtailment, sensor errors, wiring faults or module degradation. Start with a comparable baseline and normalise for irradiance or use nearby strings with similar orientation. Avoid declaring module degradation from a single monthly bill.
Look for changes that affect only part of the array
String-level or module-level monitoring can reveal whether one section is underperforming relative to its peers. A new tree shadow, failed bypass diode, connector issue or damaged module may create a local pattern while the rest of the system remains normal. Thermal imaging and IV-curve testing can help when performed by qualified technicians under suitable conditions.
Soiling should be confirmed before aggressive cleaning
Dust, pollen, bird deposits and industrial contamination can reduce output, but cleaning benefit depends on climate, tilt, rainfall and contamination pattern. Inspect safely and follow module-manufacturer cleaning instructions. Do not use abrasive methods or chemicals that can damage coatings or void warranty terms.
Shading can change over the system life
Trees grow, neighbouring structures change and rooftop equipment may be added after the original survey. Compare current shade conditions with the design model. Seasonal sun angle matters, so one midday inspection cannot represent the full year. If optimisers or microinverters are present, use their monitoring to localise recurring shade effects.
True degradation should be measured against the warranty method
If modules appear to have lost power beyond expected ageing, review the exact product and performance warranty. Manufacturers may specify test conditions, measurement uncertainty and claim procedures. Melnti’s product-versus-performance warranty guide explains why low system yield alone is not proof of a module warranty breach.
Document the diagnosis before replacing equipment
Record monitoring plots, weather context, inspection findings, test results and exact module serial numbers. If replacement is required and the original model is unavailable, use the module replacement electrical-match checklist so a substitute is evaluated for voltage, current, connectors, dimensions and temperature behaviour—not wattage alone.