Solar panel defects: how to identify hotspots, PID, and snail trails

Solar panels are warranted for 25 years because they're designed to be durable. But defects do develop — sometimes during manufacture, sometimes from installation, and sometimes from environmental stress. Most defects reduce output rather than cause complete panel failure, making them easy to miss without monitoring.

How defects typically show up

Via monitoring data: A panel or string underperforming relative to others is the first signal. Modern monitoring platforms (Fronius Solar.web, Sungrow iSolarCloud, Enphase Enlighten, SolarEdge monitoring) can show per-panel or per-string output. A string generating 15% less than expected may have one defective panel dragging the whole string down.

Via thermal imaging: A thermal (infrared) camera reveals hotspots and cell-level anomalies that aren't visible in ordinary light. Many solar installers and inspection services offer thermal scans, particularly for systems with unexplained underperformance.

Physical inspection: Some defects (delamination, snail trails, cracked cells) are visible with the naked eye from close range — though ground-level inspection misses panel surfaces.

Hotspots

What they are: Hotspots occur when one cell within a panel operates at significantly higher temperature than surrounding cells. The hot cell acts as a load rather than a generator — the electrical energy that would have been generated by that cell is instead dissipated as heat within the panel.

Causes:

• Partial shading of one cell (e.g. a single leaf or bird dropping on one cell)
• Cell damage (cracked cell absorbs less light, surrounding cells push current through it)
• Manufacturing defect in one cell (poor bypass diode connection)

Perth-specific risk: Perth's high irradiance means panel operating temperatures are already elevated. A hotspot superimposes additional thermal stress — panel surface temperatures can exceed 80°C in a hotspot area during Perth summer. Sustained high-temperature hotspots reduce panel life.

How to identify: Thermal imaging shows hotspots clearly as bright spots (high temperature) on the camera. A thermal scan costs approximately $100–$300 for a residential system. If monitoring shows consistent underperformance on one string that doesn't correlate with shading, a thermal scan identifies whether hotspots are present.

Resolution:

• If caused by persistent partial shading (e.g. a new structure casting shadow): system redesign or panel relocation
• If caused by cell damage: panel replacement under warranty (if within warranty period and manufacturer accepts the claim)
• Isolated bird dropping: clean the panel (drops are temporary)

PID (potential-induced degradation)

What it is: PID occurs when a voltage difference develops between the solar cells and the grounded metal frame of the panel. Ions migrate through the panel's encapsulant, gradually reducing cell efficiency — sometimes causing 30–50% or more output reduction in severely affected panels.

Causes:

• High string voltage combined with humidity
• Certain panel constructions are more susceptible
• Grounding system issues (improper bonding between inverter and panel frames)

Perth relevance: Perth's low annual humidity reduces (but doesn't eliminate) PID risk compared to tropical climates. High string voltages (400V+) in Perth summer, particularly in hot and humid periods, create conditions for PID development in susceptible panels.

How to identify: PID typically affects entire strings or panels within a string rather than isolated cells. Monitoring shows gradual decline in output across a string over months. Electroluminescence imaging (a specialist test, not standard residential inspection) can visualise PID.

Warranty and resolution: Many manufacturers now warrant panels against PID. Recovery of some PID degradation is possible through "PID recovery mode" available on some inverters (Fronius, SMA) — the inverter applies a corrective voltage overnight. If a panel is severely PID-affected within the warranty period, replacement is the path.

Snail trails

What they are: Snail trails are brown or black discoloration patterns on the front surface of solar panels, following the edge of silver busbars/interconnects in a pattern that resembles snail trails. They're visible to the naked eye on close inspection.

Causes: Moisture penetrates the panel's encapsulant (EVA layer) and reacts with the silver paste in the cell interconnects, causing silver compound deposits. The discoloration pattern follows the contact lines, creating the distinctive trail appearance.

Does it reduce output? Snail trails do not always reduce panel output significantly. In mild cases, the discoloration is cosmetic with minimal output impact. In severe cases, the underlying cell degradation that allowed moisture ingress has already reduced output — the snail trail is a symptom of the underlying EVA degradation rather than the cause of output loss.

Warranty: Snail trails are covered under the workmanship/product warranty in most cases (they represent a manufacturing or material defect). Document the appearance with photos and contact the installer or manufacturer for a warranty assessment.

Delamination

What it is: Delamination is separation of the layers within the panel — typically the EVA encapsulant separating from the front glass, the cell layer, or the backsheet. Visible as cloudy patches, bubbles, or areas where the panel layers have visibly separated.

Causes:

• Manufacturing defects in EVA curing
• UV degradation of backsheet materials
• Prolonged moisture ingress

Perth relevance: Perth's high UV irradiance accelerates UV degradation in lower-quality backsheet materials. Some panels with polymer backsheets (rather than glass-glass construction) show earlier delamination in high-UV environments.

Glass-glass panels: Many premium panel manufacturers (REC, some LG Jinko models) use glass-glass construction — a glass backsheet instead of a polymer film. Glass-glass panels are substantially more resistant to delamination and generally perform better in high-UV environments like Perth. They are slightly heavier and more expensive.

Resolution: Delaminated panels should be replaced under product warranty. A delaminated panel continues generating but is structurally compromised and may have moisture paths into the cell layer — an early warranty claim is preferable to waiting for complete failure.

Getting a panel inspection

If you suspect panel defects:

1. Check monitoring data for string-level underperformance patterns
2. Look at the panels from ground level for visible discoloration or surface anomalies
3. Contact your installer for a warranty inspection — this is covered by the workmanship warranty within 5 years, and any genuine panel defect is a product warranty claim against the manufacturer
4. Ask whether a thermal imaging inspection is available — useful for hotspot identification

Cost:

• Monitoring review: free (access your monitoring app)
• Visual inspection: free (contact your installer)
• Thermal scan: $100–$300 from specialist solar inspection companies

Early defect detection through monitoring and periodic inspection is more effective than waiting for complete panel failure. Most panel warranties require claims to be made while the panel is still within the warranty period — don't delay a warranty assessment.