A panel fault is a problem inside the module itself, rather than in the inverter, the wiring or the roof. Most are invisible from the ground, and some are invisible even close up.
Five come up often enough to be worth naming. Hot spots, micro cracks, snail trails, potential induced degradation and failed bypass diodes. This guide explains what each one is, what it actually costs you, and what we do when we find it.
Key Points
- Not every mark on a panel is a fault, and not every fault leaves a mark.
- Micro cracks are common and are often harmless for years. Sometimes they are not.
- Snail trails are a visible sign of an invisible problem. The discolouration itself is not known to cost output.
- Potential induced degradation usually has no visual signs at all, and in bad cases can take almost all of a module's output.
- Hot spots come from a cell being pushed into reverse bias, usually by shading, a crack or a shunt.
- A bypass diode is there to protect the module from that. When one fails it fails quietly.
- Infrared imaging finds the heat. Electroluminescence imaging finds the cracks. A visual check finds neither reliably.
| Fault | What it looks like | What it means | What we do |
|---|---|---|---|
| Micro cracks | Nothing visible | Cracked cells inside the laminate | Compare output, image the module, monitor or replace |
| Snail trails | Grey or black lines on the cell surface | Discolouration along cell edges and cracks | Treat as a marker, then look for the crack |
| Hot spot | A burn mark or discoloured patch, often on the back | A cell is being forced into reverse bias | Infrared check under load, then remove the cause or the module |
| Potential induced degradation | Usually nothing | Cell shunting, output falling across a string | Test the string, check earthing and inverter type |
| Failed bypass diode | Nothing, until the module is shaded | The module cannot bypass a shaded section | Test the module, replace the junction box or the module |
Micro cracks
Cells crack in transport, in handling, or from load on the roof. The IEA PVPS review of module failures found a pattern. The more cracked cells a module has, the higher the risk of longer and wider cracks later.
Cracks do not always cost you anything. The same review notes modules with many cracked cells showing no significant power loss after two years. It also records a solar park where nearly half the modules had lost about 10% or more after six years. A crack is a risk, not a verdict.
Snail trails
A snail trail is a grey or black discolouration of the silver paste on the front of the cell. It follows the cell edges and cracks you cannot otherwise see. It typically appears three months to a year after installation.
The review states that no case is known where the discolouration itself leads to a measurable power loss. The trail is a sign, not the damage.
Hot spots
A hot spot forms when part of a module is forced into reverse bias. Shading does it. So do cracked cells and manufacturing shunts. The reverse current concentrates and the spot heats.
At its worst that produces a burn mark. The review notes that if the current cannot be bypassed an electric arc can result, and such an arc can cause a fire. That is why a burn mark is never left alone. An infrared image taken while the system is working shows whether the area is still hot.
Potential induced degradation
Potential induced degradation, usually written as PID, is voltage driven. The review explains that losses are more pronounced the higher the system voltage is. The effect causes cell shunts and reduces the fill factor. Massive PID often has no visual effect at all.
It is not always slow. The review records field power degradation developing within several months and reaching almost 100%. Recovery is often possible within hours by applying a reverse voltage, though PID that happened at high temperature is much less reversible.
Failed bypass diodes
Bypass diodes sit inside the junction box on the back of the module, wired across a group of cells. They reduce the loss from partial shading and stop cells being reverse biased beyond what they are designed for.
They are usually Schottky diodes, which the review notes are very susceptible to static discharge and mechanical stress. A failed diode only shows itself when the module is shaded or mismatched. In one study of 1272 modules the review cites, 47% had defective bypass diodes, and 3% of those also had burn marks.
Common Misconceptions
"A mark on the glass means the panel is failing." Not necessarily. Snail trails are visible and are not known to cost output on their own.
"If it looks fine it is fine." PID often has no visual effect at all.
"One bad panel does not matter." It can pull down the string it is in, and a hot spot has a safety dimension.
"Micro cracks mean the panel must be replaced." Not automatically. Some cracked modules run for years without measurable loss.
"Bypass diodes never fail." They do, and quietly. One published study found nearly half the modules checked had defective diodes.
Real-World Advice
Our published projects are at our work, and none of them is a panel fault repair.
UK-Specific Considerations
Panels fitted here carry long performance warranties. AIKO publishes 25 years product and 30 years performance on the NEOSTAR series we fit. First year degradation is 1% or less, then 0.35% a year or less. JA Solar publishes a 30 year power warranty with an annual degradation rate of 0.3%. A genuine fault shows up as a departure from that, not as a slow slide.
Shading is the local driver worth watching. Tree growth around Wannock, Hailsham and the Wealden villages is one source. Chimneys and split roofs on the Hastings and St Leonards slopes are another. Both create the partial shading these faults depend on.
Warranty claims need evidence. Keep the commissioning paperwork, the module make and model, and your monitoring history. Without them a claim is much harder.
When This Isn't Right For You
Do not climb on the roof to inspect panels yourself, and do not open a junction box. Burn marks and arcing are electrical faults and are a job for a qualified engineer.
Do not chase a panel fault while an obvious cause is still there. Cut the branch back first, then re measure.
Before You Spend Money
Get your monitoring data first. A fault you can see in the numbers is far easier to prove.
Ask for the module make, model and serial numbers. Warranty claims need them.
Ask what test is being proposed. A visual inspection alone will not find cracks or PID.
Ask whether the array or the inverter has been ruled out. See why is my solar generating less.
Ask whether the fault is a warranty matter before you accept a quote to replace anything.
Frequently Asked Questions
Q: Are snail trails dangerous?
A: The discolouration itself is not known to reduce output. It marks cracks underneath, so it is worth investigating rather than ignoring.
Q: Can micro cracks be repaired?
A: No. A cracked cell cannot be mended inside a sealed module. The decision is whether the module still performs.
Q: How do you find a hot spot?
A: With an infrared image taken while the system is generating. A hot area that stays hot is the sign.
Q: Is PID reversible?
A: Often, at least partly. The review says recovery is often possible within hours with a reverse voltage, but PID that occurred at high temperature is much less reversible.
Q: How would I know a bypass diode had failed?
A: You probably would not from the ground. It only shows when the module is shaded, which is why testing matters.
Summary
Five faults account for most panel problems. Micro cracks are common and often harmless, but they raise the risk of later loss. Snail trails are a visible marker of invisible cracks and do not themselves cost output. Hot spots come from reverse bias and are the one with a safety edge. Potential induced degradation is voltage driven, usually invisible, and can be severe and partly reversible. Failed bypass diodes are common and only show under shading. Testing, not looking, is what tells them apart.