Hot Spots on Solar Panels: Causes, Risks, and What to Do

Hot spots are one of the more common solar panel faults reported by UK homeowners — and one of the more misunderstood. They don't always mean your panels are about to catch fire, but they do need attention. Here's what's actually going on, how to spot them, and when to act.

What is a hot spot?

A hot spot is a localised area within a solar panel that runs significantly hotter than the surrounding cells — sometimes tens of degrees hotter under full sun.

Under normal operation, all cells in a panel work together to convert sunlight into electricity. When one or more cells can't generate electricity properly — because they're shaded, cracked, or defective — the working cells try to push current through them anyway. Instead of generating power, the problem cells become resistive loads, absorbing energy and converting it to heat. That's the hot spot.

The affected area might be a single cell, a cluster of cells, or a larger zone depending on the fault type. In a well-designed panel, bypass diodes are wired in parallel across groups of cells to limit the damage: when a cell fails, its bypass diode kicks in and routes current around that cell string. But the hot spot effect can still occur, and the bypass diode itself can eventually fail.

Common causes

Partial shading

The most frequent cause. A small patch of shade — a blob of bird droppings, a fallen leaf, a chimney shadow hitting one corner of a panel — forces the shaded cells into resistive mode while the rest of the panel generates normally. Even partial shading on a single cell can affect the entire cell string it belongs to.

This is why bird fouling is worth taking seriously. A dried dropping might look insignificant, but it can create a persistent hot spot on that cell throughout the day.

Micro-cracks

Hairline cracks in the silicon cells are invisible to the naked eye and can form during manufacturing, transportation, or installation. Thermal cycling (panels heating and cooling daily) can widen micro-cracks over time. When a crack isolates part of a cell from the circuit, the affected area can no longer contribute to generation and becomes a hot spot instead.

Snail trails

The brownish, winding discolouration sometimes visible through the glass of older panels. Caused by silver paste from cell contacts reacting with moisture — usually through micro-cracks in the anti-reflective coating. The affected cells gradually lose efficiency and can develop into hot spots.

Delamination

If the encapsulant layer between the glass and the cells separates or bubbles, moisture can ingress. Wet patches within a panel lose electrical performance and can produce hot spots. Delamination is often visible as cloudy or bubbled areas under the glass.

Damaged or failed bypass diode

Bypass diodes are small components built into the panel's junction box, designed to limit the damage from hot spots. If a bypass diode fails in the open position, it can no longer protect its cell string — turning a minor shading issue into a more serious hot spot. If it fails in the short-circuit position, the entire cell string it protects is permanently bypassed, causing a noticeable generation drop.

Cell degradation

As panels age, individual cells can degrade at different rates. A cell that degrades faster than its neighbours becomes a weak point in the circuit — and potentially a hot spot site.

How to detect hot spots

Thermal imaging (IR camera)

The gold standard. A thermal imaging camera shows temperature differences across the panel surface in real time. Hot spots appear as bright warm patches against the cooler background of a working panel. Professional solar inspectors use dedicated IR cameras; some recent smartphone models have built-in thermal sensors (notably certain CAT and FLIR-integrated phones). A thermal survey during a sunny period, when panels are generating at full output, gives the clearest results.

Visible signs

In more advanced cases, hot spots leave visible marks. Look for:

• Brownish or yellowish discolouration on the backsheet (the white or black underside of the panel)
• Yellowing or browning visible through the glass surface
• Snail trail patterns (brown lines or patches visible through the glass)
• Bubbling or milky patches under the glass (delamination)

These signs indicate a hot spot has been present for some time and is causing material damage.

Generation monitoring

If your inverter provides string-level monitoring, a significant drop in output from one string versus another can indicate a problem panel in that string. Whole-system monitoring won't pinpoint the issue, but a sudden or gradual drop in yield — not explained by weather or shading — is worth investigating.

Are hot spots dangerous?

In most cases, a hot spot reduces your panel's output without posing an immediate safety risk. The bypass diode limits the damage to one cell string within the panel, and the rest of the panel continues generating. You may notice a modest drop in yield; the panel itself continues to function.

However, hot spots exist on a spectrum. At the severe end:

• Backsheet burning: Sustained high temperatures at the same location can scorch the backsheet material. This is irreversible and usually means the panel is no longer safe to operate.
• Fire risk: In rare but documented cases, severely overheating panels — particularly where backsheet material has already been compromised — can ignite. This is more likely with poorly manufactured panels or where a fault has been left unaddressed for an extended period.
• Bypass diode failure: Heat stress can cause bypass diodes to fail, which either worsens the hot spot or permanently reduces generation.

The risk level depends on severity, panel quality, and how long the fault has been present. A small, stable hot spot in a well-manufactured panel is a different concern to a large, intensifying one in an older or cheaper panel.

When is it a warranty claim?

Most panel manufacturers offer a product warranty (covering materials and workmanship, typically 10–25 years) and a separate performance warranty (guaranteeing a minimum output level at specified intervals). Hot spots that cause abnormal power loss — beyond the guaranteed degradation curve — generally fall under the product warranty.

What constitutes a valid claim varies by manufacturer, but common grounds include:

• A measurable power loss greater than the guaranteed tolerance (often 3–5% beyond the rated degradation for that year)
• Physical evidence of manufacturing defects contributing to the fault (micro-cracks, delamination, junction box failure)
• Bypass diode failure not caused by external damage

To support a warranty claim you will need:

1. Photographs — clear images of the discolouration, browning, or visible damage
2. Thermal images — even a smartphone thermal camera is useful if you have one; a professional thermal survey is stronger evidence
3. Generation data — export data from your inverter showing when output dropped and by how much
4. Installation date and panel serial numbers — these are on the label on the backsheet of each panel

Your installer is usually the first point of contact for warranty claims. They will liaise with the manufacturer on your behalf. If your installer is no longer trading, contact the panel manufacturer directly — the warranty belongs to the panel, not the installer relationship.

Prevention

You cannot prevent all hot spots — manufacturing defects and natural degradation are outside your control — but you can reduce the risk of the most common causes:

Keep panels clean

Bird droppings are one of the leading causes of partial shading and hot spots in UK installations. Regular visual checks and occasional cleaning to remove concentrated fouling can meaningfully reduce hot spot risk. You do not need to clean the whole panel every time — targeting bird droppings specifically is enough.

Bird proofing

If pigeons or other birds nest under your panels, their fouling will create persistent localised shading. Bird proofing mesh (fitted around the panel perimeter so birds cannot access the gap beneath) is worth considering if you have a nesting problem. Your installer can fit this, and it pays for itself if it prevents hot spot damage or a costly panel replacement.

Ensure adequate ventilation

Panels generate heat during operation. Installations where panels sit flush against a very low-pitch roof with no air gap beneath can run hotter overall, which slightly worsens the conditions for hot spot development. Most professional installations account for this, but it is worth noting if you ever have panels refitted.

Annual visual inspection

From the ground (or safely from a window at the same level), scan your panels periodically for discolouration, browning, or anything unusual. Catching visible signs early — before a hot spot progresses to backsheet damage — keeps your options open for a warranty claim.

Can hot spots be repaired?

No. Individual solar cells cannot be replaced in the field. Once a cell is damaged or defective, there is no patch or fix that restores it.

The practical outcomes are:

• Mild hot spot, functioning bypass diode: The affected cell string is bypassed. Output from that section of the panel is lost, but the rest of the panel continues generating. The panel is degraded but operational. Monitor it and document the loss for a potential warranty claim.
• Severe hot spot or backsheet damage: The panel should be replaced. Operating a panel with backsheet burning is a safety risk, and most installers will advise taking it out of service.
• Failed bypass diode without physical damage: Some specialist electricians can replace junction box components including bypass diodes, but this is a relatively involved job and may only make economic sense on a newer, expensive panel. It is worth getting a quote if the panel is otherwise in good condition.

The sooner a developing hot spot is caught, the more options you have. A panel showing early snail trails or a small warm patch on thermal imaging may still be within warranty — whereas one with a burnt backsheet is a harder claim to make and a more urgent safety concern.

What to tell your installer

When you contact your installer about a suspected hot spot, the more information you can provide, the faster they can assess the situation:

• When you first noticed it — a rough date, or when your monitoring showed a generation drop
• What you can see — describe or photograph any visible discolouration, browning, or unusual marks
• Which panel or string — if you know roughly where on the roof the affected panel is, or which string is showing lower output, say so
• Generation data — any figures from your monitoring app showing output before and after the change
• Any recent events — new shading source (a neighbour's tree, a new extension next door), recent heavy snow, or a period of extreme heat

A good installer will arrange a site visit, carry out a thermal survey, and advise whether the panel is a warranty claim or a monitoring job. If they suggest simply watching and waiting without any inspection, it is reasonable to ask for a thermal check before agreeing to that approach.

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