A hot spot is a localised area of a module that runs significantly hotter than the rest of it. It is one of the few photovoltaic failure modes that is both visible in a single image and genuinely indicative of a problem — which is also why it is over-diagnosed. Not every warm area is a hot spot, and not every hot spot is a defect.
This guide covers what produces one, why a small shadow can cause a disproportionate loss, how hot spots are actually found, and the distinction that decides whether a warranty claim is a product claim at all.
What a hot spot actually is
Cells in a module are connected in series. Under normal operation every cell passes the same current and each contributes power. If one cell cannot pass that current — because it is shaded, cracked, or defective — it stops contributing and instead dissipates power. It becomes a load rather than a source, and it heats up.
That is the whole mechanism. The cell is being reverse-biased by the rest of the string and converting the string’s energy into heat in one small area.
Why bypass diodes exist, and what they do not prevent
Modules contain bypass diodes across groups of cells. When a cell group cannot pass current, the diode provides an alternative path around it, so the rest of the module keeps producing and the affected group is not forced to dissipate the full string current indefinitely.
Two things are worth being clear about:
- They limit the damage; they do not eliminate the condition. The affected group is still out of production, and the diode is conducting instead. A module with an active bypass diode is underperforming, not healthy.
- They are a finite number of groups. A module’s cells are divided into a few sections, each with its own diode. If the cause affects more than one section, the protection is used up — which is why a single shadow across a wide band behaves very differently from one across a corner.
What causes it
Four categories, and they matter because only one of them is a product issue:
- Shading. The most common cause by far. A chimney, a vent pipe, a parapet edge, a neighbouring row, a tree that has grown, or accumulated soiling in a corner of the frame. The cell is fine; it is simply not receiving light.
- Soiling and debris. Bird droppings, leaf litter, and dirt built up along the lower frame edge are shading by another name, and they are persistent rather than transient.
- Cell damage. Microcracks from handling, transport or installation, or from mechanical load — a cracked cell can be partially disconnected from the circuit.
- Manufacturing defects. Genuine cell or interconnection faults. This is the smallest category and the one everyone assumes first.
Why a small shadow costs more than its area
This is the part that surprises people, and it is worth understanding before arguing about output figures.
Because cells in a string are in series, the current through the whole string is limited by its weakest point. A shadow covering a small fraction of one cell does not reduce output by that fraction — it can reduce the output of the entire string substantially, because the affected group is bypassed and its contribution is lost entirely.
So the loss is governed by where the shadow falls and by how the module’s cell groups are arranged, not by the area of the shadow. A narrow band across the module can be worse than a larger patch in one corner. This is why layout and shade analysis belong at design, and why a roof object that looks trivial in a photo can cost measurable annual generation.
What it does over time
A persistent hot spot is not static. Sustained local heating degrades the encapsulant and the backsheet, and in the worst cases it discolours or cracks the glass. The progression is slow enough to be ignored for a season and fast enough to become a safety matter rather than a performance one.
That is the reason hot spots are worth chasing even when the output loss looks modest: the end state is not “a module producing slightly less”.
How hot spots are found
Three methods, and they answer different questions:
- Infrared scanning finds the temperature difference. It is fast, it can be done from the ground or from the air, and it shows where to look. It needs irradiance and load conditions that make the difference visible, and it shows the symptom rather than the cause.
- Electroluminescence imaging shows the cell structure — cracks, disconnected areas, and the pattern of the affected cells. It is what turns “there is a hot spot” into “here is why”.
- I-V curve tracing quantifies how far the string has drifted from its expected curve, which is what you need for a claim rather than for a diagnosis.
The sequence in our guide to diagnosing underperformance starts with the cheap checks and moves to component-level testing only when those are clean. Hot spot scanning sits at the component level, not at the start.
When it is a defect, and when it is not
This is the distinction that decides the outcome, and it is the same shape as the one that applies to PID:
- Shading, soiling, debris and site objects are not product defects. The module is doing exactly what a module does when part of it is in the dark. The remedy is on the site, not in the warranty.
- Cell damage may or may not be. A crack caused in transit or during installation is a handling and installation matter; a crack with no external cause is a product matter. The two are distinguished by the pattern and by whether there is any evidence of mechanical damage to the module.
- Manufacturing faults are product defects, and they are the rarest of the three.
What settles it is evidence: infrared images showing the location and severity, EL images showing the cell structure, and I-V traces quantifying the loss. Photographs of the roof taken at the same time, showing what was casting the shadow, are often the most useful item in the file — for both sides. The assembly sequence is in our warranty claims guide.
What to specify and document
Reference table
Swipe horizontally to view all columns →
| Item | What to do | Why it matters later |
|---|---|---|
| Shade analysis | At design, including objects that will grow or be added | Most hot spots are designed in, not manufactured |
| String layout | Keep strings out of mixed shading conditions | Series current is limited by the weakest point |
| Commissioning scan | Infrared at handover, with the images filed | Without a baseline, later findings cannot be dated |
| Soiling regime | Defined cleaning interval for the site | Debris shadows are persistent, not transient |
| Handling records | Packaging and installation method noted | Separates transit damage from product faults |
| Diagnostic method | Which of IR, EL and I-V, and in what order | Cost escalates quickly; symptoms and causes differ |
| Evidence set | Images plus site photographs at the same time | The shadow’s cause is usually visible in the photo |
How we supply
For the selected photovoltaic module, agree the manufacturer’s datasheet, available flash-test records and applicable certification documents in the order documentation schedule. Selection criteria including degradation and test data are covered in the module buying guide; the cell-technology properties that influence susceptibility to these failure modes are set out in our technology comparison.
We will not tell you that a warm area on your array is a defect before anyone has looked at it. The cause is on the roof far more often than it is in the module, and a claim built on the wrong premise wastes the time of everyone involved — including yours. Send the infrared images and the site photographs together, and the answer comes back against what the images show.
BUYER CHECKLIST
Buyer considerations for this scope
Do the shade analysis at design, not after handover
Most hot spots are designed in rather than manufactured — a vent pipe, a parapet edge, a neighbouring row, or a tree that has not grown yet. Once the array is built, the remedy is a layout change that has already been paid for once.
Get an infrared baseline at handover
Without a commissioning scan on file, a hot spot found in year three cannot be dated, and the question of whether it was there from the start becomes an argument. The scan is cheap at handover and expensive to reconstruct later.
Send the infrared images and the site photographs together
A thermal image shows that something is warm; a photograph taken at the same moment usually shows what is casting the shadow. The two together settle the defect-versus-site question faster than either does alone, whichever way it goes.
PRACTICAL ANSWERS
Questions buyers ask about this
Is every warm area on a module a hot spot?
No. Infrared imaging shows temperature differences, and there are benign reasons for a module or a section to read warmer — reflection, viewing angle, irradiance conditions at the moment of the scan, or a roof surface behind the array. A hot spot is a localised area dissipating power because it cannot pass the string current. The scan points to where to look; it does not by itself establish the cause.
Do bypass diodes prevent hot spots?
They limit the damage rather than eliminating the condition. When a cell group cannot pass current, the diode provides an alternative path so the rest of the module keeps producing. The affected group is still out of production, and a module with an active bypass diode is underperforming rather than healthy. If the cause spans more than one cell group, the protection is used up.
Is a hot spot covered by the module warranty?
It depends entirely on the cause. Shading, soiling and site objects are not product defects — the module is doing what a module does when part of it is in the dark. Cell damage may be, depending on whether there is evidence of handling or installation damage. Genuine manufacturing faults are the rarest category and the clearest to claim.
Why does a small shadow cause a larger loss than its area?
Because cells are connected in series and the string current is limited by its weakest point. A shadow over part of one cell does not reduce output proportionally — the affected cell group is bypassed and its contribution is lost entirely. The loss depends on where the shadow falls and on how the module's cell groups are arranged, not on the shadow's area.
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