A solar module hot spot usually starts when one cell cannot keep up with the cells around it. Shade, cracks, or an internal cell defect can reduce the current that one cell can produce. If the rest of the series circuit keeps carrying higher current, that weak cell can be pushed into reverse bias. It then uses electrical power instead of producing it, and that power turns into heat.[1]
Uneven current → current mismatch → reverse bias → heat
There is another type of heating too. A loose connector, damaged solder joint, ribbon, or junction-box connection can heat up because its electrical resistance is too high. It may look similar on a thermal camera, but the fault is different.
What Is a Hot Spot?
A solar module will get hot in strong sunlight. That is normal.
A hot spot is different. It is one cell, part of a cell, or one electrical point that is much hotter than nearby areas under the same conditions.
You may see:
- one whole cell running hot;
- half of a cell heating up;
- a small bright point on a thermal image;
- a hot line along a ribbon or busbar;
- a hot junction box or connector.
For example, imagine most cells are around 62–65°C, but one cell reaches 91°C. The useful number is the difference: that cell is about 26–29°C hotter than the cells beside it.
That does not tell you the cause yet, but it tells you where to look.
Hot Spot vs Hot Cell
A hot spot and a hot cell do not always look the same.
A classic hot spot may be very small. Heat is packed into one tiny part of the cell. A hot-cell effect can spread across most of the cell surface.
IEA PVPS reports that older multicrystalline cells often show more localized breakdown, while modern monocrystalline cells can heat across a wider area under reverse bias. PERC, TOPCon, and silicon heterojunction cells also behave differently when reverse voltage becomes high.[2]
So do not expect every serious hot-cell problem to look like one tiny bright dot.
Tongwei's high-efficiency cell range includes N-type cell technologies, which is another reason thermal results should be read with the actual cell type in mind.
Why Reverse Bias Happens
Cells in the same series path carry almost the same current.
Now picture this: most cells can produce 10 A, but one shaded cell can only produce 2 A.
The current through that weak cell may still be much higher than 2 A because the module is working as part of a larger electrical circuit.
If the weak cell cannot supply that current, its voltage can fall below zero. That is reverse bias.
At that point, the cell is no longer helping the module produce power. It is being driven by the rest of the circuit.
sunlight → healthy cell → useful power
electrical power → reverse-biased cell → heat
How Much Heat?
Reverse-bias power can be estimated with a simple formula:
Power = Current × |Reverse voltage|
| Current | Reverse Voltage | Power Turned Into Heat |
|---|---|---|
| 4 A | 5 V | 20 W |
| 8 A | 5 V | 40 W |
| 10 A | 5 V | 50 W |
At the same reverse voltage, twice the current means twice the electrical power is being dumped into the weak cell.
The numbers above are calculation examples. Real reverse voltage depends on the cell and the fault.
Power also does not tell you the final temperature by itself. A cell with good airflow may run cooler than the same fault on a hot roof with little air behind the module.
Why Heat Can Stay in One Spot
A weak cell does not always spread reverse current evenly.
A defect can create an easier path for current. Engineers often call this a shunt. Think of it like water finding one crack in a pipe: much of the flow may end up going through one small place.
Possible causes include:
- crystal or junction defects;
- crack intersections;
- damaged metal current-collecting lines;
- manufacturing defects;
- electrically damaged cell areas.
Take the earlier 72 W example and spread that power over different areas:
| Heated Area | Power Density |
|---|---|
| 200 cm² | 0.36 W/cm² |
| 40 cm² | 1.8 W/cm² |
| 10 cm² | 7.2 W/cm² |
Shrinking the heated area from 200 cm² to 10 cm² increases the power density 20-fold. That is why a tiny defect can become surprisingly hot.
What Makes It Worse?
The worst hot spots usually happen when several things line up at once.
- High current: more current means more power can be turned into heat.
- Higher reverse voltage: this also raises the power being lost in the weak cell.
- Small heated area: the same power packed into a smaller area creates a higher local heat load.
- Hot starting temperature: a module already running hot has less room before temperatures become severe.
- Long exposure: a shadow that stays for hours is more serious than one that passes in seconds.
- Poor cooling: low airflow makes it harder for the module to dump heat.
Bypass design matters too. IEA PVPS notes that fewer cells per bypass-protected section can reduce the reverse voltage and hot-cell temperature seen by a weak cell.[3]
Does Shade Size Matter?
Shade size alone tells you very little.
A wide shadow may mainly cut power. A much smaller object can sometimes create more trouble if it covers one cell while the rest of the substring is still carrying strong current.
The result depends mainly on:
- which cells are shaded;
- how much current is still flowing;
- how the cells and bypass diodes are arranged.
NREL testing has shown that shade position and bypass-diode grouping can change how a partially shaded PV system behaves.[4]
Narrow Shadows
Poles, cables, antennas, pipes, railings, nearby rows, and even tall weeds can cast narrow shadows.
A thin shadow running along one group of cells can affect the module differently from the same shadow crossing several groups.
Modern solar modules use different cell formats and layouts, so the actual module design matters when shade is unavoidable.
Bird Droppings and Debris
Light dust usually cuts sunlight over a broad area. Bird droppings, leaves, or dried mud can block a much smaller area almost completely.
That small blocked area may become the weak point while the rest of the module stays fully lit.
The same object can also cause very different heating in the morning and at midday.
| Example Condition | Current | Power at 5 V Reverse Bias |
|---|---|---|
| Morning | 4 A | 20 W |
| Midday | 10 A | 50 W |
At 10 A, the weak cell is dissipating 2.5 times as much electrical power as it was at 4 A, assuming the same 5 V reverse-voltage magnitude.
This is why a leaf or bird dropping may look harmless early in the day and become much more obvious on a thermal image around noon.
Cell Cracks
A clean module can still develop a hot cell.
Cracks can form during manufacturing, transport, installation, hail, wind or snow loading, or years of daily heating and cooling.
A crack becomes electrically important when it cuts off part of the cell from the current-collecting network.
crack → inactive cell area → current mismatch → possible reverse-bias heating
The front glass may still look perfect. Electroluminescence imaging is useful here because it can reveal cracks and inactive cell areas that are hard to see by eye.[5]
High-Resistance Heating
A bad electrical connection creates heat in a different way.
P = I²R
Assume a connection resistance of 0.05 Ω:
| Current | Heating |
|---|---|
| 8 A | 3.2 W |
| 10 A | 5.0 W |
| 12 A | 7.2 W |
| 15 A | 11.25 W |
Current rises from 10 A to 15 A by 50%, but heating jumps from 5 W to 11.25 W, an increase of 125%.
That extra heat may be concentrated in a very small connector or solder joint.
Common locations include:
- solder joints;
- cell ribbons;
- busbars;
- junction-box terminals;
- DC connectors;
- cable terminations.
Current ratings differ across module types. Tongwei's module application data can be used to check the actual current of the module being inspected.
Bypass Diodes
Many crystalline-silicon modules split their cells into groups protected by bypass diodes.
During normal operation, current goes through the cells and the diode stays off. If one cell group becomes weak enough, the voltage across that group can fall until the diode starts conducting.
The current then has another route around the weak cells. The module loses some voltage, but the affected cells see less reverse stress.
A bypass diode responds to substring voltage, not to shade directly, so a weak cell can enter reverse bias before the diode switches on.
IEA PVPS identifies bypass diodes as the standard way to limit excessive heating in shaded cell groups.[6]
Bypass Diode Faults
| Condition | What Can Happen |
|---|---|
| Open circuit | The bypass route is lost, so shaded cells may see more reverse stress. |
| Short circuit | The substring may stay bypassed, reducing module voltage and power. |
| Normal conduction | The diode may run warm because it is carrying current. |
If the junction box is hot, check module voltage and shading before assuming the diode has failed.
Modern Cell Technology
PERC, TOPCon, and silicon heterojunction cells do not behave exactly the same way under reverse voltage.
IEA PVPS reports that monocrystalline PERC cells can enter avalanche breakdown below about -20 V, while TOPCon and HJT cells may need even more negative reverse voltage. Modern monocrystalline cells can also show broader hot-cell heating instead of one tiny hot point.[7]
TOPCon and HJT can still develop hot cells under mismatch. Their reverse-bias behavior is simply different.
Tongwei also publishes information on its PV technology development and module reliability work.
What Can Be Damaged?
| Part | Possible Damage |
|---|---|
| Cell | Permanent electrical changes, larger inactive areas, or crack growth |
| Encapsulant | Browning, poor adhesion, or delamination |
| Interconnections | Higher resistance or solder-joint damage |
| Backsheet | Browning, blistering, deformation, or insulation damage |
| Glass | Extra stress caused by a large local temperature difference |
IEA PVPS reports typical module operating temperatures of about 50–70°C. Severe partially shaded hot cells can exceed 150°C, where polymer materials can start to suffer serious damage.[8]
Power Loss Can Be Small
A hot cell does not always cause a dramatic drop in total module output.
Take a hypothetical 500 W module. If one local defect is dissipating 20 W, that is only:
20 W ÷ 500 W = 4%
A 4% change may be hard to spot in normal monitoring data, especially while clouds and temperature are changing. But 20 W packed into a small cell area can still create a serious local temperature rise.
If a bypass diode removes a full substring from normal operation, the module may lose much more voltage and power even while the stressed cell itself becomes cooler.
How to Find Hot Spots
Infrared thermography is the main field tool for finding abnormal heat while a PV system is running.
IEC TS 62446-3 covers outdoor infrared inspection of operating PV modules and plants.[9]
| Thermal Pattern | Likely Direction to Check |
|---|---|
| Whole hot cell | Cell mismatch, reverse bias, or internal cell defect |
| Small intense point | Local shunt or breakdown point |
| Part of one cell | Crack, local shade, or inactive area |
| Narrow hot line | Ribbon, solder joint, busbar, or other connection |
| Hot junction box | Bypass diode or junction-box connection |
| Hot connector | High contact resistance or connector damage |
IR Test Conditions
IR inspection works best under stable sunlight and enough operating current to make the fault heat up.
IEA PVPS O&M guidance says plane-of-array irradiance should be at least 600 W/m² during module infrared inspection.[10]
- Wind: strong airflow can cool the module and hide a temperature difference.
- Clouds: fast changes in sunlight make current and temperature unstable.
- Reflection: PV glass can reflect the sky, sun, buildings, or even the person holding the camera.
- Low current: a fault may not heat much when current is low.
- Comparison: compare the suspect cell with nearby cells under the same sunlight and airflow.
Confirm the Cause
| Method | What It Shows |
|---|---|
| Visual inspection | Leaves, bird droppings, mud, cracked glass, burn marks, loose wiring, and visible damage |
| Infrared imaging | Where abnormal heat is located |
| Electroluminescence imaging | Cracks and electrically inactive cell areas |
| I-V curve testing | How the fault changes current, voltage, and power |
| Electrical testing | String voltage, current, insulation, connectors, and bypass-diode behavior |
IEA PVPS treats IR and EL as complementary methods for field inspection.[11]
IEC Hot-Spot Testing
IEC 61215-2:2021 includes a hot-spot endurance test as part of PV module design qualification.[12]
IEC testing checks the module design under controlled test conditions. Damage can still appear later from cell cracks, new shading, corrosion, poor connections, bypass-diode faults, transport damage, or years of outdoor stress.
Reduce the Risk
- Check fixed and seasonal shade before installation.
- Use the actual cell and substring layout when shade cannot be avoided.
- Keep vegetation below the module edge.
- Remove leaves, bird droppings, and other opaque debris that keeps covering the same cells.
- Do not stand or kneel on modules unless the manufacturer allows it.
- Use compatible connectors and support cables so they do not pull on terminals.
- Inspect modules again after hail, transport damage, or a major storm.
Tongwei publishes installation and maintenance manuals for its PV modules.
When to Check a Module
Closer inspection makes sense when you see:
- the same hot cell appearing again under similar sunlight;
- a large temperature difference from nearby cells;
- browning, blistering, delamination, or burn marks;
- unexpected voltage or power loss;
- a junction box that stays unusually hot;
- a hot connector;
- visible electrical or insulation damage.
There is no single temperature that means every module must be replaced. Use the thermal pattern, visible damage, electrical loss, and manufacturer limits together.
What Users Should Avoid
PV modules can stay energized whenever enough light reaches them.
Do not unplug DC connectors under load, open a junction box to test a bypass diode without proper training, or cover cells on purpose to recreate a suspected hot spot.
Owners can check the module visually and review monitoring data. Energized DC testing should be left to qualified personnel.
Conclusion
Hot spots are mainly a current-mismatch problem, not simply a “hot module” problem. A weak cell can enter reverse bias and turn electrical power into heat; at 9 A and an 8 V reverse-voltage magnitude, that would be 72 W of electrical dissipation. IEA PVPS reports normal module operating temperatures around 50–70°C, while severe hot cells can exceed 150°C.[13] Compare the suspect cell with nearby cells, then use IR, EL, I-V, or electrical testing to confirm the cause before deciding whether the module needs repair or replacement.