Key Takeaways
- Solar panel hot spots happen when one cell is forced to absorb power instead of producing it, usually because shading, soiling, or a crack chokes the current in that string.
- Left alone, a hot cell can reach 80-100 C, scorch the backsheet, brown the encapsulant, and permanently cut a panel's output.
- Most hot spots on a healthy roof come from avoidable causes: bird droppings, leaf litter, dust cake, or a shadow you can trim away.
- A cheap infrared thermometer or a thermal camera pass on a clear afternoon is the fastest way to find one before it does lasting damage.
- Cleaning and de-shading fix soiling hot spots; cracked or diode-failed panels usually need replacement, and any DC work should be done with the array isolated.
The first time I met a hot spot, I found it with the back of my hand. A client's 4 kW array in Pune was down about 12 percent for no obvious reason, and while walking the roof I brushed one panel and nearly burnt my palm on a patch that was far hotter than its neighbours. That single overheating cell was quietly cooking the panel from the inside. Solar panel hot spots are one of the most common and most misunderstood faults on Indian rooftops, and the good news is that most of them are cheap to find and cheaper to prevent. Here is how they form, how to spot one, and what to actually do about it.
What a hot spot actually is
A solar panel is a long chain of cells wired in series, so every cell has to pass the same current. When one cell cannot keep up, because it is shaded, cracked, or dirty, it stops acting like a battery and starts acting like a resistor. The healthy cells behind it keep pushing current through, and that trapped energy turns into heat right at the weak cell.
That is the hot spot: a small area, sometimes a single cell, that runs far hotter than the rest of the panel. Bypass diodes inside the junction box are meant to route current around a struggling group of cells, but they only trigger when the mismatch is bad enough, and they cover a whole sub-string, not one cell. So a partial problem can sit below the diode threshold and simmer for months.
The temperatures involved are not trivial. A normal cell on a summer afternoon in India might sit at 55-65 C. A reverse-biased hot cell can climb past 90 C, and lab studies have recorded cells above 150 C in severe cases. That heat browns the EVA encapsulant, can delaminate the backsheet, and in rare cases starts scorch marks you can see from the ground.
What causes solar panel hot spots
Almost every hot spot traces back to one cell being unable to carry the string current. The reason it cannot is what varies, and knowing the cause tells you whether you are looking at a five-minute clean or a panel swap.
Shading and soiling
This is the big one on Indian rooftops, and it is the most fixable. A bird dropping, a fallen leaf, a smear of cement dust, or the shadow of a new water tank or TV antenna can block a single cell. That cell drops out, the string keeps driving current, and the covered cell heats up. Dust cake after a dry Delhi winter or a Chennai coastal salt-and-grime layer does the same thing across patches of the array.
Micro-cracks and cell damage
Cells crack during rough transport, careless installation, hail, or someone walking on the glass. A crack may be invisible to the eye but it isolates part of the cell, so the working section has to carry the current for the dead section and overheats. These hot spots do not clean off, and they tend to spread as thermal cycling widens the crack.
Solder failures and bad bypass diodes
Poor soldering at the cell interconnects raises resistance at that joint, and resistance means heat. A failed or short-circuited bypass diode can also cook a whole sub-string, which usually shows up as one third of the panel running hot in a thermal scan. These are manufacturing or ageing faults, not something you clean away.
Manufacturing defects and PID
Mismatched cells, contamination in the silicon, or potential induced degradation (PID) in humid conditions can all create cells that underperform from day one or drift over a few years. Cheap panels with loose quality control are far more likely to develop early hot spots, which is one reason I steer clients toward tier-one modules with a proper IEC 61215 hot-spot endurance test in their datasheet.
How to spot a hot spot before it does damage
The frustrating thing about hot spots is that they rarely trip an alarm. Your inverter shows a small production dip, not a fault code, so you have to go looking. Here is the sequence I run on a service call.
- Check your monitoring first. If a string is running a few percent below its neighbours on clear days, that is your cue. My write-up on how I diagnosed a solar system's underperformance walks through reading those numbers.
- Pick a hot, sunny afternoon. Hot spots show up best when the array is under full load and the temperature gap is widest.
- Do a visual pass. Look for browned or bubbled patches on the backsheet, discoloured cells, or scorch marks near the junction box.
- Scan with heat. A handheld infrared thermometer costs a few hundred rupees and lets you compare cell temperatures. A thermal camera, or even a clip-on phone thermal sensor, gives you the full picture in one shot.
- Mark and note. A cell running 15-20 C above its neighbours is a confirmed hot spot. Photograph it and log the panel position.
Tip: Scan the array roughly two hours either side of solar noon on a clear day. On a cloudy morning the temperature difference between a healthy cell and a hot cell shrinks, and you can walk right past a real fault.
Causes and fixes at a glance
| Cause | How it looks on a thermal scan | Fix | Cost in India |
|---|---|---|---|
| Bird dropping or leaf | Single bright cell | Clean the panel, trim overhanging branches | Free to nominal |
| Dust or salt cake | Several warm patches | Wash with soft water and a soft brush | Free to ₹1,500 per service |
| Fixed shadow (tank, antenna) | Repeating hot cell at same time daily | Relocate the object or re-string the array | Varies |
| Micro-crack | Hot line or corner that will not clean off | Monitor, then replace the panel | ₹6,000-14,000 per module |
| Failed bypass diode | One third of panel hot | Replace junction box or panel | ₹1,500-14,000 |
| Solder or PID defect | Persistent hot cell, no surface cause | Warranty claim or replacement | Warranty dependent |
How to fix and prevent hot spots
The fix depends entirely on the cause, and this is where a lot of homeowners overspend. Roughly three quarters of the hot spots I find on well-installed systems are soiling or shading, and those cost nothing but a wash and a pair of pruning shears.
For soiling, wash the array early morning or late evening with plain water and a soft brush, never cold water on hot glass, which can thermal-shock the panel. In dusty regions I tell clients to rinse monthly through the dry season. For a fixed shadow from a tank, chimney, or new construction next door, either move the object or ask your installer to re-string the array so the shaded modules sit on their own MPPT input.
Cracked cells, failed diodes, and PID damage are different. You cannot clean or shade-manage those away, and a panel that keeps a hot cell after cleaning is on borrowed time. If it is inside its product warranty, usually 10-12 years on the module, raise a claim with the manufacturer using your thermal photos as evidence. Out of warranty, budget for a replacement module of the same wattage and voltage so it matches the string.
Warning: A solar array produces dangerous DC voltage whenever there is daylight, and there is no simple on-off switch at the panel. Never remove a junction box cover, touch connectors, or swap a module with the string live. Isolate the array at the DC isolator, and leave panel replacement or diode work to a qualified installer. Rooftop DC arc faults are a real fire risk.
Prevention is mostly boring maintenance done on schedule. Keep the array clean, keep vegetation trimmed, and do a thermal scan once a year, ideally at the same time you check torque on the mounting clamps. Catching a browning cell early is the difference between a wash and a replacement. The same habit protects against slow output loss over the system's life, which I covered in my notes on tracking panel degradation over three years.
Repair or replace: how to decide
My rule of thumb is simple. If the hot spot disappears after cleaning and the cause was clearly external, the panel is fine and you just prevented damage. If a hot cell persists after a proper clean, the fault is internal, and you are choosing between a warranty claim and a paid replacement.
Do not try to save a panel with visible backsheet burns or delamination. That damage is permanent, the module will keep underperforming, and a scorched backsheet is a genuine fire and shock hazard. One weak panel also drags down every other module in its series string, so leaving it in place costs you far more energy than the panel is worth. When you weigh the numbers, factor in the whole-string loss, not just the one module. The wider trade-offs of running versus retiring ageing gear sit inside our solar troubleshooting guide, which is worth a read before you spend on replacements.
For the technically curious, the US National Renewable Energy Laboratory has published detailed field research on photovoltaic module reliability, including hot-spot and PID failure modes, and the IEC 61215 standard defines the hot-spot endurance test that quality modules must pass.
Frequently Asked Questions
Can a solar panel hot spot cause a fire?
It is uncommon but possible. A severe hot spot can reach temperatures that scorch the backsheet and, combined with a DC arc fault at a damaged connection, has started rooftop fires. This is why you never ignore a browned cell and never work on a live DC array yourself.
How much output does a hot spot actually cost me?
A single shaded or hot cell can pull an entire panel's output down by 20 to 30 percent, and because panels sit in series, that weak module drags the whole string with it. On a small array that can mean a noticeable dent in your monthly generation and bill savings.
Do I need a thermal camera to find hot spots?
No. A basic infrared thermometer costing a few hundred rupees lets you compare cell temperatures and catch obvious hot spots. A thermal camera or a phone clip-on sensor is faster and shows the whole array at once, but it is a convenience, not a requirement for a small rooftop system.
Will cleaning always fix a hot spot?
Only if the cause is on the surface, like dust, droppings, or leaves. Cleaning does nothing for a micro-crack, a failed bypass diode, or PID damage inside the module. If a hot cell survives a thorough wash on a sunny day, the fault is internal and the panel likely needs replacement.
How often should I check for hot spots in India?
An annual thermal scan is enough for most homes, ideally before the peak summer when temperatures and output are highest. If you live in a dusty or coastal area, pair it with more frequent cleaning, since soiling is the leading cause of hot spots on Indian rooftops.
Keep an eye on the heat
Hot spots are one of those faults that reward a little attention and punish neglect. A clean array, trimmed branches, and one thermal scan a year will catch almost every hot spot while it is still just a warm cell and not a burnt panel. The tools are cheap, the habit takes an afternoon, and the payoff is a system that holds its output for its full life. If you want to build the wider diagnostic routine that finds problems like this early, start with our solar troubleshooting guide and work through the checks one by one.
