Solar panel hotspots explained for plant owners
"Hotspot" is the word most people know from solar thermography, and it is often used loosely to mean any warm patch on a module. In an inspection report it means something specific, and the difference between a hotspot, a failed diode and an offline string decides what your O&M team should do next.
What a hotspot is
Solar cells in a module are wired in series. When one cell produces less current than the others, because it is shaded, soiled, cracked or degraded, the rest of the string tries to push current through it anyway. The weak cell becomes a resistor and turns that energy into heat instead of electricity. That localised heating is a hotspot.
Bypass diodes exist to limit this. Each module has a few of them, each protecting a group of cells. When a group is dragged down far enough, its diode conducts and the group is bypassed, sacrificing its output to protect it from overheating.
Common causes
- Soiling and bird droppings on a single cell or a corner of the module. Cheap to fix, and the hotspot disappears after cleaning.
- Partial shading from vegetation, structures, cables or the row in front at low sun angles.
- Cell cracks from transport, installation, hail or people walking on modules. Often invisible to the eye but clear under infrared.
- Cell mismatch and degradation, where one cell simply ages faster than its neighbours.
- Delamination and moisture ingress, which in a tropical climate progress from a small defect to a large one within a few seasons.
Why it matters
An individual hotspot cell costs relatively little yield. The reasons owners care are cumulative and long-term:
- Accelerated degradation. Repeated heating damages the cell, the encapsulant and the backsheet. A hotspot module fails earlier than a healthy one.
- Fire risk. Severe hotspots, and especially heating in junction boxes and connectors, can reach temperatures that ignite backsheets and nearby material. On rooftops above occupied buildings this is the finding that gets fixed first regardless of yield.
- Warranty evidence. A pattern of hotspots across a batch of modules is exactly the kind of evidence a manufacturer warranty claim needs.
- Diagnosis. The pattern tells you the cause. Random single cells point to soiling or damage. Every module at the bottom of the row points to shading or soiling build-up. Clusters by pallet or batch point to manufacturing.
Hotspot, failed diode or offline string?
| What the inspection shows | What it usually is | Typical action |
|---|---|---|
| One or a few hot cells | Hotspot: soiling, shading, crack or cell degradation | Clean or clear shading first; monitor or replace if it persists |
| One third (or two thirds) of a module uniformly warmer | Bypass diode conducting or failed, or a sub-string fault | Check the junction box; usually a module warranty item |
| Whole module uniformly warmer than neighbours | Module open-circuit or disconnected | Check connectors and cabling |
| Whole string of modules uniformly warmer | String offline: blown fuse, tripped breaker, failed connector | Often the biggest recoverable loss on the plant; fix immediately |
| Small, very hot point at the junction box or a connector | High-resistance connection | Safety priority; repair regardless of yield impact |
What a good report does with hotspots
It does not just count them. It tells you which module each one is on, how severe it is, what it is probably caused by, roughly what it is costing you, and whether it is new since the last survey. That turns a list of warm patches into a cleaning schedule, a warranty claim and a repair order, each aimed at the right modules.