Hot Spot

Key Takeaways

• Hot spot = localized cell overheating from shading + reverse bias.
• Bypass diodes mitigate but failed diodes risk fire.
• Detected via IR thermography.
• Long-term hot spots damage encapsulant and cell.
• Inspect quarterly on utility plants.

How a Hot Spot Forms

A module’s cells are wired in series, so every cell in a string normally carries the same current. When one cell is shaded by a leaf, bird droppings, a chimney, or a neighboring row, or is damaged by a cell microcrack, it can no longer pass that current on its own.

Instead of generating power, the affected cell is forced into reverse bias and starts dissipating the rest of the string’s current as heat, concentrated in a small area of glass and encapsulant.

A properly functioning bypass diode routes current around the shaded segment before this gets severe, but if the diode has failed or is undersized, the cell can run tens of degrees hotter than its neighbors, the same threshold picked up during bypass diode activation, just without the protection kicking in.

Detecting and Preventing Hot Spots

Because a hot spot rarely shows up on a monitoring dashboard until output has already dropped, the standard check is an IR thermography scan of the array while it is operating under load. Utility-scale plants typically run this on a quarterly cadence; residential systems are checked opportunistically during service visits.

Prevention starts earlier, at the design stage: a shading study that accounts for row spacing, obstructions, and seasonal sun angles reduces how often cells fall into partial shade in the first place. Tools built for exactly this kind of pre-construction check are covered in our review of shading analysis software and in the workflow described for the PVsyst 3D near-shading scene builder.

Related Reading

Hot spots are one of several module-level failure modes that engineers weigh alongside string design and layout decisions. If shading is a recurring issue on a project, it usually traces back to the same site geometry that drives shading analysis during design, which is why the software walkthroughs linked above are worth reading before finalizing a layout.

Hot spots are also easy to confuse with unrelated faults picked up during commissioning, for a look at how a different class of problem shows up on the inverter side rather than the module side, see this solar inverter troubleshooting guide from Qbits Energy.

Many hot spots trace back to installation choices; see common solar installation defects and design errors.

Disclosure: Qbits Energy is a Heaven Designs group company.