What Causes Solar Panel Hotspots and How They Can Permanently Burn Your PV Modules

When inspecting rooftop solar systems with thermal imaging cameras, engineers frequently encounter bright glowing red spots on individual solar cells where temperatures exceed 140°C to 180°C. These dangerous localized thermal defects are known as Solar Cell Hotspots.

A severe hotspot doesn't just reduce generation—it physically melts the EVA encapsulant film, shatters the tempered glass, burns the backsheet, and poses a catastrophic rooftop fire hazard. Here is the technical breakdown of how hotspots form and how to eliminate them.

The Physics of a Hotspot: Reverse Bias Overheating

In a standard solar panel string, silicon cells are connected in series. When all cells receive uniform sunlight, they all produce matching DC current.

• The Shading Bottleneck: If a single cell is covered by a dry leaf, bird dropping, or chimney shadow, it stops generating current and acts as an obstruction.
• Reverse Bias: The remaining 50+ fully illuminated cells in the string force their high current through this shaded cell. The shaded cell is forced into reverse bias, acting like an electrical resistor heating element.
• Instead of generating power, the cell dissipates high wattage purely as concentrated heat, creating a scorching hotspot.

The Role of Bypass Diodes (And Why They Fail)

Quality solar panels integrate 3 Bypass Diodes inside the junction box to divert current around shaded cell strings. However, bypass diodes fail under two common scenarios:

1. Sub-standard Junction Boxes: Cheap non-certified diodes overheat and short-circuit under sustained partial shading, leaving the string permanently exposed to reverse bias.
2. Internal Micro-cracks: Micro-cracks caused by installers walking on solar panels during installation create localized resistance points that bypass diodes cannot bypass.