Hot spots on solar panels: what they are and how to detect them

A hot spot is a cell heating significantly more than the others on the same panel. Causes: persistent partial shade, point soiling, factory-defective cell, or internal microcrack. Undetected, it burns the panel.

Why it heats

All cells in a panel are in series. A shaded cell stops producing and absorbs current from the others like a resistor. Hits 300-390 °F in minutes. Burns encapsulant and breaks the panel.

Prevention: bypass diodes

Each panel has 3 bypass diodes (one per cell strip). If a cell fails, the diode reroutes current, preventing hot spot. If the diode also fails: hot spot guaranteed. That's why Tier 1 panels use premium diodes.

How to detect: thermal camera

Pocket thermal camera (Flir One Pro $400, Hti HT-19 $230) shows the whole panel. A cell at 122 °F when others are at 95 °F: hot spot. Annual inspection recommended on any >5 kWp install.

Indirect symptom: production drop

Without camera, you notice the panel produces 30-50% less than neighbors. With microinverters or optimizers, you see it panel-by-panel in app. No monitoring: measure each string current with DC clamp meter.

What to do when you find one

Point soiling: clean. Fixed shade: remove shade source (branch, obstacle) or accept the loss. Internal defective cell: panel is broken, RMA under warranty or replace.

Cost of a burned panel

Catch it late: heat burns EVA encapsulant (visible brown stain), cracks glass or delaminates. Panel unusable. $200-400 loss per panel + RMA time. Annual thermal inspection: $60 professional service worth it.

More on degradation in PID and LID degradation.