What is a hot spot
A hot spot in a solar PV module is a region of localised excessive heating, typically affecting one or a few cells, caused by the cell being forced to operate in reverse bias (acting as a load instead of a generator) while string current flows through it. The cell dissipates the entire string current at high voltage, generating significant heat that can reach 100-200°C.
Hot spots originate from any condition that reduces one cell’s current generation below the string current: partial shading from external objects, cell defects from manufacturing, cell cracks from thermal stress or handling, bird droppings, leaves, soiling concentrated on one cell, or simply mismatched cell aging.
Modern PV modules include bypass diodes in the junction box. When string current cannot flow through a shaded or damaged cell, the diode provides an alternative path, limiting the hot spot energy. Without bypass diodes, hot spots could permanently damage encapsulant, back sheet and cells. Field technicians confirm suspected hot spots the same way described in this solar inverter low-output diagnostic guide: an infrared thermal scan during peak sun that reveals failed cells generating heat instead of current.
Why hot spots matter
Module reliability. Severe hot spots damage encapsulant and back sheet.
Yield loss. Shaded modules produce less than potential.
Fire risk. Extreme hot spots in poorly designed modules can ignite encapsulant.
Insurance. Hot spot damage may be insurance excluded.
O&M priority. Detecting hot spots through IR thermography is standard O&M.
How hot spots form
- One cell in a string receives less light (shading, soiling, defect).
- String current is determined by the strongest cells.
- Shaded cell cannot supply this current.
- Shaded cell goes into reverse bias.
- Current still flows through the shaded cell.
- Power dissipated in shaded cell = String current × Cell reverse voltage.
- Localised heating.
- Bypass diode (if present) opens path around shaded sub-string, limiting damage.
Technical deep dive
Bypass diode protection
Standard module: 60-72 cells in series, 3 bypass diodes (one per 20-24 cells).
When sub-string current cannot flow through shaded cell:
- Bypass diode conducts in forward direction
- Sub-string output bypassed
- Hot spot energy limited to bypass diode rating
- Module continues operating at reduced power
Without bypass diodes
| Aspect | With bypass | Without bypass |
|---|---|---|
| Hot spot temperature | 60-80°C | 150-200°C |
| Module survives | Yes | Limited |
| Module damage | Minor | Severe |
| String output | Sub-string lost | Whole string compromised |
Common bypass diode count
| Configuration | Diodes | Sub-strings |
|---|---|---|
| 60-cell module | 3 | 3 × 20 cells |
| 72-cell module | 3 | 3 × 24 cells |
| Premium 60-cell | 6 | 6 × 10 cells |
| Half-cut 120 cells | 3 | 3 × 40 half-cells |
Hot spot test (IEC 61215)
Module covered with various opacity patterns at one or a few cells for 1 hour. Module must continue working without damage, encapsulant browning or back sheet cracking.
Real-world applications
O&M IR thermography surveys.
Identifying soiled, shaded or damaged modules.
Manufacturer warranty claims.
Insurance documentation.
Site monitoring.
Common hot spot causes
Partial shading (chimneys, trees, water tanks).
Bird droppings.
Leaf accumulation, which is why a pre-monsoon inspection that clears debris before the wet season also reduces hot spot risk.
Cell micro-cracks.
Module micro-mismatch in old arrays.
Cell defects from manufacturing.
Insufficient bypass diodes for module size.
Best practices
Annual IR thermography during O&M, or the simple hand-hover check described in this monsoon prep checklist for spotting a warmer panel before it is investigated formally.
Clean modules regularly.
Manage trees and obstructions.
Use modules with sufficient bypass diode and junction box protection.
Use microinverters or DC optimisers in shading-prone sites.
Replace damaged modules promptly.
Comparison: hot spot risk
| Application | Risk level |
|---|---|
| Utility tracker plant | Low (uniform conditions) |
| Utility fixed-tilt | Low |
| Unshaded rooftop | Low |
| Partial-shade rooftop | High |
| Tree-shaded rooftop | Very high |
| Microinverter system | Lowest (per-panel isolation) |
Indian market context
Indian solar plants periodically experience hot spots from:
- Tree shading
- Bird droppings (rural and semi-urban)
- Dust and soiling on specific cells
- Construction debris adjacent to arrays
Annual IR thermography is standard O&M practice for utility and large C&I plants.
ALMM-listed modules meet IEC 61215 hot spot tests.
Standards and certifications
| Standard | Scope |
|---|---|
| IEC 61215 | Module qualification includes hot spot test |
| IEC 61730 | Module safety |
| IEC 62716 | Hot spot test method |
| IS 14286 | Indian module standard |
Key takeaways
Hot spots are localised excessive heating in solar cells caused by partial shading or defects forcing one cell into reverse bias. Bypass diodes limit hot spot damage but do not eliminate it. Modern modules pass IEC 61215 hot spot tests. Annual IR thermography during O&M detects developing hot spots. Microinverters and DC optimisers reduce hot spot risk through per-panel operation. Prevention through shading management, cleaning and bypass diode coverage is the practical approach.
Further reading
For how Hot Spot plays out in real projects, these guides go deeper:
- How Shadow on One Solar Panel Kills Your Generation (And How to Fix It)
- Solar Inverter Failure: Safe First Actions
- Solar Shading Analysis Software 2026: Guide
- ALMM List-II Exemption Window for Net Metering & Open Access
Sources
- IEC 61215, Module qualification.
- IEC 61730, Module safety.
- IEC 62716, Hot spot test method.
- IS 14286, Indian module standard.
- IEC TS 63209, Extended stress testing.
- NREL Module Reliability Studies.
- IEA PVPS Task 13 Report on PV Performance.
- Sandia National Laboratories Hot Spot Studies.
- Manufacturer hot spot test data.
Frequently Asked Questions
What is a hot spot in simple words?
What causes hot spots?
How hot does a hot spot get?
What prevents hot spots?
How many bypass diodes?
Do hot spots cause permanent damage?
How are hot spots detected?
Can bird droppings cause hot spots?
Do microinverters prevent hot spots?
What is IEC 61215 hot spot test?
How does cell crack cause hot spots?
Are hot spots common?
What is the typical bypass diode rating?
Are hot spots monitored?
Are hot spots covered in warranty?
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