Assessment: 87 Hotspot-Damaged Modules Cause 14% Yield Loss on 420 kWp System

Assessment type: Yield assessment / Defect evaluation System size: 420 kWp polycrystalline Region: Cologne/Bonn area Period: May – Jul 2024

Assessment summary:

420 kWp commercial rooftop system (built 2017) with 14% yield deviation from P50 forecast
Drone thermography at 820 W/m² identifies 87 modules with hotspot temperatures >85 °C
Causes: 62x defective bypass diodes, 18x cell cracks with short circuit, 7x solder joint fractures
Maximum hotspot temperature: 142 °C (module backsheet) — fire risk per IEC 61215 exceeded
Recommendation: Replacement of all 87 modules, economic damage EUR 94,200 incl. lost yield

How was the hotspot problem identified?

The operator of a 420 kWp rooftop system on a logistics building in the Cologne/Bonn area noticed a continuous yield decline through PV monitoring. Compared to the yield forecast (specific yield: 980 kWh/kWp, P50), actual generation in 2023 was only 843 kWh/kWp — a deviation of 14%. The operator commissioned an expert assessment to determine the cause.

Drone thermography: Test conditions

Parameter	Value
Irradiance during measurement	820–870 W/m²
Module temperature (reference, undamaged)	48–52 °C
Wind speed	<2 m/s
IR camera	FLIR Vue Pro R 640, NETD <50 mK
Flight altitude / resolution	15 m / 3.8 cm/px
Modules tested	1,120 (entire system)

What did the thermographic analysis show?

The IR images identified 87 modules with local temperature anomalies significantly above the ambient temperature of unaffected modules:

Temperature difference ΔT	Number of modules	Risk assessment
ΔT 20–40 K (68–92 °C absolute)	41	Elevated — monitoring recommended
ΔT 40–60 K (92–112 °C absolute)	33	High — replacement recommended
ΔT >60 K (>112 °C absolute)	13	Critical — immediate replacement

The hottest module showed a backsheet temperature of 142 °C at an irradiance of 850 W/m². This value exceeds the hotspot threshold defined in IEC 61215 (module temperature + 50 K) by 40 K and represents an acute fire risk.

Root cause analysis of hotspots

Further investigation (I-V curve, EL spot check, bypass diode test) revealed:

Cause	Count	Mechanism
Defective bypass diodes (short circuit)	62	Diode permanently diverts current → cell becomes a resistor → heating
Cell cracks with local short circuit	18	Crack area forms ohmic resistance → local power dissipation
Solder joint fractures (cell connectors)	7	Interrupted current path → current concentration at remaining cross-section

Safety assessment: 13 modules with temperatures above 112 °C represent an immediate fire risk. At summer irradiance levels above 1,000 W/m², temperatures of 165–180 °C can be calculated — above the ignition temperature of EVA encapsulation (approx. 200 °C). The assessment recommended immediate shutdown of the affected strings.

What was the economic damage?

Damage calculation:

Item	Amount
Cumulative yield loss 2020–2024 (progressive degradation)	EUR 47,800
Module replacement 87 modules incl. installation	EUR 26,100
Downtime loss during replacement (2 weeks)	EUR 4,300
Thermography + IV measurement + assessment	EUR 16,000
Total damage	EUR 94,200

Expert assessment: The analysis indicates a manufacturing defect in the bypass diodes (batch 2016, identical module type). The high failure rate of 5.5% of bypass diodes after only 7 years of operation is significantly above the expected failure rate (<0.5% after 10 years). A recourse claim against the module manufacturer appears justified.

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Last updated: 2026-06-16 | Author: Christoph S. Prestele, TUV-certified expert assessor | PV-BESS-Assessor.com