Solar Module Hotspot: Causes, Detection & Remediation

Hotspots are among the most common and most dangerous defects in photovoltaic systems. They occur when individual cells or cell groups overheat severely – and can, in the worst case, cause fires. This article explains what hotspots are, how they develop, when they become critical and how thermographic inspection reliably makes them visible.

Was ist ein Hotspot?

A hotspot is a local overheating within a solar module, where individual cells or cell groups reach significantly higher temperatures than the rest of the module. The temperature difference can range from a few degrees to over 100 kelvin depending on the cause. In the infrared image a hotspot appears as a brightly glowing area on an otherwise evenly tempered module.

Hotspots occur whenever a cell can no longer generate or conduct the same amount of current as the other cells in the same string. Since the current in the string remains constant, the weaker cell becomes a consumer rather than a generator – it absorbs electrical energy and converts it into heat. This mechanism is known as reverse-bias operation bezeichnet.

Causes of hotspots

The most common causes fall into three categories:

Mechanical damage: Microcracks from transport, installation or hail damage lead to increased internal resistance in the affected cell. Microcracks are invisible to the naked eye – in the infrared image they show up as a characteristic heat pattern. After hailstorms microcracks are particularly frequent and widely distributed.

Soiling: Bird droppings, leaves or dust on part of the module create partial shading. The shaded cells are driven into reverse-bias operation by the rest of the string. Bird-dropping hotspots are often intense and concentrated – ΔT of 30–60 K is not unusual.

Manufacturing defects and degradation: Cell mismatch from different electrical characteristics within a module, faulty solder joints on busbars and progressive PID degradation all lead to hotspots. These causes develop slowly over years and are often only discovered during a systematic thermographic inspection.

Temperatures and severity levels

IEC TS 62446-3 classifies hotspots by the temperature difference (ΔT) between the affected cell and the reference temperature (adjacent intact cells or module rear):

The classification is a guide value – whether a hotspot requires immediate action also depends on the cause, the system environment and the module type. An experienced thermographer assesses every finding in its overall context.

Fire risk from hotspots

Hotspots are the most commonly identified cause of PV fires in Germany. Two scenarios are particularly dangerous: overheated junction boxes, whose seals and cable insulation are thermally destroyed by sustained hotspots above 80°C, and hotspots from faulty bypass diodes, where temperatures of 100–150°C can develop in the centre of the module.

The risk increases with system size and age. Systems that have been in operation for 10 or more years and have never had a thermographic inspection have a statistically significantly higher probability of critical hotspot findings. VdS guideline 2858 explicitly recommends thermographic inspection as a fire protection measure.

How are hotspots detected?

There are fundamentally three methods for hotspot detection, which differ considerably in effort, accuracy and site coverage:

Monitoring analysis: Yield drops at string level can indicate hotspots – but the method is blind to individual modules. A 100 kWp string with 5% yield loss from a hotspot is barely identifiable in the monitoring because the signal is too small.

Visual inspection: Discolouration, glass bubbles or burn marks are visible signs of advanced hotspots. These findings only show damage that has already occurred, however – thermography detects hotspots much earlier.

Thermographic inspection: The only method that reliably makes every hotspot of any size visible at module level. Drone thermography allows every module to be inspected across the entire site within a short time.

Thermography as a detection method

Infrared thermography uses the heat radiation of a hotspot to make it visible in the image. A calibrated IR camera with sufficient resolution (min. 640×512 pixels for professional inspections) captures the temperature distribution across the entire module surface in real time.

For a standards-compliant inspection to IEC TS 62446-3 defined measurement conditions must be maintained: at least 600 W/m² irradiance, wind below 4 m/s and the system must be operating at at least 20% of its rated power. Only under these conditions are the thermograms reproducible and usable for insurers and expert reports.

In drone thermography every module of the system is systematically surveyed – without gaps and geo-referenced. The report classifies all hotspots found by ΔT and action priority and states which module at which position in the system is affected.

Remediation and prevention

Remediation depends on the cause. For soiling a targeted cleaning of the affected module suffices. For microcracks or internally defective cells the module must be replaced. For bypass diode defects, depending on the module type, only the diode can be replaced or a full module replacement is necessary.

For prevention regular thermographic inspection on a two-year cycle is recommended per VdS recommendation. Anyone who does not wait after a hailstorm or a noticeable yield drop can detect and remedy damage before it worsens. A commissioning inspection directly after installation documents the initial condition and serves as the reference for all subsequent inspections.

What does a hotspot diagnosis cost?

The cost depends on system size and chosen package. The Basic package includes the complete drone thermography with a standards-compliant report – hotspots are geo-referenced and classified at module level. The Complete package adds the hand thermography of BOS components, which is necessary for hotspot findings at junction boxes and inverters. Specific prices are available in the price calculator on the homepage.

Frequently asked questions