The Sky is the Limit for EL Drone Testing
by Jörg Althaus and Andreas Fladung
This article was originally published in pv magazine - September 2025 Edition.
Infrared drone inspections are now routine for solar projects, but post-installation electroluminescence (EL) testing remains misunderstood and under-utilized, argue Jörg Althaus of Clean Energy Associates (CEA) and Andreas Fladung of Aerial PV Inspection. The pair examine EL’s ability to detect microcracks, early-stage degradation, and installation induced damage, revealing how good planning and execution can lead to meaningful results.
EL testing by drone can be used to identify cracks, microcracks and other module defects.
EL testing by drone can be surprisingly cost effective with the right planning. Infrared inspections, another option that can be carried out by drone, rely on thermal imaging to identify hot spots or signature patterns related to defective modules.
While efficient, infrared cannot reveal a wide range of problems that don’t generate heat, or that exist well before symptoms appear. Microcracks, for example, can form during transportation or installation. They may remain electrically inactive, producing no abnormal thermal signal until further deterioration occurs.
If only thermal methods are used, early- stage potential-induced degradation – a process by which electrical stress causes leaks of current that reduce power output over time – and subtle installation defects such as cell or glass fractures can escape detection entirely.
Field experience shows these early, invisible threats are surprisingly common. If left unaddressed, they often compound over time, leading to long-term energy loss, warranty disputes, or costly, reactive repairs.
How it works
Unlike contactless infrared inspection, EL testing involves activating module circuits at night and capturing images of their electroluminescent light emissions. This requires connecting an external power supply to the panels to activate their circuits.
Every plant design, string configuration and physical site layout influences the complexity of the process. There is no one-size-fits-all approach, and this is where most misunderstandings begin. Requests for “an EL drone inspection” often overlook key planning realities. Factors include module density, accessibility, terrain, local airspace or night flight restrictions, string wiring, combiner box locations and inverter placement. The mechanical configuration – whether the system uses single-row trackers or densely packed fixed racks – directly impacts measurement strategy and platform selection.
In some installations, drone-based EL imaging is feasible. In others, ground based vehicles or multi-camera tripod systems are the better choice.
“By embracing EL testing, the industry can unlock new levels of risk management”
Planning is everything
Achieving meaningful EL test results starts with a technical assessment of the plant’s electrical system. It requires confirming safe access to strings or combiner boxes, the availability of auxiliary power for nighttime activation, and choosing the right imaging platform to overcome site and system-specific obstacles.
A clear definition of the project scope is needed. Are you sampling selected module groups or aiming for 100% system coverage? Which defect types and severity thresholds are most critical for the site owner? What is the tolerance for longer, higher-resolution campaigns compared with faster, lower-detail overviews? These choices determine logistics, scheduling and technician skill requirements.
Skilled personnel, equipment transport, O&M team coordination, and local permitting all need to be aligned well before night falls and inspection work begins. Overlooking even one piece – such as permit timing or crew certification – can bring the process to a halt, resulting in wasted effort or missed anomalies.
In the field
On one recent project, a team only discovered on arrival that the plant’s electrical layout did not match the provided plans. Before starting the campaign, the team had to update the string plans to make sure they were powering up the right strings to be inspected. Using multiplexers and switches makes such last-minute changes easy, but expecting the unexpected is advisable.
Nighttime inspections in winter can require more than 1,500 V due to the negative temperature coefficient of the voltage. Here, teams may decide to split strings to lower the voltage.
The lesson is clear – successful EL testing is less about the specific tool or platform and more about understanding each site’s technical and logistical requirements.
The industry’s tendency to seek standard “per module” pricing for EL inspections overlooks a fundamental truth that every plant is unique, and so too is the planning required.
AI and automation
Artificial intelligence and advanced image analytics are helping EL testing reach new levels of consistency and insight. Neural networks can now perform automated defect detection, cell-level fault classification, and rapid image segmentation with growing reliability. However, automated systems depend on quality input data, which in turn depends on the steps outlined above. Until training data is broadly available for each module type and configuration, analysis and fault identification remain a partially manual process.
Developments in intelligent planning platforms allow us to harness AI to assess site characteristics, forecast required resources, and propose tailored inspection strategies in advance. While these technologies promise to streamline EL project setup and quotation in the future, the real value today still comes from expert analysis and site-specific adaptation.
Best practice
At its core, EL testing is not simply an advanced “add-on” to routine infrared inspection. It is a fundamentally different diagnostic tool that can reveal vulnerabilities and hidden risks, enabling owners and operators to plan repairs and maintenance before performance is compromised.
Lessons learned from early adopters clearly show that proactive, well-planned EL programs pay for themselves many times over by extending asset life, reducing unscheduled down time and supporting evidence-driven warranty claims. Industry best practice is shifting accordingly from generic requests for “EL drone surveys” to consultative engagement, tailored scope definition and alignment of a company’s technology, logistics and business objectives.
With planning as the foundation, post-installation EL testing can deliver real value as part of a comprehensive PV system health management strategy. By embracing EL testing, the industry can unlock new levels of risk management, performance assurance and long-term value for solar assets.
Jörg Althaus is Global Director of Quality Assurance Services and Regional Manager for Europe, the Middle East, and Africa at Clean Energy Associates. Based in Cologne, Germany, he has more than 25 years’ experience in photovoltaics – including module manufacturing, plant construction, and supply-chain oversight.
Andreas Fladung, Managing Director of Aerial PV Inspection GmbH, has worked in the PV industry since 1988, including project planning, PV system installations, and management. In 2005, he founded Fladung Solartechnik GmbH, which offers consulting, planning, installation, and services for PV systems.
