The Key Points First
Electroluminescence (EL) testing is a kind of X-ray for solar modules. It reveals damage inside the cells that neither a visual inspection nor, reliably, thermography will pick up: micro-cracks caused by transport, installation or hail, broken cells, interrupted contacts and PID degradation.
This is decisive for building a case: anyone who wants to assert a module defect against the installer, manufacturer or insurer needs proof for each module. The EL image provides it – with serial number, position and picture.
Cost: private systems from €400 net (€476 incl. 19% German VAT), large plants and solar parks with drone from €3,000 net. You receive the exact price after a free initial assessment.
How Does Electroluminescence Testing Work?
A solar cell converts light into electricity. EL testing reverses this process: a direct current is fed into the module in the forward direction, in the order of magnitude of its short-circuit current. The cells then emit a faint light in the near infrared that is invisible to the eye. A camera sensitive to this light records it.
- Intact cell areas glow evenly bright.
- Damaged areas – cracks, detached cell fragments, poor contacts – remain dark because no current flows there.
- The result is a greyscale image of each module in which damage patterns can be clearly identified.
Because the signal is very weak, measurements are traditionally taken in darkness (night-time EL). Newer methods also work in daylight by deliberately shifting the modules' operating point and subtracting the ambient light from the images. The methodological basis for EL imaging of PV modules is the technical specification IEC TS 60904-13.
What Does an EL Image Show? Typical Damage Patterns
| Finding | Appearance in the EL image | Typical cause | Significance |
|---|---|---|---|
| Micro-cracks | fine dark lines in the cell | transport, installation (stepping on, bending), snow load, hail | often no power loss at first, but can develop into cell fractures through temperature cycling |
| Cell fracture / inactive areas | sharply defined dark areas | advanced cracks, mechanical overload | direct power loss, risk of hotspots |
| Hail damage | star-shaped or web-like crack patterns, often on many modules at once | hailstorm | proof for the insurer, module by module |
| Contact and soldering defects | dark stripes along the cell interconnectors, interrupted contact fingers | manufacturing defects, ageing | indication of a manufacturer defect (guarantee claim) |
| PID (potential-induced degradation) | dark cells mainly at the module edge, checkerboard pattern | voltage to earth, moisture, unsuitable modules | gradual, sometimes considerable yield loss; partly reversible if detected early |
| Failed substring | one third of the module completely dark | short-circuited bypass diode in the junction box | power loss of around one third of the module |
EL, Thermography or I-V Curve Measurement – Which for What?
The three methods do not replace each other; they complement each other. In a robust expert report they are combined in a targeted way.
| Method | Shows | Does not show | Typical use |
|---|---|---|---|
| Electroluminescence (EL) | micro-cracks, cell fractures, contact defects, PID patterns – even before any power loss is measurable | the power loss in watts, faults outside the cells (connectors, cables) | proof of damage per module, hail, transport, guarantee claim |
| Thermography (IEC TS 62446-3) | hotspots, warm connectors, failed substrings during operation | micro-cracks that do not generate heat | fire risk, quick overview of large areas |
| I-V curve measurement | actual power per string or module | where in the module the damage is located | quantifying yield shortfall, proof of performance under the guarantee |
Rule of thumb from practice: thermography and yield data show where something is wrong. EL shows what has happened inside the module. The I-V curve shows how much power is missing.
EL Testing of Small Systems: Single-Family Homes and Commercial Roofs
For rooftop systems, measurements are usually taken at dusk or at night. The procedure:
- Preparation: review the string plan and module data, define measuring points, clarify safe access to the generator junction box or inverter.
- Isolation and energising: the string is disconnected from the inverter and energised via an adjustable power supply. This happens at night – so no yield is lost.
- Imaging: each module is recorded with the EL camera – from the ground, from the roof or by drone, depending on roof shape and accessibility.
- Assignment: each image is assigned to the module position and, where accessible, the serial number. Without this assignment, an EL image is of little value as evidence.
- Evaluation and report: classification of the findings per module, assessment of the cause and recommendation on how to proceed.
The test is particularly worthwhile right after installation: micro-cracks caused by stepping on modules or improper transport otherwise only show up years later as a yield shortfall – usually after the warranty period has expired.
EL Testing of Large Plants and Solar Parks: Drone Instead of Individual Images
For ground-mounted systems with thousands of modules, individual imaging from the ground is not economical. This is where drone EL comes in:
- Night-time EL by drone: several strings are energised simultaneously and the drone flies along the module rows with the EL camera. Highest image quality, but only possible in darkness.
- Daylight EL by drone: imaging during operation by deliberately shifting the operating point. Less effort on site, but somewhat lower image resolution – generally sufficient for detecting performance-relevant damage.
- Module-level assignment: the images are assigned to each module via the flight path and the layout plan – a prerequisite for replacement, guarantee processing and proof for the insurer.
Full Survey or Sample?
Not every question requires imaging all modules. For a condition assessment or due diligence, a statistically justified sample is often sufficient, supplemented by targeted images of conspicuous areas identified by thermography or monitoring. For hail damage or serial defects, where every affected module has to be proven to the insurer or manufacturer, a full survey of the affected fields makes sense. The scope of testing is agreed in advance.
When Does EL Testing Pay Off?
| Occasion | Purpose of the test |
|---|---|
| Acceptance of a new system | document transport and installation damage before the warranty period starts to run |
| After hail, storm or snow load | prove the damage per module to the insurer |
| Before the warranty or manufacturer's guarantee expires | assert hidden defects in good time |
| Unexplained yield shortfall | find the cause in the module when inverter and cabling show no abnormalities |
| Purchase, financing, due diligence | know the actual module condition before the purchase price or credit decision |
| Dismantling, relocation, repowering | decide which modules can be reused |
What Does EL Testing Cost?
| System type | Method | Price |
|---|---|---|
| Private system (single-family home, small rooftop system) | night-time EL from the ground, from the roof or by drone | from €400 net (€476 incl. 19% German VAT) |
| Large plant, commercial roof, solar park | drone EL (night-time or daylight), full survey or sample | from €3,000 net |
The final price depends on the number of modules, accessibility, travel and the desired scope of documentation. If the result has to stand up in court or with an insurer, the EL test is integrated into a damage or yield assessment. An overview of expert report costs can be found on the page Cost of a PV expert report.
Cost and Value: When Does EL Testing Pay for Itself?
EL testing itself does not generate any yield. Its value lies in making claims provable and preventing costly wrong decisions:
- Warranty and guarantee: a module with a cell fracture that keeps running without proof loses yield for years. With EL evidence, replacement at the expense of the party responsible can be demanded – in good time, before deadlines expire.
- Insurance: after hail, the insurer only settles proven damage. Without an image per module, the outcome is often a flat-rate partial compensation.
- Cost recovery: if defects are found, the testing costs may under certain circumstances be claimed from the party responsible as part of the damage.
- Purchase and repowering: the module condition determines the purchase price, remaining service life and whether modules are reused or replaced.
Rule of thumb: the larger the potential claim and the closer a deadline, the more likely the test is to pay off. For an unremarkable system with no damage event and plausible yields, on the other hand, EL testing is usually not necessary – a visual inspection, monitoring and, if needed, thermography are then sufficient.
Limits of Electroluminescence
- Snapshot: an EL image shows the condition on the day of measurement. Whether a micro-crack continues to spread can only be proven by comparison with a later image.
- A crack does not equal power loss: not every micro-crack is already costing yield today. Assessing which finding is relevant requires experience and, where appropriate, an additional I-V curve measurement.
- Only the inside of the module: EL does not reveal defective connectors, cables or inverters – thermography and electrical measurements are needed for that.
- Intervention in the system: for night-time EL, strings have to be isolated and energised. This may only be carried out by qualified personnel in compliance with the safety rules.
Frequently Asked Questions on EL Testing
What does electroluminescence testing cost?
For private systems from €400 net (€476 incl. 19% German VAT), for large plants and solar parks with drone from €3,000 net. The exact price depends on the number of modules, accessibility, travel and the scope of documentation.
Is EL testing by drone as meaningful as testing from the ground?
For detecting performance-relevant damage such as cell fractures, inactive areas and PID, generally yes. For assessing very fine micro-cracks on individual modules, an additional close-range image can be useful.
Does the system have to be shut down for the test?
For night-time EL, the strings are disconnected from the inverter and energised via a power supply. As the measurement takes place at night, there is no loss of yield. Daylight EL works while the system is in operation.
Does EL testing detect hail damage?
Yes. Hail produces typical star-shaped or web-like crack patterns that are clearly recognisable in the EL image – even when the glass appears undamaged from the outside. This makes it possible to prove the damage per module to the insurer.
When is the best time for EL testing?
At the acceptance of a new system, immediately after a damage event such as hail, and before the warranty or manufacturer's guarantee expires. In the case of an unexplained yield shortfall, when inverter and cabling show no abnormalities.
Isn't thermography sufficient?
Not for proving micro-cracks. Thermography only shows damage once it generates heat. EL shows cracks and cell fractures before that. For a complete picture, both methods are combined.
Related Topics
- Identifying and documenting damage to photovoltaic systems
- PV acceptance inspection by an independent expert – checklist and costs
- PV damage assessment for insurers and courts
- Yield assessment: proving a yield shortfall and finding the cause
- Warranty: securing defects before the deadline expires
- Technical due diligence and tendering for solar parks
Request EL Testing
Send us the key details – system size, module type, location and occasion (acceptance, hail, yield shortfall, purchase) – and you will receive a free initial assessment of the appropriate scope of testing and the costs from Dipl.-Ing. Andreas Benz, independent PV expert in Germany.
Email: info@gutachterpv.org