EL Testing of Solar Panels: How It Finds Hidden Defects (Like an X-Ray)

A solar panel’s glass can look undamaged even when the cells underneath have tiny cracks or faulty electrical connections. These defects can escape a visual inspection and may affect the panel’s performance over time. EL testing solar panels, where EL stands for Electroluminescence, helps identify defects without opening or damaging the panel.

Often compared to an X-ray for photovoltaic (PV) modules, EL testing of solar panels involves supplying electricity to the solar cells so they emit faint near-infrared light. A specialized camera then captures this light. Dark lines or patches in the resulting image indicate cracks or electrically inactive areas that are otherwise invisible to the naked eyes.

In this guide, we’ll walk you through how EL testing solar panels works, what an EL tester for solar panel inspection actually does, which defects it catches, and when it makes sense to use it.

What Is EL Testing of Solar Panels?

EL testing of solar panels is a non-destructive inspection that reveals hidden defects in solar cells. It produces images that help identify micro-cracks, inactive cell areas, and faulty electrical connections that a visual check may miss.

These images show differences in the light emitted by the cells during testing. For example, a crack can interrupt the flow of current to part of a cell. If that area receives little or no current, it emits less light and appears darker than the surrounding areas.

Since other electrical faults can also reduce brightness, experts assess the shape, location, and extent of the dark areas to identify possible causes.

Why Do People Call an EL Test for Solar Panels an X-Ray?

The comparison isn’t just marketing. An X-ray lets a doctor see a fracture that’s invisible from the outside. An EL image does the same for a solar cell. A crack, a broken connection, or a dead patch shows up clearly in the image even when the glass surface looks flawless.

The difference is that EL uses no radiation. It’s simply the panel’s own light, captured by a camera.

The Science Behind EL Testing Solar Panels, in Plain Words

A solar cell normally converts sunlight into electricity. EL testing uses the opposite effect, where electricity is first supplied to the cell, causing it to give off a faint light. This effect is called electroluminescence.

The principle is similar to a light-emitting diode (LED), which glows when electricity flows through it. Inside these devices, electrons can fill gaps left by missing electrons. This releases energy, some of which appears as light.

For silicon solar cells, the light is mainly near-infrared, which is invisible to human eyes. A camera sensitive to near-infrared light captures this glow and produces an image of the cells for inspection.

How Does an EL Test for Solar Panels Work?

The process is simpler than the science sounds. This section walks through the steps, the equipment, and the different ways the test is carried out in factories and in the field.

Here’s a step-by-step walkthrough of the EL testing process

1. Darkness first: The solar module is placed in a dark room, a closed hood, or tested at night outdoors. Any stray light would drown out the faint EL signal.
2. Forward bias: In this method, an external power supply is connected to the module and pushes current through it in the forward direction. A solar cell is essentially a large diode, so this is the reverse of what happens when sunlight is generating power, and the cells respond by giving off a faint glow of near-infrared light. You can’t see this glow, so a specialised camera captures it, and any cracks or inactive areas in the cells show up as dark patches. The International Electrotechnical Commission (IEC), the global body that sets standards for electrical and electronic equipment, covers this method in IEC TS 60904-13, which applies specifically to modules tested with a power supply that places the cells in forward bias.
3. Image capture: An infrared-sensitive camera photographs the glowing module. Exposure and current are adjusted until the picture is clear.
4. Analysis: Software, or a trained technician, studies the image for dark lines, patches, and uneven brightness. Researchers have even built automated methods that count and measure cracks; one NREL (National Renewable Energy Laboratory) study reported detecting over 90% of cracks longer than 3 cm with an optimized filter.
   Source: NREL via OSTI
5. Reporting: Each solar panel defect is logged with its location and severity, so a buyer, manufacturer, or insurer can decide what to do next.

What’s Inside an EL Tester for Solar Panel Inspection?

An EL tester for solar panel work is a combination of a few parts working together. Knowing them helps when you’re reading a quote or an inspection proposal. The table below lists each component and the job it does in the test.

Main components of an EL tester:

Testers come in several forms, from inline units built into a factory production line, to standalone offline benches, to portable kits used on site. Some are paired with other checks such as thermography.

Factory, Field and Drone: Where EL Testing Is Done

The same principle works at very different scales. Here’s how it’s applied in practice.

• In the factory: Manufacturers run EL imaging after cell stringing and again after lamination, so cracks and soldering problems are caught before modules are packed. Inline testers sort modules into pass and fail grades.
• On site: EL testing of solar modules can be done where they are installed. The modules usually don’t need to come off the roof or structure, and testing is often done at night so the plant’s daytime output isn’t disrupted.
• By drone: For large solar plants, drone-mounted EL cameras are used to speed up inspection. A 2025 peer-reviewed review covers indoor, outdoor, daylight, and drone-based EL imaging, along with AI-driven defect detection.
  Source: Micromachines (MDPI), 2025

What Hidden Defects Can EL Testing Find?

EL testing reveals defects invisible to the eye: micro-cracks, broken or dead cells, soldering faults, finger interruptions, shunts, potential-induced degradation (PID), and uneven cell quality, often before output visibly drops.

Each of these leaves its own signature in an EL image, and learning to read those signatures is what makes the test so useful. The table below pairs each signature with what it means for the module, and the sub-sections after it take the most serious defects one at a time.

Common defects and how they appear in EL images:

The table gives you the pattern at a glance, but a pattern only helps if you know what sits behind it. These groups of defects deserve a closer look here, since they’re the most likely to affect a module’s output over time. We’ll start with micro-cracks, the most widespread of them.

1. Micro-Cracks

Micro-cracks are one of the most common solar panel problems in India. They are small fractures in the silicon wafer. They can form during manufacturing, packing, transport, or installation, and sometimes from stress after installation, such as hail. Many don’t affect output on day one. The risk is that thermal cycling and mechanical load can make them spread, which can cut off parts of a cell and, in some cases, activate bypass diodes or create hotspots.

2. Soldering and Interconnect Problems

A solar cell is only useful if current can leave it. Poor solder joints or weak ribbon connections can leave part of a cell electrically isolated. In EL images, these usually show up as dark cells or dark regions near the connection points. They’re hard to spot by eye, but they can cause local heating and lower the output of the whole string over time.

3. Dead Cells, Shunts and PID

This last group covers the following defects that either switch cells off or slowly drain their performance.

• Dead or inactive cells look completely dark and directly reduce the module’s output.
• Shunts are leakage paths that appear as dim or dark spots and can raise hotspot risk.
• Potential-induced degradation (PID) is a gradual loss of performance linked to leakage currents, and it can show up as dark cell patterns in EL images.

How Do You Read an EL Image?

An EL report is only useful if you know what you’re looking at. This section explains what a healthy module looks like and which signs should raise questions.

What a Healthy Module Looks Like

A good module shows even, consistent brightness across its cells. One thing that catches beginners out: monocrystalline modules usually look quite uniform, while multicrystalline (polycrystalline) modules naturally show patchy grain patterns. That patchiness is the crystal structure, not necessarily a defect.

Red Flags to Look For

When you or an inspector review an image, these are the signs worth flagging:

• Dark lines, especially branching or multiple-direction cracks
• Fully dark cells
• Dark patches that look cut off from brighter areas of the same cell
• Dark zones lining up with ribbons or busbars
• Sharp brightness differences between neighboring cells
• The same pattern repeating across many modules in a batch, which points to a manufacturing or handling problem

Severity matters. Not every crack calls for replacement, so a trained engineer should grade the findings.

What Should You Do If EL Testing Finds a Defect?

If EL testing of solar panels reveals a defect, share the report with your installer and request a professional assessment. The next step depends on the defect’s severity and its effect on the panel’s performance or safety.

Work through these steps with your installer:

1. Understand the finding: Ask the testing professional to explain which panel is affected, how serious the defect is, and whether it needs immediate attention. If the panel has not yet been installed, have the issue assessed before installation.
2. Arrange additional checks if needed: The engineer may recommend current-voltage (I-V) testing to check the panel’s electrical output or thermal imaging to look for overheating. These checks help establish how the defect affects the panel.
3. Check warranty coverage: Ask whether the panel warranty or your installation agreement covers the proposed work. Keep the EL report, panel serial number, purchase invoice, and warranty documents ready. Coverage depends on the cause of the defect and the applicable terms.
4. Agree on the recommended action: Your installer should explain whether the panel needs replacement or can remain in use with monitoring. If monitoring is advised, ask what changes to watch for and when the panel should be checked again.
5. Check the result afterward: Following replacement or other corrective work, ask your installer to verify that the identified issue has been addressed. Keep the service report and any follow-up test results with your original EL report.

When Should You Do EL Testing of Solar Modules?

Timing decides how useful the result is. This section lays out the key points in a project where an EL test pays off.

The before-and-after approach is the real value. If you have an EL image from the factory and another from the site, you can tell where the damage happened, and that’s often what decides who is responsible for a claim.

Why Does EL Testing Matter for Rooftop Solar Panels in India?

EL testing of solar panels helps rooftop owners identify hidden damage that could affect electricity generation. It provides a check of the panels supplied for installation and can reveal problems before a noticeable drop in output becomes the first sign of trouble.

Let’s explore what makes it so important.

1. It Checks the Condition of the Panels Supplied for Your Roof

A model may meet Bureau of Indian Standards (BIS) requirements and appear on the Approved List of Models and Manufacturers (ALMM), yet individual panels can still be damaged during transport or installation.

EL testing after delivery or installation helps check the condition of those particular panels. This gives you additional information about the panels being fitted on your roof, beyond their certification and listing.

2. It Helps Identify Defects That Could Reduce Electricity Generation

Some cell cracks have little effect on a panel’s output initially. Temperature changes and physical stress can worsen existing cracks, potentially leaving parts of a cell unable to contribute to electricity generation.

EL testing can reveal these defects while they are still difficult to identify through visual checks or electricity generation records alone. This allows your installer to assess the problem earlier and recommend action to limit avoidable losses.

What are the Limits of EL Testing (What It Can’t Do)?

EL testing solar panels is powerful, but it’s not magic. Being honest about its limits helps you use it well, and this section covers the main ones.

• It’s an image, not a power measurement: It shows where problems are, but you still need I-V testing to measure actual power loss.
• It needs darkness and a power source: That means night work, hoods or enclosures, and disconnecting modules.
• It needs interpretation: Not every dark line is serious, so a trained engineer should grade results.
• It isn’t practical for every small home system: Running a separate EL test on each panel of a typical household rooftop rarely makes sense. The realistic approach is to buy certified modules from a reputable installer and check for handling damage.
• The screening usually happens before the panels reach you: At SolarSquare, we either EL-test modules ourselves or obtain an EL test report from the manufacturer, so only high-quality modules reach our customers. If you’re comparing installers, ask whether they do this and whether they can share the report.
• It adds cost and time: It’s most worthwhile for larger projects, bulk purchases, suspected damage, and warranty disputes.

Planning Rooftop Solar? Start with Quality

EL testing shows why module quality, careful handling, and proper installation matter as much as the sticker on the panel. Whether you’re a homeowner, a housing society, a business, or a farmer, the goal is the same: a system that keeps performing year after year.

At SolarSquare, we help Indian homes, housing societies, and businesses install rooftop solar systems with end-to-end support. We provide EL images of all the solar panels installed on your roof to make sure that our homeowners are informed that there are no defects in the panels and that the panels installed are A-grade. If you’d like to understand your options, talk to our team, and we’ll walk you through the right system for your roof and your needs. Talk to a SolarSquare solar expert.

Frequently Asked Questions

Can EL testing be done on installed solar panels?

Yes. EL testing of solar modules can be carried out on site, often at night, without removing panels from their mounting structures. Modules generally need to be disconnected from the array and powered individually or in small groups, and drone-based systems are used on larger plants.

Is EL testing mandatory for solar panels in India?

No. India’s ALMM and BIS framework focuses on certification and listing of modules rather than on an EL image for every individual panel, as far as MNRE’s published orders go. Requirements can change, so check the latest MNRE notifications. Even where it isn’t required, many buyers, lenders and insurers use EL reports as quality evidence for larger projects.

What is the difference between EL testing and thermography?

Thermography uses an infrared camera on operating modules to find hotspots and heat-related faults. EL testing, on the other hand, powers the module in the dark to reveal internal cell defects, including ones that haven’t started to cause heating yet. The two methods complement each other.

How do you know if a solar panel is defective?

A defective panel usually shows lower output than identical panels, visible damage such as cracks, browning or burn marks, or a hotspot under thermal imaging. Rule out dust and shading first, since they cause most output loss in India. If the drop persists after cleaning, compare against another string or the same season’s earlier data, then confirm with I-V testing for actual power loss and EL testing for hidden cracks and dead cells. Ask your installer for the test report, as defects found early are easier to claim under warranty.

Can EL testing solar panels find snail trails?

Not directly, but it finds what causes them. Snail trails are surface discolouration, while EL shows the micro-cracks underneath that are commonly linked to them. They’re generally believed to form when moisture reacts with the silver paste along those cracks. EL tells you how serious the problem is: dark patches near the trail mean part of the cell is disconnected and the loss is real, while a trail over a tight crack may cost little output.