6 Ways to Identify Solar Panels with Low Power Output

When a solar power plant continues to experience low generation, it is necessary to check not only the plant-wide weather conditions and equipment outages but also the possibility that output is reduced in some panels or strings within the plant. If low-output conditions are left unaddressed, they can not only cause continued generation losses but also lead to output imbalances between strings, localized heating, and rework during inspections.

However, it is dangerous to conclude that a panel is producing low power based solely on visual inspection. There are multiple possible causes—soiling, shading, wiring, connectors, junction boxes, power conditioners, measurement equipment, communication failures, and so on. The important thing is to narrow the scope step by step from the overall generation trend, safely verify on site, record the measurement results, and make a judgment.

Table of Contents

• What to check before identifying low-output panels

• Method 1: Compare power generation data to narrow down abnormal ranges

• Method 2: Check voltage and current at the string level

• Method 3: Visually inspect for shading, dirt, and damage

• Method 4: Use infrared inspection to find overheated panels

• Method 5: Use electrical measurements to isolate causes of low output

• Method 6: Accumulate records to identify panels prone to recurring issues

• Precautions when identifying low-output panels

• Summary: Narrow down causes of low power generation step by step

What to check before identifying panels with low power output

When investigating panels with low output, it's important not to assume from the outset that the panel itself is faulty. Solar power output is influenced by many factors: solar irradiance, temperature, orientation, tilt, shading, soiling, wiring conditions, and measurement conditions. Even within the same solar power plant, output can vary depending on the installation location and the surrounding environment.

For example, rows that are shaded only in the morning, areas that receive shadows from trees or buildings in the evening, lower edges where rain streaks or sand and dust tend to accumulate, and locations prone to bird damage can make power generation appear low even if the panels themselves are not malfunctioning. Also, if the power generation data are sampled at coarse intervals or communications are interrupted, the low readings may be due to gaps in the records rather than an actual drop in generation.

Therefore, the basic order of investigation is to progressively narrow down from the entire power plant, to the power conditioner unit, to the junction box unit, to the string unit, and finally to the panel unit. Inspecting all panels at once takes time and increases the workload. First narrow down the abnormal range using generation data, and by performing on-site checks and measurements within that range, you can efficiently get closer to the cause.

Safety checks are essential during on-site work. Solar panels continue to generate electricity as long as they are exposed to sunlight, and even after shutdown operations there may still be voltage on the DC side. Do not touch wet areas, damaged cables, loose terminals, or parts with burn marks, and make sure to establish arrangements for qualified personnel or specialist contractors to carry out inspections as needed.

Identifying low-output panels is not simply about finding defective panels. It is the work of combining data, on-site conditions, and electrical measurements to determine where losses are occurring. Below, I explain six practical investigation methods that are easy to use in the field, presented in order.

Investigation Method 1: Narrow Down Abnormal Areas by Comparing Power Generation Data

The first thing you should do is compare the generation data. When you feel the power output is low, instead of immediately inspecting panels one by one on site, first check the recorded generation and current values to narrow down which range is showing abnormalities.

The basic principle of comparison is to compare equipment under the same conditions. Within the same power plant, if there are sections with the same orientation, the same tilt, the same capacity, and similar installation conditions, compare their power generation trends. By checking whether only specific power conditioners are underperforming, only some connection boxes are underperforming, or only particular strings are underperforming, you can significantly narrow the scope of the investigation.

When comparing power generation, looking only at a single day's numbers can lead to incorrect conclusions. On cloudy days or days with rapidly changing weather, output can vary greatly due to short-term shading and passing clouds. Therefore, examining the generation curve on clear days, trends over multiple days, and comparisons for the same time of day makes it easier to determine whether an anomaly is present. In particular, if output remains consistently low over a specific range during periods when it is sunny and solar irradiance is relatively stable, an on-site inspection should be given high priority.

When reviewing generation data, I check not only power output but also current, voltage, shutdown history, fault history, and communication status. Even if the generation appears low, it may simply be missing data due to communication loss. Also, if output is being limited by control or shutdown on the power conditioner side, inspecting the panels alone will not reveal the cause.

At power plants with string monitoring, differences in current values between strings provide important clues. When strings with the same configuration show low current only on a specific string, focus inspections on the panels, connectors, cables, shading, soiling, and other components within that string. On the other hand, if multiple strings are low at the same time, suspect common combiner boxes, collection paths, the surrounding environment, or the extent of shading.

In data comparisons, it is important to carefully examine the boundary between normal and abnormal. In solar power generation, not all panels will produce exactly the same output. Some variation naturally occurs due to installation position, temperature differences, and differences in how dirty they are. Therefore, rather than concluding a fault just because output is slightly lower, check whether the noticeably lower condition persists compared with surrounding panels, and whether the same trend appears regardless of time of day or weather.

The first step in identifying low-output panels is not to wander around the site, but to use data to narrow down the area where abnormalities are likely. The more carefully this stage is carried out, the shorter the subsequent on-site inspection will be, and the easier it will be to avoid unnecessary panel replacements and incorrect repairs.

Method 2: Check the voltage and current at the string level

Once you've narrowed down the abnormal range using generation data, the next thing to check is the voltage and current at the string level. In solar power plants, multiple panels are often connected in series to form a string. Therefore, a fault in a single panel, partial shading, or a poor connection can affect the output of the entire string.

When checking at the string level, first compare strings with the same configuration. If they have the same number of modules, the same orientation, the same tilt, and the same irradiance conditions, their current and voltage generally show similar patterns. If only a particular string has a lower current, there may be shading, soiling, open circuits, poor contacts, panel degradation, or operation of bypass circuits within that string.

If the current is low, possible causes include insufficient solar irradiance, dirt on the panel surface, partial shading, or problems inside the panel or at the connections. On the other hand, if the voltage is significantly different, you need to check factors such as a difference in the number of connected panels, an open-circuit condition, wiring errors, poor contacts, a broken panel connection, or the condition of protective devices. Which areas you should inspect depends on whether the abnormality is in the current or the voltage.

However, measured values change depending on the time of day. At dawn, dusk, or during cloudy conditions, solar irradiance is unstable and may be unsuitable for comparison. If possible, take measurements during times when solar irradiance is relatively stable and compare multiple strings at the same time. If you measure sequentially while irradiance is changing, the values measured first and later may not be directly comparable.

Before taking measurements, it is also important to verify the circuit configuration. If the string numbers on the drawings do not match the labels on site, you may trace a fault to the wrong location. At sites where wiring has been changed by past renovations or replacements, the drawings may differ from the actual setup. During inspections, proceed while cross-checking the numbers inside junction boxes, cable labels, on-site row numbers, and panel layout to make later record-keeping easier.

Even if an anomaly is found in string-level measurements, it is premature to conclude that a specific panel has failed at that point. Consider it as an indication that the cause is more likely somewhere within the string, and next proceed to visual inspection, infrared checks, and detailed measurements as necessary. Conversely, even when anomalies are hard to detect at the string level, an individual panel may still exhibit localized heating or soiling.

String measurements are an effective way to narrow down and find low-output panels. This is because they allow you to confirm abnormalities at the local circuit level that are hard to see from plant-wide data alone. To efficiently identify the cause of low power generation, it is practical to compare the current and voltage of each string, pinpoint the circuits with anomalies, and then proceed to panel-level inspections.

Method 3: Check for shadows, dirt, and damage by visual inspection

When an area suspected of abnormality at the string level becomes apparent, conduct an on-site visual inspection. Visual inspection is a basic task but is important for identifying panels with low power output, because the cause of reduced output is not necessarily an electrical fault.

The first thing to check is shading. Even partial shading of solar panels can reduce their output. Causes of shading include trees, weeds, buildings, utility poles, fences, mounting racks, adjacent rows of panels, anti-bird devices, and debris buildup. Because the position of shadows changes with the time of day, shadows may occur in the morning or evening or vary by season even if none are visible during inspection. If generation data shows reduced output only during specific times, conducting an on-site check at those times makes it easier to identify the cause.

Next, check for soiling. Soiling on the panel surface can include sand and dust, pollen, bird droppings, fallen leaves, mud splashes, mineral deposits, and rain streaks. If the soiling is uniformly and thinly distributed, it may appear as a reduction in output across the entire plant, but if the soiling is localized, only specific panels may exhibit low output. In particular, under installation conditions where dirt tends to accumulate at the lower edge of the panels, part of the cells can remain continuously covered, which can cause reduced power generation and heating.

We also check for damage and external abnormalities. We look for cracked glass, deformed frames, surface scratches, internal discoloration, bubble-like appearances, burn marks, abnormalities around the terminal box, damage to cable sheathing, and disconnected or loose connectors. If any abnormalities are visible on inspection, we check not only that panel but also the surrounding wiring and how it is secured. Damage can also occur later due to typhoons, snowfall, flying debris, or grass-cutting work.

During visual inspections, it is important to check not only the panel surface but also the rear side and the surrounding environment. Even if the surface looks clean, cables on the back may be sagging, contacting the mounting structure, or connectors may be located where water tends to accumulate. There are cases where cables are damaged after grass cutting, and cases where the sheathing is damaged by small animals. It is not uncommon for low power generation to be caused not by the panels themselves but by the connections or wiring.

Also, always record the results of visual inspections. By documenting, with photographs, which column and which-numbered panel had what kind of soiling, shading, or damage, you can later cross-check them against power generation data and measurements. If the records are vague, you won’t be able to recheck the same locations during the next inspection, making it difficult to trace the cause.

Visual inspections may look simple, but in practice they are a process where performance can easily differ. When looking for panels with low power output, rather than just staring at them, it is important to check, in order, shadows, soiling, damage, wiring, and the surrounding environment for the areas narrowed down by data. If an obvious cause is found visually, cleaning, weeding, removing the source of shading, wiring repairs, and similar actions can often lead to relatively quick countermeasures.

Method 4: Use infrared inspection to locate panels that are generating heat

Infrared temperature checks are also effective for identifying panels with low power output. When a solar panel is faulty, areas that are hotter than their surroundings may appear. By locating localized hot spots, you can narrow down the panels or cells that may be causing the low output.

Representative anomalies detected by infrared inspection include cell-level heating, heating of part of a panel, heating around the junction box, and heating at connector and cable connection points. Behind the heating are possible causes such as localized loading due to dirt or shading, cell damage, faults in internal connections, and increased contact resistance. However, because a high temperature does not necessarily indicate a failure, infrared inspection should be evaluated together with other inspection results.

When performing infrared inspections, conditions are important. During periods of weak solar radiation or cloudy weather, temperature differences may not appear clearly and abnormalities can be overlooked. Conversely, when the wind is strong the panel surface can be cooled, reducing temperature differences. Also, due to shooting angle and reflections, something may appear hotter than it actually is or heat from another object may be reflected into the image. Therefore, it is important not to judge based solely on infrared images, but to document them together with visible-light images and on-site location information.

Infrared inspection is particularly effective at large power plants. Measuring every panel electrically and individually takes time, but if infrared inspection first identifies hotspots, you can narrow down the locations that require detailed measurement. If a heated panel is within a string suspected of low output based on the data, that panel becomes a priority candidate for inspection.

However, there are also precautions for infrared inspection. If bird droppings or fallen leaves adhere to part of a panel, that area can appear to be heated. In such cases, the cause is dirt, not panel failure. Also, immediately after a shadow has been cast or solar irradiation conditions have changed, temporary temperature differences can occur. Before judging a condition as abnormal, it is necessary to check the surface condition, the presence of shadows, the time of measurement, and the weather together.

If a hot spot is found, it is also important not to touch it or unplug any connectors immediately. The heated area may have poor contact or an electrical fault, and careless handling is dangerous. First, record the location, confirm the corresponding string or panel number, and then follow safe procedures to perform electrical measurements and inspections.

Infrared inspection is a powerful method for efficiently locating candidate underperforming panels. In particular, it helps detect localized anomalies that are difficult to discern from appearance alone. However, infrared images do not determine the cause; they are clues that indicate locations that may be abnormal. Using them in combination with power generation data, visual inspection, and string measurements improves the accuracy of the assessment.

Investigation 5: Isolate the Causes of Low Output Using Electrical Measurements

When potential anomalies have been narrowed down using power generation data, string measurements, visual inspections, and infrared checks, perform detailed electrical measurements as needed. Electrical measurements are important for identifying panels with low power output, but because the work involves hazards, they need to be carried out only after understanding the system configuration and safety procedures.

In electrical measurements, check the open-circuit voltage, short-circuit current, operating voltage and current, insulation condition, and connection condition. By comparing these values with panels or strings under the same conditions, you can narrow down the locations of anomalies. For example, if only a specific panel or string shows a lower current under the same irradiance conditions, issues such as a problem with the light-receiving surface, internal degradation, or poor connections are suspected. If the voltage differs significantly from the expected value, check for broken wiring, poor connections, an incorrect number of modules, or terminal problems.

When inspecting an individual panel, removal or disconnection may be required. Because this work carries a risk of electric shock and arcing, do not open or close DC circuits that are generating power casually. It is important to follow site procedures, equipment specifications, and shutdown procedures, and to have specialized personnel handle the work as necessary. In particular, damaged panels or connections with burn marks require more cautious handling than usual.

In electrical measurements, it's important not to view a measured value in isolation. Because a solar panel's output is affected by solar irradiance and temperature, the measured value will change if the conditions at the moment of measurement differ. When comparing, verify under the same time, the same irradiance conditions, and the same measurement method as much as possible. Also record the weather at the time of measurement, the stability of the irradiance, the panel temperature, and the measurement location to make later evaluation easier.

Low output may sometimes be caused not by the panel itself but by connectors, cables, or junction boxes. Increased resistance at connection points can cause heating and voltage drops, which may lead to reduced output of the entire string. Do not conclude that the panel is defective based solely on measured values; it is necessary to inspect the terminals, connection points, protective devices, and cable routing.

In electrical measurements, having a normal reference is also important. Comparing within the same model, the same installation conditions, and the same string configuration makes it easier to detect abnormal values. If past measurement records are available, comparing them with current values allows you to determine whether the decline is gradual or sudden. If the decline is sudden, prioritize suspecting soiling, shading, poor contacts, or damage; if the decline is long-term, consider degradation and environmental factors.

Detailed measurements can provide decisive information for identifying panels with low power output. However, the measurements themselves are not the objective. Based on power generation data and on-site conditions, it is important to decide which measurements to perform and to isolate causes from the measurement results. By limiting measurements to the necessary scope while ensuring safety, you can efficiently narrow down the causes of low output.

Investigation Method 6: Determine panels prone to recurrence by compiling records

To accurately identify panels with low power generation, it is important to judge based on accumulated records rather than a single inspection result. Because the output of solar power generation is affected by weather and seasons, a panel that appears to have low output on one day may look normal on another. Conversely, even abnormalities that seem minor require attention if they occur repeatedly in the same location.

Items to be recorded are: power generation data, inspection date, weather, time period checked, equipment numbers with abnormalities, panel positions, visual inspection results, measured values, infrared inspection results, measures implemented, and subsequent power generation status. By recording these in a single continuous flow, the relationship between causes and countermeasures becomes easier to see. For example, if output recovered after cleaning, it can be assumed that soiling had a significant impact. If improvement occurred after connector repair, it is more likely that the problem was at the connection.

Finding panels or areas that are prone to recurring problems is also important. If power generation consistently falls in the same season each year, shadows from changes in solar altitude, fallen leaves, snow accumulation, pollen, yellow sand, or the growth of weeds may be involved. If abnormalities occur at specific locations after rain, you should check for water pooling, the waterproofing of terminals, and cable routing. If abnormalities increase during windy periods, also inspect cable movement, the condition of fixings, and the load or stress on connection points.

By keeping records over time, it becomes easier to determine whether panel replacement is truly necessary. If low power generation is caused by dirt or shading, replacing the panels will not address the root cause. Conversely, if cleaning and shading countermeasures do not improve performance and measurement values or temperature anomalies persist, you should more strongly suspect defects in the panel itself or its connections. Keeping the evidence behind your decision on record makes it easier to use for internal explanations and maintenance planning.

It is also important to link the panel layout within the plant to the records. If column numbers, row numbers, orientation, string numbers, and combiner box numbers are not organized, it will take time to reach the relevant location on site even when an anomaly is found. If inspection records match the on‑site labeling, the efficiency of subsequent inspections and repair work will be greatly improved.

When identifying low-output panels, it is essential not only to find momentary anomalies but also to observe continuous trends. By accumulating records, it becomes easier to determine whether an issue is a one-off soiling, a seasonal shadow, or a malfunction on the equipment side. If you identify locations that are prone to recurrence, you can reflect that in inspection schedules, weed-control plans, and cleaning plans, leading to management that prevents declines in power generation.

Precautions When Identifying Low-Output Panels

When identifying panels with low power output, you should avoid relying on a single piece of information. Drawing conclusions from only generation data, only a visual inspection, only infrared images, or only measurements can lead to misdiagnosing the cause. Causes of reduced output are often complex, and dirt and shading, poor connections and overheating, or communication failures and actual output drops can appear to overlap.

One thing to be especially careful about is immediately assuming that low power generation is caused by panel failure. In practice, the cause is sometimes not the panel itself but factors such as shading, dirt, weeds, cables, connectors, junction boxes, measurement errors, and communication failures. Even if you replace the panels, if the root cause is shading or dirt, the power generation at the same location will be low again.

Also, attention must be paid to the conditions during inspections. Under cloudy or rainy weather, in the morning or evening, or during periods when solar irradiance is changing rapidly, it becomes difficult to compare measurements. To determine whether power generation is low, you should compare systems under as similar conditions as possible and record the environmental conditions at the time of measurement. If you compare only the measurement values while ignoring solar irradiance conditions, you may mistakenly judge a normal panel to be faulty.

From a safety standpoint, handling DC circuits requires caution. Because solar panels generate power when exposed to light, some voltage may remain even after equipment has been shut down. Plugging and unplugging connectors, checking cables, working inside junction boxes, and measuring individual panels must be carried out with an understanding of the equipment’s configuration and procedures. If there is abnormal heating or scorch marks, consider it more hazardous than usual, and it is important to refrain from touching it.

Also, pay attention to how you record information. If you only keep photos of abnormal areas but cannot tell which string or which panel they belong to, the data will not be useful later. By recording the panel location, shooting direction, date and time, weather, measurements, and work performed together, you create records that can be used for future decisions. Relying on memory at the site makes it easy to get things mixed up after inspecting multiple locations.

Identifying low-output panels is the entry point to restoring power generation. If you don't correctly isolate the cause, you may end up replacing areas that only need cleaning or overlooking wiring that should be checked. To proceed efficiently, it is important to follow a flow of narrowing down with data, confirming on-site, backing findings with measurements, and making decisions based on records.

Summary: Narrow down the causes of low power generation step by step

To identify panels with low power output, it is important to narrow down the cause step by step rather than looking only at plant-wide figures. First, compare generation data to find the range where anomalies are occurring. Next, check voltage and current at the string level to determine which circuit is likely problematic. Then perform visual inspections to check for shading, soiling, damage, and wiring condition, and, if necessary, carry out infrared inspections and electrical measurements.

The causes of low power generation are not limited to faults in the panels themselves. Shadows, dirt, weeds, bird droppings, cable damage, poor connector contact, abnormalities in the junction box, communication failures, and measurement errors are among the various possible factors. That's why it's important not to judge based on a single result but to combine multiple pieces of information.

A decrease in power generation is not over once it has been detected. After cleaning or repairs, you need to confirm whether power generation has recovered and whether the same location has not experienced a recurrence. By accumulating records, you can identify locations prone to recurrence and seasonal trends, improving the accuracy of inspection and maintenance plans.

For field personnel, the important thing is not to waste the limited time available wondering where to start checking. If you suspect low power output, first narrow down the area using data, check the site for shading and soiling, and establish a workflow to take detailed measurements only where necessary; this makes it easier to identify the cause.

If you want to streamline identifying panels with low power output, organizing on-site survey results, and understanding declining trends, it is effective to establish a management system that continuously monitors the power plant’s condition and connects to inspection and maintenance decision-making. By linking daily generation data with on-site records and managing them together, it becomes easier to detect drops in generation early and to work on preventing recurrence.