Six items to review for panel dirt and shading that cause reduced power generation

When you feel the power output of a solar power system is low, it is premature to immediately assume equipment failure. Power output fluctuates due to many factors such as weather, season, solar irradiance, temperature, equipment condition, and the surrounding environment. Among these, two factors that are often overlooked on site yet easy to check during routine inspections are dirt on the surface of the solar panels and the effects of shading.

Soiling and shading on panels may seem to cause only small differences over short periods, but when daily power generation accumulates, they can lead to a decline that cannot be ignored. In particular, if generation is lower than in the same month of the previous year, or if only certain sections within the same site are showing reduced generation, it is important to carefully review the panel surfaces and the surrounding environment.

Table of Contents

• Approach to Confirming Declines in Power Generation from Dirt and Shading

• Review 1: Check the panel surface for dirt at each location

• Review 2: Check for localized deposits such as bird droppings and fallen leaves

• Review 3: Check for shadows on the lower edge caused by weeds and vegetation

• Review 4 Check the shadows of fixed objects such as buildings and utility poles by time of day

• Review 5 Check how solar altitude and shadow length change with the seasons

• Review 6: Confirm changes in power generation after cleaning or weeding by checking records

• Continuous monitoring and visualization are essential for quickly detecting declines in power generation.

Approach to verifying declines in power generation from soiling and shading

When power generation is low, you naturally want to suspect a major failure or equipment anomaly first. However, in practice the cause is not necessarily a single one. For example, there may have been consecutive days with lower solar irradiance than the previous year, higher ambient temperatures that made panel temperatures more likely to rise, periods when the power conditioner was offline, or differences in the conditions under which measurement data were acquired — multiple factors can overlap and make the power generation appear low.

Among these, panel soiling and shading are factors that are easy to detect by on-site inspection. Even before using measurement instruments or conducting detailed analyses, you can narrow down the presence of anomalies to some extent through visual inspection, photographs, inspection records, and comparison of generation data. In particular, if low generation is observed in only part of the installation—such as a string, an array, or a section—rather than across the entire system, localized soiling or shading may be involved.

Solar panels' power generation is affected by the condition of the surface that receives sunlight. If the surface has deposits such as sand and dust, mud, pollen, bird droppings, fallen leaves, sap, or moss-like growths, it becomes harder for light to reach the panels. Also, if part of a panel is shaded, it can affect not only that portion but the output of the entire connected circuit. The effect of shading varies with system configuration and circuit design, so it is important not to assume that localized shading is harmless.

However, just because dirt or shadows are found, it is risky to conclude that they alone are the cause of reduced power generation. You should confirm whether the period when the dirt was present coincides with the period of reduced generation, whether the shadows actually occur during power-generating hours, and whether the generation trend changed after cleaning or weeding. Rather than looking only at the result that generation is low, it is important to connect on-site conditions with the data when making a judgment.

Also, during inspections, prioritize safety. On roofs, at heights, on slopes, in mud, or in areas with tall grass, avoid undue close approach and working alone. Actions such as stepping on panel surfaces, carelessly touching wet equipment, or getting too close to energized electrical equipment are dangerous. If cleaning or internal inspection of equipment is required, it is a prerequisite to follow site rules and the judgment of the specialist in charge.

When checking for a decrease in power generation, rather than proceeding straight to complex analyses, it's easier to organize the causes by reviewing, in order, solar irradiance, operational status, soiling, shading, the surrounding environment, and work history. In this article, from the perspective of field personnel who can easily check on site, we explain six items focused on panel soiling and shading.

Review 1: Check panel surface contamination by location

When power output is low, the first thing to check is dirt on the panel surface. Because solar panels are installed outdoors, sand dust, yellow sand, pollen, soil dust, exhaust-derived grime, rain streaks, and mud splashes can adhere to the surface. Ordinary rain can wash some of this away, but in locations with a shallow tilt, where wind easily causes dust to accumulate, or where there is unpaved ground or farmland nearby, dirt tends to remain.

When inspecting, it's important not to view the entire facility as a single surface but to look for differences by location. Even within the same power plant, soiling varies in rows near roads, near slopes, close to drainage ditches, in sections near trees, or in locations that are often downwind. If you notice differences such as only certain rows appearing whitish or cloudy, mud remaining only on the lower edges, conspicuous streaks where rainwater has flowed, or deposits accumulated near the frames, compare those areas with their power generation output.

Dirt on the panel surface can be noticeable not only around noon on clear days, but also when viewed at an oblique angle. Even light dirt that is hard to see head-on may reveal surface haze or streak-like deposits when viewed from the side or at a low angle. When taking photos, regularly photographing the same section from the same direction makes it easier to compare the progression of soiling and changes after cleaning.

However, a slightly dirty surface does not necessarily cause a large drop in power generation. The impact varies depending on the degree of soiling, the area covered, solar irradiance conditions, panel tilt, the frequency of rainfall, and the system configuration. Therefore, do not judge based on visual inspection alone; you need to view it together with power generation data. Checking whether there is a difference in power output between a visibly soiled section and a relatively clean section on days with similar solar irradiance can help narrow down the cause.

When inspecting for dirt, carefully assess whether cleaning is necessary. If you assume simply that using water or scrubbing is sufficient, you may damage the panel surface, frame, or areas around wiring. Avoid actions such as scrubbing with hard tools, applying strong pressure, suddenly applying water when there is a large temperature difference, or casually using detergents. If cleaning is necessary, it is important to carry it out in accordance with equipment specifications, installation conditions, and safety procedures.

In practice, it is important not to end the inspection of soiling with only “soiled” or “no problem.” Record which section, the exact position, what kind of soiling was observed, and to what extent. If you retain photographs, the date and time they were taken, the weather, the inspector, and the section inspected, these records will be easy to use later for comparing power generation or deciding on cleaning. When you receive inquiries about low power output, having soiling records also makes it easier to explain.

Review 2 Check for localized deposits such as bird droppings and fallen leaves

Among the types of soiling on panel surfaces, the conspicuous yet often underestimated ones are localized deposits such as bird droppings, fallen leaves, branches, insect nests, pieces of plastic, and clumps of dirt. Unlike cases where the entire surface is thinly soiled, these distinctly cover only part of the panel and can create conditions similar to localized shading.

Bird droppings tend to occur when there are power lines, trees, buildings, or structures where birds like to perch around a power plant. If they are concentrated around specific rows or racks, check their relationship to bird flight paths and perching sites. Fallen leaves occur not only in autumn but also after strong winds or pruning, and the timing varies depending on the types of surrounding trees. When moisture-laden fallen leaves remain at the lower edges of panels or near frames, dirt and dampness can persist for a long time.

When inspecting localized soiling, it's important to check not just on a per-panel basis but also at the string and row levels. Even if it appears to be present only on part of a single panel, it can still affect the output of the same circuit. Because how the impact manifests depends on the system configuration, you cannot simply assume that a visually small area corresponds to a small effect on power generation.

Also, it is important to confirm when the deposits appeared. If there was a day when power output suddenly dropped, and shortly before that there had been strong winds, heavy rain, bird damage, nearby construction, grass cutting, or sediment inflow, localized deposits may be involved. Conversely, if the deposits have been present for quite some time and do not coincide with the period of reduced power output, other causes should also be suspected.

During inspections, take photographs as soon as any deposits are found, and, if possible, record the condition both before and after cleaning. If you can confirm how much power generation recovered after cleaning, that will serve as a reference for future decisions. However, it is safer to avoid drawing conclusions from just a single day immediately after cleaning. Because power generation varies with weather and solar irradiance, it is easier to judge by comparing days with similar irradiance conditions or by making relative comparisons with nearby sections.

If leaves and twigs repeatedly accumulate, you need more than a one-time cleaning—you also need to manage the surrounding environment. If tree branches overhang the panels, the terrain causes leaves to gather in strong winds, or debris tends to build up in drainage channels, the same problems can keep occurring. To prevent a drop in power generation, it's important not only to remove the deposits but also to understand the conditions that make them likely to occur.

Checking for localized deposits is a quick inspection to perform, but its practical value depends greatly on how it’s recorded. Recording which panels had what attached, the extent of the affected area, when it was removed, and whether generation changed afterward turns a simple field note into information that can be used to analyze causes of reduced power output.

Review 3: Check for shadows on the lower edge caused by weeds and plantings

In ground-mounted solar power systems, shadows from weeds or vegetation can cause a reduction in power generation. In particular, when grass grows up to the lower edge of the panels and casts shadows, it can be hard to notice from a distance and may be overlooked during inspections. Even when the grass appears only slightly tall, shadows can stretch long and fall onto the panel surface during early morning and late afternoon when the sun is low.

When checking for shadows cast by weeds, it's important not to judge based only on the time of inspection. Even if there are no shadows around midday, shadows can appear in the early morning or evening. If you can identify times when power generation is low, inspecting the site during those times makes it easier to find the cause. For example, if a particular row shows low output only in the morning, shadows from grass or structures on the east side may be responsible.

Weed growth varies significantly with the seasons. Areas that grow rapidly from spring into summer, locations that quickly proliferate after rain, and spots where cuttings are repeatedly missed are management points that tend to cause reduced power generation. Rather than weeding the entire site at the same frequency, it is more efficient to prioritize areas prone to shading — the front of panels, the south side, the east and west edges, around drains, and along fence lines.

It is also necessary to check plantings and surrounding trees. Even trees that weren’t a problem at the time of installation can grow branches and cast shadows after a few years. Trees on neighboring properties and plantings near property boundaries require attention regarding the scope of management and how to respond. Because you may not be able to cut or prune them without permission, it is practical to document the shadow conditions with photos and timestamps and keep them in a form that can be used to coordinate with the relevant parties.

Shadows cast by weeds and other plantings affect not only power output but also the ease of maintenance inspections. When vegetation grows tall, it becomes harder to inspect the wiring, racking, foundations, and drainage under the panels. It can become a habitat for pests and small animals and increase footing risks for inspectors. Checking the state of weed control is important not only to identify causes of reduced power output but also from the perspective of overall equipment management.

After weeding, in addition to recording that the work is complete, check for changes in power output. By checking whether the power output during the time periods that were shaded before weeding has improved, or whether the differences within the same section have decreased, it becomes easier to estimate the impact of shading. Because power output is affected by weather, simply comparing the day before and the day after weeding can be insufficient. Using days with similar solar irradiance conditions or comparisons with adjacent sections yields a more stable assessment.

Weed control is more effective when planned before plants grow than when rushed after power generation has declined. Include locations where shadows appeared in the past, areas prone to missed cuts, and slopes or narrow passages that are difficult to work on in the next weed-control plan. To avoid repeated drops in power output, it is essential to carry the shadow information discovered on site through to the next maintenance work.

Review 4 Check the shadows of fixed objects such as buildings and utility poles by time of day

Fixed objects that cast shadows—such as buildings, utility poles, power lines, fences, signs, adjacent equipment, warehouses, trees, lightning protection equipment, and monitoring poles—may be present around solar power generation systems. Even if these were accounted for during design, additions to equipment or changes in the surrounding environment can later alter shading conditions. When power generation is low, not only the panels themselves but also the surrounding fixed objects should be reassessed.

Shadows from fixed objects change position depending on the time of day. If you conclude there is no problem because no shadow is visible during an inspection, you may overlook shadows that appear in the morning and evening. In particular, shadows cast by utility poles, support posts, and fences, which create long, narrow shadows, move across the panel surface depending on the time of day. Even a shadow that looks thin at first glance can be worth checking for its impact on power generation if it falls across multiple panels or recurs at the same time every day.

The basic principle for checking is to match periods of low power generation with the times when shadows occur. If you only look at daily generation, the impact of shadows can be obscured. If you can view hourly or shorter-interval generation data, identify the times when output drops and inspect the shadow conditions at the site during those periods. Determining whether the drop occurs in the morning, in the afternoon, or only during certain seasons will make it easier to identify the cause.

What to watch for with shadows from fixed objects is that, even if they look the same day to day, their length and angle change with the seasons. A fixed object whose shadow didn’t reach the panels in summer can cast a shadow on them during periods when the sun’s altitude is lower. Conversely, if power generation is lower only in a particular season, you can suspect a shadow specific to that period. When comparing with the same month in the previous year, checking for shadows also makes sense to ensure you’re comparing the same seasonal conditions.

Also check whether anything has been newly installed around the equipment. Temporary materials, construction signs, stored items, mobile equipment, newly added support posts, and the like can unintentionally cast shadows. If the person responsible for managing the power generation equipment is different from the personnel working on site, items may be placed without sharing information about their impact on power generation. Particular caution is necessary when periods of low power generation coincide with times of site changes.

When shadows from fixed objects are detected, some can be removed immediately while others cannot be moved easily. If they are temporarily placed materials, relocation may improve the situation, but buildings, utility poles, and neighboring structures are not easily dealt with. Even in such cases, recording the time the shadow occurs, its extent, the season, and the sections suspected to be affected will help explain decreases in power generation and aid future retrofit considerations.

Shadows cast by fixed objects are elements whose overall impact is difficult to grasp from a single site visit. By taking photos at different times of day, checking the same location across seasons, and cross-referencing with the times when generation data dips, you can turn them from mere impressions into information usable for management decisions.

Review 5: Check how solar altitude and shadow length change with the seasons

If you feel your power output is low, judging only by comparisons with the previous year or the previous month can lead you to mistake normal seasonal variations for abnormalities. Solar power generation is affected by solar irradiance, hours of sunshine, solar altitude, and temperature. In particular, shadows tend to extend longer when the sun’s altitude is low, so obstacles that were not a problem in summer can cast shadows in winter.

When examining seasonal differences, it is important not to compare only the total amount of generated power. In winter, daylight hours are shorter and the sun is lower in the sky, so even with the same equipment generation tends to decrease. On the other hand, lower temperatures can sometimes be advantageous for panel output. In other words, seasonal changes in generation are not one-directional but result from a combination of factors. Therefore, you should avoid attributing low generation solely to the season, and you should also avoid ignoring seasonal differences and hastily labeling the situation as abnormal.

When reviewing shadows, it is useful to check the same location in spring, summer, autumn, and winter. Trees or buildings whose shadows do not reach the site at one time of year can cast shadows on the lower edge of the panels at another time. In particular, morning and evening shadows extend long, so short inspections during the day can easily miss them. If drops in power generation are concentrated in winter or in specific months, inspect the site while paying attention to the sun’s path during those periods.

Also, when comparing with the same month of the previous year, check for differences in weather. Even if power generation is lower than in the same month of the previous year, a higher number of cloudy or rainy days will reduce generation. Conversely, if solar irradiance conditions have not changed significantly but only a particular section is lower, you should suspect soiling, shading, or differences in equipment condition. Combining not only power generation data but also solar irradiance, weather, outage history, inspection history, and cleaning history improves the accuracy of your assessment.

It's not only shadows that change with the seasons. Pollen, yellow sand, fallen leaves, snowfall, freezing, debris blown in after typhoons, and residual dirt during the rainy season all affect the condition of panel surfaces. In spring, pollen and dust; in autumn, fallen leaves; after strong winds, branches and blown-in debris; and during periods with little rain, the accumulation of surface dirt can become noticeable. Organizing which kinds of soiling are likely during periods of low power generation makes it easier to set inspection priorities.

What is important in seasonal checks is to use the same perspective every year. If inspection times or the range of inspection differ from year to year, comparisons become difficult. Keeping fixed-point photographs, recording the inspection time, recording the area affected by shadows using consistent wording, and linking power generation data to inspection dates makes it easier to assess conditions in subsequent years. Even when there is an inquiry about low power generation, having records from the same period in past years makes it easier to explain whether it is a seasonal trend or an anomaly specific to this time.

Understanding how shadows lengthen with the seasons is useful not only for post-installation maintenance but also when planning expansions or the placement of nearby equipment. When adding new equipment or storage, shadows that don’t fall now may extend in another season. To prevent a decrease in power generation, it is important to manage based on seasonal changes as well as current conditions.

Review 6: Confirm changes in power output after cleaning or weeding by checking records

After inspecting panels for dirt and shading and performing cleaning or weeding, it is important to verify in the records how those actions affected power generation. On site, people tend to assume that cleaning must have led to improvement or that cutting grass must have resolved the issue, but in reality, because of differences in weather and solar irradiance, changes in power generation should be assessed carefully.

The basic principle of verification is to compare the before-and-after conditions under as similar circumstances as possible. If it was cloudy before the work and clear and sunny afterward, an increase in power generation cannot be attributed solely to cleaning or weeding. Conversely, if it rains or remains cloudy after the work, power generation may appear not to have improved. Comparing on days with as similar solar irradiance conditions as possible, at the same time of day, and in the same section makes it easier to judge the effect of the work.

When monitoring power generation, check not only the total for the entire facility but also changes in the specific sections or circuits in question. If soiling or shading is limited to part of the installation, the change may appear small in the total power output. However, looking at the output of the affected section can reveal an improving trend. When investigating the causes of low power generation, it is effective to use data that allows comparisons at the finest possible granularity.

The items that should be recorded are not limited to the work date alone. Record the areas cleaned, the areas weeded, the condition of dirt and shadows before work, photos after work, the weather, the time of work, the inspector, the methods used, and any locations where abnormalities remain. Even when cleaning or weeding is outsourced, it is desirable to ensure that the information necessary for power generation management is retained. If records are vague, it will be difficult to determine whether past work was effective the next time the same problem occurs.

Also, if power output does not recover after cleaning or weed removal, you should suspect another cause. This provides grounds for moving on to further checks — for example, that dirt on the panel surface was not the primary cause, that the impact of shading was limited, that there is another fault on the equipment side, or that there is a problem with acquiring measurement data. In other words, checking power output after the work is important not only to confirm the effect but also to help isolate the cause.

When verifying the effectiveness of work, it is important not to draw conclusions based solely on short-term changes. Solar power generation is heavily influenced by weather variations, so data from a single day can lead to inconsistent judgments. By observing trends over several days to a certain period and confirming whether improvements continue under similar solar irradiation conditions, you can make more practical decisions.

As you accumulate records of cleaning and weeding, trends for each power plant become apparent. If you know when dirt tends to increase, which sections are prone to shading from weeds, and how much improvement typically occurs following cleaning and weeding, you can shift from reactive, after-the-fact responses to preventive management. Rather than responding after you notice reduced power output, you can plan inspections and work before the periods when output tends to decline.

To detect declines in power generation early, continuous visualization is essential

Panel soiling and shading that cause reduced power generation are easy to check on-site, but if discovery is delayed, identifying the cause becomes difficult. If you don't know when the soiling began to accumulate, when the shading started occurring, or whether cleaning and weeding had any effect, it becomes hard to explain why power generation is low. That's why it's important to visualize daily power output and site conditions.

What is important in visualization is not treating power generation data and on-site records separately. Looking at only a power generation graph does not reveal the condition of soiling or shading. Conversely, looking at only on-site photos does not reveal how much they affected power generation. By linking inspection photos, work records, weeding records, cleaning records, weather, and changes in power generation, you obtain information that can be used for root-cause analysis.

Especially important for operations personnel is a system that can detect anomalies early. If you only notice a drop after aggregating power generation at the end of the month, you have to trace back to when soiling or shading occurred. Being able to check daily and hourly changes makes it easier to find days or time periods with low power generation. If on-site photos or inspection notes are available, you can narrow down possible causes more quickly.

Panel soiling and shading can each reduce power generation on their own, but they can also appear together with other factors. For example, during periods of low solar irradiance there may be accumulated soiling, or in plots where grass casts shadows there may be connection faults, making the observed drop in generation more complex. Therefore, it is important to repeatedly carry out on-site inspections and data checks and to methodically isolate each condition one by one.

The six items we checked this time are all things that can be easily reviewed on site when power output is low. Inspecting panel surface soiling by location, checking for localized deposits such as bird droppings and fallen leaves, looking for shading at the lower edge caused by weeds or plantings, checking shadows from fixed objects like buildings and utility poles at different times of day, considering seasonal changes in solar altitude and how shadows lengthen, and recording and confirming changes in power output after cleaning or weeding are the basics for sorting out the causes of decreased power generation.

To avoid leaving low power generation unaddressed, it is essential not to check only when visiting the site but to continuously link daily generation data with on-site conditions. Dirt and shading are easier to deal with, and their causes are easier to explain, if detected early. Managing the plant’s condition with photos, location information, inspection history, and work history, and establishing a system that enables correlating changes in power generation with on-site conditions, leads to early detection of generation declines and prevention of recurrence.