4 Checks to Identify Bird Droppings That Cause Reduced Power Output

When you feel that the power output of a solar power generation system is low, there are many elements to check: weather, season, solar irradiance, equipment shutdowns, shading, and aging. One often overlooked element is localized soiling from bird droppings. Bird droppings may look like small stains when judged only by area, but because they heavily cover the light-receiving surface of PV modules, they can, under certain conditions, contribute to reduced power generation. In particular, at sites where birds tend to perch in the same spot or where there are nearby power lines, trees, signs, or roof protrusions, similar soiling tends to recur.

However, the presence of bird droppings does not necessarily mean a significant drop in power generation is occurring. The magnitude of the impact varies depending on solar irradiance conditions, the location of the soiling, the extent of the coverage, the configuration of modules and strings, how shadows fall, and overlap with existing dirt or fallen leaves. What is important is not to judge by appearance alone, but to check, in order, the generation data, the actual soiling pattern on site, the surrounding environment, and the changes after cleaning. This article explains four checks to distinguish bird droppings, presented in a format that is easy for site personnel to use, aimed at practitioners investigating causes using the search term "low power generation".

Table of Contents

• Understand why bird droppings lead to a decrease in power generation

• Check 1: Examine the drop in power generation data and weather-related differences

• Check 2: Inspect the module for white adhered contamination and confirm its location

• Check 3: Confirm surrounding environments that are likely to attract birds and the potential for reattachment.

• Check 4: Observe post-cleaning recovery trends and the persistence of other causes

• Points to note when assessing bird droppings stains

• Summary

Understanding Why Bird Droppings Lead to Reduced Power Generation

In solar power generation, electricity is produced when sunlight reaches the surface of photovoltaic modules. Therefore, if there is dirt or shading on the surface, those areas receive less light. Bird droppings differ from thin, spreading contaminants like dust or pollen: they tend to concentrate in specific spots as adhered deposits composed of white, gray, and dark components. They may be washed away by rain, but once they dry and harden they tend to remain, and if they accumulate repeatedly in the same location they can create shading patches that are darker than they appear.

When considering the causes of low power generation, the first thing to check is not to judge solely by the area of soiling. Solar photovoltaic (PV) modules are composed of multiple cells and circuits, and even if only part is obscured, the impact on power generation can be limited in some cases, while under certain conditions the effect can be more noticeable than the soiled area would suggest. In particular, when bird droppings heavily cover a cell, that area can become difficult for sunlight to reach even though the surroundings are sunny, creating a condition close to a local shadow. Rather than assuming a small amount of dirt is not a problem, you need to check where, to what extent, and for how long the soiling has been present.

Bird droppings can be suspected as the sole cause of reduced power output, but they may also overlap with other factors. For example, if bird droppings are attached on top of overall soiling from yellow sand or pollen, local shading is added to an already degraded light-receiving condition across the entire surface. If leaves or branch shadows, shadows from nearby buildings, residual snow, or water stains from poor drainage are present at the same time, it is dangerous to judge bird droppings alone as the cause. At sites, multiple small factors can accumulate and result in the state of "low power generation."

Moreover, bird droppings affect not only power output but also inspection and maintenance decisions. When looking only at power generation data, it can be difficult to tell whether a drop in output is due to reduced solar irradiance from the weather, an equipment shutdown, or soiling. Therefore, it is important to consider on-site photographs, daily and hourly generation data, comparisons between sections within the same installation, and past cleaning records together. Although bird droppings are easy to spot visually, their impact on power output can sometimes appear small in the data, so it is essential to verify both visually and with data.

Additionally, it is important to note that bird droppings tend to recur. Even if cleaning seems to restore power generation, similar soiling can reappear within a short time if there are nearby spots where birds like to perch. Therefore, rather than treating bird droppings as a one-off issue, checking where they originate, which areas they concentrate in, and whether there is a tendency for reattachment will help achieve more stable power generation management.

Check 1: Examine drops in power generation data and weather differences

The first clue to suspect bird-dropping soiling is the way power generation data declines. When you feel the output is low, first check whether daily generation has dropped suddenly or is falling gradually. Bird droppings can cause part of the output to decrease from the moment they adhere, and depending on the extent and location of the soiling, the growth in generation may appear weaker from a specific day. However, because daily generation also fluctuates with solar irradiance and cloud cover, it is not appropriate to conclude bird droppings solely because the value is lower than the previous day.

In practice, it is important to restrict comparisons to clear-sky days as much as possible. On cloudy or rainy days there is inherently less solar irradiance, so even if generation is low it is difficult to judge whether the cause is soiling. If solar irradiance data are available, check those as well, and line up several days with similar clear-sky conditions to observe generation trends. Because the sun’s altitude and sunshine duration change with the seasons, comparisons with the previous month or the same period in the previous year are also useful. Rather than relying solely on simple monthly generation totals, examining the generation curves for days with similar irradiance conditions makes it easier to narrow down the likelihood of soiling-related declines.

The impact of bird droppings may not be apparent when looking only at the facility’s total power generation. In large installations or systems divided into multiple arrays, local declines can be masked by the overall output and thus may not stand out. In such cases, compare as finely as possible within the same installation—by area, by circuit, by string, etc., within the range you can inspect. If a section shows a weaker start-up on sunny days, a slower increase around midday, or is lower than an adjacent section under the same conditions, it is worth checking that section for surface soiling.

Time-of-day generation curves are also important. If bird droppings are present on specific cells or modules, the reduction in power output can appear different at different times of day depending on the sun’s position and the state of the circuit. Output may be low all day from morning to evening, or the difference may be most noticeable during periods of strong irradiance. In particular, if the rise near the peak is muted despite clear skies, if the peak height is lower compared with another section of the same capacity, or if generation seems to plateau during times of sufficient irradiance, these are signs that soiling or shading should be suspected.

However, you cannot definitively attribute the shape of a power generation curve to bird droppings alone. Similar appearances can be caused by power conditioner control, output curtailment, voltage conditions, temperature rise, shading, equipment malfunctions, or missing communication data. To distinguish bird-dropping soiling, it is important to connect anomalies in the data to on-site inspections. When you find a low power output condition, first check whether power generation has stopped or whether there are any equipment alarms; then proceed to check the module surfaces for soiling to more reliably isolate the cause.

Also, bird droppings can be diluted by a brief rain. If power generation recovers somewhat after a heavy rain but does not return completely, it may be that some surface dirt was washed away while stubborn, adhered dirt remains. Conversely, if there is no change after rain, you need to check for causes other than bird droppings. By carefully examining the relationship between weather and power generation, it becomes easier to determine whether bird droppings are the primary cause or a contributing factor.

Check 2: Confirm white adhered contamination on the module and its location

On-site visual inspection is indispensable for distinguishing bird droppings. Bird droppings often appear as residues made up of mixed white, gray, and dark components, and they adhere to the surface and remain after drying. Unlike dirt that spreads thinly like sand dust, they tend to remain more concentrated in spots or streaks.

When confirming the cause of low power generation, it is important not only to look at the entire module array from a distance, but also to record which column, which row, and which module have concentrated soiling.

The first things to check are the darkness and the extent of the soiling. If bird droppings are dry, white, and thickly adhered, they can block sunlight. If there are only light marks, the impact may be small, but if thick, adhered deposits cover the cells they can contribute to reduced power output. In particular, when multiple spots of soiling are close together, when several are attached to the same module, or when rain has left streak-like residues, photographing them for records will be useful for later comparison.

Next, check the location of the soiling. The effect can differ depending on whether it is lightly attached to the edge of the module or covering the center of a cell. If soiling is only accumulating near the frame at the edge, the impact on power generation may be limited. On the other hand, if it is heavily deposited on a cell or spread across multiple cells, caution is required. Check not only the visible area but also how much of the light-receiving surface involved in power generation is being covered.

Bird droppings can leave traces that have flowed downward due to drainage or the slope. If white streaks extend along the module’s tilt, deposits have congealed near the frame where they flowed, or dirt continues onto lower modules, it may not be a single instance of fouling but repeated soiling or movement caused by rain. Such contamination not only worsens the overall appearance of the surface but can also mix with dust and mineral deposits and become difficult to remove.

During on-site inspections, it is also important not to force yourself onto roofs or mounting racks. Even if you want to quickly find the cause of low power generation, you should avoid close-up checks in areas that carry a risk of falling, electric shock, or damage. Choose methods appropriate to the equipment conditions, such as using binoculars or telephoto photography from a safe position, inspecting from maintenance walkways, or engaging a professional inspector when necessary. Because rubbing the module surface directly or applying strong force can damage the glass and surrounding components, it is safer not to perform cleaning casually.

When distinguishing bird droppings from other types of dirt, look not only at color and shape but also at how they are distributed. While yellow sand (Asian dust), pollen, and windblown dust tend to spread fairly uniformly over wide areas, bird droppings tend to concentrate under and around places where birds are likely to perch. Fallen leaves, tree sap, insect marks, and water stains can also appear whitish, so if it is difficult to determine from photos alone, check the surrounding environment and recurrence patterns as well. To narrow down the causes of a decline in power generation, it is important not to assume the type of dirt and to observe while keeping multiple possibilities open.

Also, when performing inspections, keeping photos taken from the same angle, the same distance, and the same time of day makes later comparisons easier. By organizing photos taken before and after cleaning, before and after rain, and on days when low power generation is noticed, you can determine how much bird droppings remain, whether they are spreading over time, and whether cleaning has improved the situation. On-site photos are an important record that can supplement causes not visible from power generation data alone.

Check 3: Confirm nearby environments that tend to attract birds and potential re-deposition

To distinguish bird-dropping soiling, inspect the surrounding environment as well as the module surface. This is because the approach to countermeasures differs depending on whether the droppings were deposited only once or are recurring due to conditions that attract birds. If the reduced power output is temporary, cleaning may improve it, but at sites where re-deposition continues, the same problem can occur again shortly after cleaning. In maintenance practice, it is important not to overlook this recurrence.

Places where birds tend to perch include the tops of mounting structures, roof ridges, nearby power lines, antennas, signs, fences, trees, and protrusions on buildings. If modules are located beneath or around these, bird droppings tend to concentrate in specific areas. In particular, if soiling appears in a straight line, only the modules in the same row are dirty, or contamination is concentrated on specific ends or corners, check whether there are bird perches above or nearby. Cleaning only the surface soiling may result in recurrence if bird behavior does not change.

When checking the surrounding environment, pay attention to bird movements by time of day. Locations where birds tend to gather in the morning and evening, places with nearby feeding or water sources, and areas where birds can get into the shadows of buildings or gaps in roofs may not be fully assessed by daytime inspections alone. If bird droppings are suspected as a cause of reduced power generation, refer not only to what is observed at the moment of inspection but also to interviews with the site manager, past photographs, and regular inspection records. Even if you do not see birds directly, repeated soiling in the same location can be evidence suggesting a bird-related impact.

To confirm re-deposition, it is effective to monitor the site after cleaning. Even if it looks clean immediately after cleaning, if white, adherent deposits return to the same spot within a few days to a few weeks, birds may be using that location routinely. In such cases, simply increasing the cleaning frequency can raise the workload without leading to a fundamental improvement. If necessary, consider creating an environment that makes it difficult for birds to perch and reviewing management methods for the surrounding area. However, bird-control measures must take into account laws, local rules, and building-management constraints, and you should choose safe and appropriate methods.

Check whether locations with concentrated bird droppings correspond to spots where power generation data has declined. For example, if the section with low power output matches the section where bird droppings are concentrated, it is more likely that soiling is the cause. Conversely, if locations where droppings are prominent do not correspond to circuits with low power output, there may be another cause or the effect of the soiling may be limited. Cross-referencing the on-site spatial relationships with the electrical system layout improves the accuracy of the assessment.

Also, bird droppings require consideration for hygiene and work safety. Avoid rubbing or otherwise disturbing dry droppings in a way that could spread them, and address them only after preparing appropriate personal protective equipment and work procedures. When using water for cleaning, assume the presence of electrical equipment and check the equipment’s shutdown status, the work area, access and scaffolding/footing, drainage, and the potential for splashing to surrounding areas. It is important that on-site personnel do not make unilateral judgments and undertake unreasonable work, but respond in accordance with equipment management rules.

Bird droppings are often treated as merely a cosmetic issue, but when they repeatedly occur in the same location they represent an ongoing risk to power generation management. Rather than finding the soiling once when output is low and leaving it at that, keeping a continuous record of whether re-deposition occurs, the surrounding environment, cleaning intervals, and changes in power output will make future decisions easier.

Check 4: Observe recovery trends after cleaning and the persistence of other causes

Changes in power output after cleaning are an important clue when determining whether bird droppings caused the reduction in generation. If output improves under similar sunny conditions after the soiling is removed, surface contamination—including bird droppings—may have been one of the factors responsible for the decline. However, if the weather, temperature, solar irradiance, or the presence of output control differ before and after cleaning, a simple comparison is not possible. Even if generation increases on the day after cleaning, it may simply be because there was more sunlight.

For post-cleaning verification, compare sunny days under as similar conditions as possible. Place a sunny day from before cleaning and a sunny day from after cleaning side by side, and check the daytime power generation curves, the rise near the peak, and the differences with other sections within the same installation. If bird-dropping soiling was the main cause, the generation of the affected section may become closer to that of the surrounding sections after cleaning. Because changes can be easily masked when looking only at the installation’s total monthly generation, it is easier to judge if you can view daily, time-of-day, and section-by-section data.

On the other hand, if cleaning does not appreciably restore power output, consider causes other than bird droppings. Reasons for low power output are diverse, including equipment shutdowns, alarms, communication failures, string faults, problems at connection points, shading, output control, missing cumulative data, and deviations from design assumptions. When bird droppings are found, there is a tendency to attribute the problem to them, but visible soiling does not always correspond to reduced output. If a decline remains after cleaning, consider that the soiling was only a partial factor or not the primary cause, and proceed to the next checks.

When evaluating the effect of cleaning, pay attention to the scope of cleaning. If only part of the modules is cleaned, the effect may be difficult to discern in the total power generation. Conversely, when the entire installation is cleaned, not only bird droppings but also sand and dust, pollen, yellow sand, and water stains are removed simultaneously, so even if power generation recovers you cannot conclude that the change was due solely to bird droppings. In practice, it is safer to state that surface soiling, including bird droppings, was likely related to the decrease in power generation.

Also, even if output temporarily recovers after cleaning, if power generation drops again within a short period, check for re-deposition or other causes. Consider separately whether bird droppings have returned to the same spot, a different type of soiling is spreading, seasonal reductions in solar irradiance are occurring, or whether there are control actions or shutdowns on the equipment side. Especially when low power generation appears intermittently, it is necessary to check not only soiling but also equipment operation and grid conditions. Do not judge based on a single post-cleaning data point; looking at trends over several days to weeks will reduce the chance of overlooking the cause.

Keeping a record of cleaning is also important. Summarizing the cleaning date, scope of cleaning, condition of soiling, methods used, before-and-after photos, weather on the day, and power generation after cleaning makes it easier to assess things at the next inspection. Bird droppings are easy to spot visually, but it can be difficult to quantify their impact on power generation. By accumulating records, site-specific trends become apparent, such as "this location is prone to bird droppings," "this level of soiling is unlikely to have a large impact," and "only this section recovered after cleaning."

Even if power generation recovers after cleaning, it is important not to stop measures there. At sites where reattachment of bird droppings is expected, review the frequency of regular inspections, the methods for checking monitoring data, and the criteria for deciding on cleaning. Rather than rushing to check after noticing low power output, it is easier to prevent delays by identifying sections prone to soiling in advance and managing changes in power output together with photographic records.

Precautions When Assessing Bird Dropping Stains

When attributing an issue to bird droppings, it is important not to be overly definitive. Low power output may not have a single cause; weather, solar irradiance, temperature, shading, equipment shutdowns, settings, grid conditions, aging, data acquisition faults, and so on can coincide. When bird droppings are conspicuous, it's easy to focus only on them, but in practice you should regard them as "possibly related to the decrease in power output" and concurrently check other factors.

Particularly important to pay attention to are the conditions under which power output is compared. If the weather, solar irradiance, season, or temperature differ between before and after cleaning, comparing power output alone may not lead to accurate conclusions. Solar power generation can vary even on sunny days due to thin clouds, temperature, wind, and solar altitude. To assess the impact of bird droppings, it is important to match the comparison conditions as closely as possible. Measures to minimize differences in conditions are necessary, such as comparing nearby sections within the same installation, comparing similarly clear days, or analyzing by time of day.

Safety precautions are also indispensable. Bird droppings are easy to spot visually, so on site people may think "just wipe it a little." However, solar power generation equipment is electrical equipment and is often installed on roofs or at heights, so careless work is dangerous. Working on wet surfaces, on steeply pitched roofs, in places without scaffolding, or around energized equipment carries risks of falls, electric shock, and equipment damage. On-site responses should follow the equipment manager’s rules and safety procedures, and, when necessary, request specialized inspection and cleaning personnel.

Care should also be taken with cleaning methods. Scrubbing with strong chemicals or hard tools can damage the module surface, frame, or surrounding components. Also, improperly applying high‑pressure water may raise concerns about effects on connections and nearby equipment. Even if bird droppings have become hardened, it is important not to forcefully scrub them off but to address them using methods appropriate for the equipment. Before cleaning, check the manufacturer's and installer/manager's handling instructions and the site's safety standards, and keep a record of the work for reassurance.

When explaining the relationship between bird droppings and reduced power generation, be mindful of wording for internal and customer communications. Rather than stating categorically "bird droppings are the cause," phrases such as "bird droppings were observed, and the post-cleaning power generation trend suggests they may have had an impact" and "since a reduction remains after cleaning, we will continue to investigate the equipment side" better reflect the uncertainty on site. Because multiple factors can affect power output, making stepwise judgments based on records helps prevent problems.

Bird droppings contamination also leads to revising maintenance plans. Inspection frequency and criteria for cleaning differ between sites where the fouling is naturally washed away a few times a year and sites where it reaccumulates in a short period. Instead of checking only when power generation is low, it is important to take a long-term view that includes the seasons when birds tend to gather, changes in the surrounding environment, nearby construction or tree growth, and the positions of power lines and structures. At sites prone to recurrence, it is reassuring to operate by combining monitoring data with on-site inspections so decreases in power generation can be detected early.

Summary

To distinguish bird-dropping soiling that reduces power output, it is important not to judge by appearance alone but to check, in order, the power generation data, the location of the soiling, the surrounding environment, and the recovery trend after cleaning. Even small-looking bird droppings can, if they adhere densely to the cells, cause localized shading and under certain conditions may be a factor in reduced power output. At the same time, because power output can also fluctuate due to weather, equipment downtime, shading, output curtailment, or other soiling, you should avoid concluding that bird droppings are the sole cause.

During the initial check, review daily and hourly power generation to confirm whether a particular section remains consistently low during sunny conditions. Next, record white adhered deposits on module surfaces, their locations, concentrations in multiple spots, and any traces of being washed by rain. Also check the surrounding environment for areas where birds are likely to perch and whether re-deposition has occurred, and observe how power generation changed after cleaning. Following this sequence makes it easier to determine whether bird droppings are the primary cause or whether they overlap with other causes.

In practice, when you notice low power generation, it is important not to hastily decide the cause but to isolate the issue step by step while keeping records. Avoid performing visual inspections or cleaning in locations where safety cannot be ensured, and use specialized inspection teams as needed. If you retain photos before and after cleaning, power generation data, weather, the scope of cleaning, and the condition of any re-deposition, future assessments and decisions will be faster.

Bird droppings can be a one-off stain, or they can become a persistent challenge for power generation management. To avoid missing causes of reduced power output, it is essential to link on-site soiling with data and identify locations prone to recurrence. By managing daily power output changes, on-site photos, and cleaning records together, you can detect impacts from bird droppings more quickly and more easily distinguish them from other causes.