6 Indicators to Determine a Decrease in Power Generation from Yellow Sand and Pollen Soiling

When power output is low, it's tempting to immediately suspect equipment failure or panel degradation, but from early spring through early summer the adherence of yellow sand, pollen, and fine dust can be one cause. In particular, at solar power plants, even with the same equipment the way panels become dirty changes with the season, wind direction, whether it has rained, and the surrounding environment. To correctly assess a drop in power output, it's important not to conclude based only on visible dirt, but to consider generation data, the weather, string-level trends, and recovery after rainfall together.

Table of Contents

• Summarize the relationship between reduced power output and contamination from yellow dust and pollen.

• Guideline 1 Check whether the decline in power generation is overall or only in part

• Guideline 2: Check whether the power generation curve on a clear day shows a rounded dip.

• Guideline 3: Check whether the dispersal and decline periods of yellow sand and pollen overlap

• Guideline 4 Check whether power generation recovers after rainfall

• Guideline 5: Check the adhesion condition of the panel surface from a safe position.

• Guideline 6: Check the surrounding environment and the likelihood of dirt recurrence

• Inspection procedures to avoid confusing dirt with malfunctions

• Points to consider when deciding on cleaning or inspection

• In order to continuously monitor low power output.

Clarifying the relationship between reduced power output and yellow sand and pollen soiling

The power output of a solar power system varies due to multiple factors, such as solar irradiance, ambient temperature, equipment condition, and the cleanliness of the panel surface. For that reason, it is dangerous to immediately conclude "yellow sand is the cause" or "pollen is the cause" based solely on a low power output result. Dirt from yellow sand or pollen is one of the causes of reduced power generation, but it can overlap with other factors such as power conditioner shutdown, string disconnection, blown fuses, shading, communication failures, snowfall, and vegetation growth.

Yellow sand and pollen tend to become problematic because fine particles can spread thinly across the panel surface and obstruct the sunlight the panels receive. Even if they don't coat the surface thickly like mud, a faint whitish film or a yellowish dust left behind can affect power generation. Dirt tends to remain particularly on panels installed at low tilt angles, on edges where rainwater does not run off easily, and in locations where wind causes dust to accumulate.

However, the effects of yellow sand and pollen vary from site to site. Even within the same region, power plants with fields or unpaved roads nearby, power plants close to forests or plantings, and power plants affected by sea breezes or strong winds will become soiled differently. Also, while substantial rain can wash away some of the dirt, pollen can absorb moisture and adhere, and yellow sand can remain in streaks along the lower edge of panels. You should avoid simple judgments such as "it must be clean because it rained" or "if dirt is visible, power generation will necessarily drop significantly."

What operational staff should look at is not the mere presence of soiling, but the relationship between soiling and power generation decline. Confirm in order: since when the output has been low, over what range it is low, what the generation curve looks like on sunny days, whether it recovered after rainfall, and whether the soiling visible on-site matches the data trends. Following this sequence makes it easier to determine whether cleaning is necessary, whether to prioritize equipment inspection, or whether to continue monitoring a little longer.

Guideline 1: Determine whether the decrease in power generation is widespread or localized

The first indicator to suspect soiling from yellow sand or pollen is whether the decrease in power output is widespread across the entire plant. Because yellow sand and pollen are carried by the wind over wide areas, if they are having an effect, similar trends tend to appear across multiple arrays or multiple strings. If only one particular system is experiencing a large drop, you should suspect not only soiling but also equipment malfunctions in that system, poor connections, shading, broken wiring, blown fuses, etc.

For example, when comparing sections with the same orientation, the same tilt, and the same capacity, if the values are generally lower it becomes easier to consider common factors such as weather or soiling. On the other hand, if only some strings are extremely low, the possibility that yellow dust or pollen is the cause is not zero, but in many cases it is better to first check for localized shading or equipment-side issues. This is because dirt tends to spread across surfaces, whereas electrical abnormalities can suddenly appear on a string or circuit basis.

The important point here is not simply to see “lower than yesterday” but to ensure you are comparing like with like. Comparing a sunny day with a cloudy day does not allow you to judge soiling. If possible, compare days with similar solar irradiance, similar temperatures, past data from the same season, adjacent plots, and installations of the same capacity. If you have a pyranometer or weather data, looking at generation corrected for irradiance makes it easier to determine the impact of soiling.

In practice, attention tends to focus only on the magnitude of the drop in power output, but it is important to look at how that drop is distributed. Check whether the whole system is slightly lower, or only a part is extremely low, whether differences widen at certain times of day, or whether it is slowly worsening day by day. Soiling from yellow sand and pollen tends to build up gradually when scattering or adhesion continues, and it is uncommon for this to cause output to suddenly drop to near zero. If there is a sudden and large decline, do not assume it is only soiling—raise the priority of equipment inspections that include safety checks.

Also, if there are sections within the power plant that have different tilt angles or installation orientations, compare them separately. Panels with lower tilt angles tend to retain dirt more because rainwater does not wash it away as easily, and are more likely to hold yellow dust and pollen. Conversely, in locations with greater tilt where rainwater runs off more readily, the same amount of airborne deposition may leave less residue. Ignoring differences in installation conditions and looking only at the overall average can lead to overlooking the effects of soiling or misidentifying another anomaly as soiling.

Guideline 2: Check whether a rounded dip appears in the power generation curve on clear-sky days

The power generation curve on a sunny day is a useful reference for judging contamination from yellow sand or pollen. On a normal sunny day, generation ramps up from the morning, reaches a high around midday, and then declines toward the evening, forming a single-peaked (mountain-shaped) curve. When contamination is present, this entire peak may be slightly lower, and it can appear that power output is suppressed broadly from morning through the afternoon, not just at the peak.

At this time, check whether the shape of the power generation curve is significantly distorted. Dust from yellow sand or pollen tends to spread thinly across the entire panel, so the whole curve tends to decline gradually. Conversely, if it suddenly drops only during specific time periods, it may be due to shadows from buildings, trees, mounting racks, utility poles, or nearby equipment. If there is a large dip only during a certain time of day, or if output is low only in the morning or only in the afternoon, you need to check for shadows, orientation differences, or the effects of equipment control rather than dirt.

Also, when the generation curve suddenly cuts off midway or levels off at a constant value, it is difficult to explain this solely by yellow dust or pollen. It may be related to power conditioner shutdown, output control, communication loss, protection operation, or grid-side restrictions. Decreases caused by soiling often appear as a failure to reach expected generation even during periods with sufficient solar irradiance, and should be considered distinct from the sudden interruption of generation seen with equipment shutdown.

A common oversight in practice is making a judgment based only on data from cloudy days. Under overcast or rainy conditions, solar irradiance becomes unstable and power output can fluctuate greatly just from cloud movement. In such situations, it is difficult to detect numerical differences caused by yellow sand or pollen. If you want to assess soiling, it is effective to choose sunny days with as few clouds as possible and compare them with past data from similar clear days.

Additionally, we consider the effect of temperature. Because solar panels' output decreases at high temperatures, generation efficiency can decline on clear summer days even without soiling. Even during early-spring yellow sand and pollen seasons, there is a difference in power generation between warmer and cooler days. Therefore, looking not only at simple generation figures but also at solar irradiance, temperature, and historical performance for the same period makes assessments of soiling more reliable.

When looking at a generation curve, it's important to check not only the total daily generation but also the shape throughout the day. Even if the total generation is low, the list of possible causes changes depending on whether the entire curve is smoothly lower, whether part of it is missing, or whether the peak is capped. If you suspect soiling from yellow sand or pollen, check whether the whole curve is slightly lower on sunny days and whether a similar trend appears across multiple sections.

Guideline 3: Check whether the dispersal and decline periods of yellow sand and pollen overlap

The third indicator for judging whether dirt is from yellow sand or pollen is whether the period when power output declined overlaps with the season when they are likely to be dispersed. Yellow sand tends to arrive mainly in spring, and depending on the region and weather conditions it can be noticed as hazy visibility or as dust deposited on vehicle surfaces. Pollen is also prominent in early spring, and in areas with many trees or grasslands nearby it can remain on panel surfaces as yellowish powder or as sticky deposits.

However, do not assume the cause just because the season fits. Spring is a time when solar irradiance increases, and it is also a period when other factors tend to coincide, such as strong winds, rain, frost, temperature fluctuations, weed growth, and setting changes after maintenance work. Just because a day of low power generation is close to an Asian dust event does not mean that soiling is the primary cause. What is important is to look at these together: how long the decline continued after the event, whether the low level remained even on sunny days, and whether it recovered after rainfall.

If yellow dust or pollen are involved, you may see power output gradually decline after days with heavy dispersion or strong winds. In particular, when a prolonged period with little substantial rainfall continues, particles on the panel surface are not washed away and tend to remain as a thin film. Conversely, if heavy rain falls immediately after the dispersal and power output recovers on the following sunny day, the effect of soiling may have been limited.

In field operations, keeping power generation data together with weather notes makes assessments easier. For example, recording days with strong winds, hazy skies, heavy dust on vehicles or mounting racks, noticeable pollen, or substantial rainfall lets you later check their relationship to reductions in power output. Even without detailed measuring instruments, simple on-site observation records can help narrow down causes.

Also, during periods of yellow sand or pollen, dirt can adhere to equipment other than the panels. If similar dust is present on combiner boxes, racking, fences, surveillance camera covers, or walkway handrails, it increases the likelihood that deposits have settled across the entire power plant. Rather than judging by the panels alone, assessing how surrounding objects are soiled provides a more realistic understanding of the impact of airborne deposits.

On the other hand, if the decrease in power generation occurs completely independently of the dispersal season, or if it does not recover after rain and only certain systems show lower output, you should prioritize investigating causes other than yellow sand or pollen. Seasonal factors are a useful clue, but they are not decisive for pinpointing a single cause. It is safer to take an approach that combines timing, data, and on-site inspection to progressively increase the likelihood that soiling is responsible.

Guideline 4: Check Whether Power Generation Recovers After Rainfall

When judging whether panels are contaminated by yellow sand or pollen, the recovery of power output after rainfall is an important clue. If the fine dust adhering to the panel surface is light, a sufficient amount of rain can wash it away, and power output may improve on the next sunny day. If power output had remained low and then, after a substantial rainfall, output on a sunny day approaches its previous level, it becomes easier to suspect that contamination had been affecting it.

However, rain does not remove all dirt. Light rain or short-duration rainfall can cause yellow dust and pollen to absorb moisture and spread, leaving film-like or streak-like residues after drying. Runoff residues are particularly likely to occur at the lower edges of panels, around frames, on gently sloped surfaces, and in areas where rainwater tends to accumulate. Even when a surface appears clean after rain, thin soiling may remain, so it is important to verify recovery by checking the power generation data.

When checking after rainfall, do not look only at the day after the rain; check several days of clear-sky data. If the day after is cloudy, a low power output cannot be taken as evidence that dirt remains. Choose a day after the rain with sufficient solar irradiance and compare it with a day before the rain that had similar irradiance conditions. If irradiance data are not available, align conditions as much as possible—for example, compare near-clear-sky days, compare with nearby installations of similar size, or compare with past performance in the same month.

One point to note is that even if power output does not recover after rainfall, you should not immediately conclude that the equipment has failed. Pollen can adhere due to moisture, yellow sand can remain as a muddy residue, and bird droppings, fallen leaves, and soil dust can mix in, so natural rainfall alone may not remove them sufficiently. Also, after rain, dirt can accumulate at the lower edge of the panels and create shadow-like effects on parts of the cells. If power output does not recover after rainfall, carry out an on-site inspection and, using safe methods, visually observe the surface to confirm how the dirt remains.

On the other hand, if, after rainfall, only certain strings remain low, the power generation curve is interrupted, abnormal indications appear, or the decline is much larger than in the past, you should suspect electrical anomalies as well as soiling. Soiling acts on the generation surface, so it is generally seen as a suppression of the power increase corresponding to solar irradiance. Equipment faults may appear in different forms, such as shutdowns, sudden drops, abnormalities affecting only one side, or missing communication values. Using post-rainfall recovery checks as an entry point to distinguish soiling from faults is appropriate.

The same approach can also be used to verify the effects after cleaning. Compare similar sunny days before and after cleaning and check whether power output and the generation curve have improved. Even if power output looks higher immediately after cleaning, differing solar irradiation conditions can lead to mistaken judgments. Recording the effects of cleaning makes it easier to decide in the future how much soiling should trigger action.

Guideline 5: Check the adhesion condition of the panel surface from a safe position

When reduced power output is suspected to be caused by yellow sand (Asian dust) or pollen, an on-site visual inspection is essential. However, inspection of panel surfaces must be carried out with safety as the top priority. Do not approach, climb onto panels, or touch them with bare hands in high locations, on steep slopes, on wet walkways, or near live equipment. In practice, check the visible area from a safe position on the ground and, if necessary, use appropriate inspection methods.

Yellow sand and pollen deposits may appear on panel surfaces as a whitish haze, yellowish powder, a thin film, rain streaks, or an accumulation of dirt along the lower edge. Even when the soiling is not severe, the surface can look uniformly dulled depending on the way light falls on it. Dirt can be especially noticeable in oblique morning and evening light, but because reflection changes the appearance, note that impressions can vary with observation conditions.

During visual inspection, check the distribution of soiling as well as viewing the whole at once. Look to see whether it is uniformly covering the entire panel surface, accumulated at the lower edge, biased to the windward or leeward side of the row, or remaining near steps in the mounting structure. If the contamination is from wide-area airborne material such as yellow sand or pollen, similar soiling is often observed across multiple rows. If there are localized bird droppings, fallen leaves, soil splashes, or mud puddles, only that part may be affected by a different cause.

Also, pay attention to dirt at the panel edges and around the frame. As rainwater flows, yellow dust and pollen can collect at the lower edges and, after drying, remain as band-like stains. Even if it is only a very small part of the panel surface, depending on the cell positions and the pattern of soiling, it can affect power generation. In particular, if you see dirt repeatedly at the lower ends of panels in the same column, it is possible that the soil did not wash away after rainfall.

However, visible soiling and reductions in power output do not necessarily correlate directly. Panels that look dirty may sometimes be little affected, while a thin, film-like layer of dirt can reduce power output. Therefore, visual inspection should be treated only as supporting information to reinforce data-driven assessments. Only when reduced power output, the clear-day curve, recovery after rainfall, and the on-site soiling pattern all align can the effects of yellow sand or pollen be strongly suspected.

When taking photos on site, shoot them regularly from the same position, direction, and distance to make comparisons easier. If you take pictures from different angles each time, differences in reflections and brightness will make it hard to tell whether dirt has increased or decreased. Choose representative points visible from safe walkways and record dust accumulation on the panel surface, lower edges, surrounding equipment, and walkways or mounting racks so they can be easily correlated with power generation data later.

Guideline 6 Check the surrounding environment and the likelihood of dirt recurrence

When assessing a reduction in power generation caused by yellow dust or pollen, the power plant’s surrounding environment is also an important indicator. Soiling is not resolved simply by cleaning once; the likelihood of recurrence changes with wind direction, terrain, vegetation, roads, and nearby work activity. Even if the cause of low output is treated as temporary contamination, if the same conditions persist, deposits can form again and similar declines may recur.

If there are unpaved roads, development sites, fields, or dry bare land nearby, not only yellow sand but also soil dust tends to mix in. When dust kicked up by passing vehicles or strong winds adheres to the panels, it can appear as a thin film of dirt similar to pollen or yellow sand. If there are many trees or plantings nearby, pollen, sap, fallen leaves, and insect residue can mix in, creating dirt that is difficult to remove with rain alone.

Wind flow paths are also important points to check. On power plants located on slopes, in valley topography, on open plains, or near buildings or windbreaks, dust accumulation patterns vary. In some cases the upwind rows are dirtier, while in others particles blown by the wind fall and remain on the downwind side. If the same sections within the plant tend to get dirty year after year, those areas may be structurally prone to accumulating dirt.

Additionally, the tilt angle and arrangement of the panels also affect recurrence. The shallower the tilt, the less easily rainwater washes away dirt, and the more likely dust remains after drying. If there is a frame step at the lower edge of the panels, or the structure causes rainwater flow to stop midway, dirt can remain in linear streaks. If the same spots become dirty again in a short time after cleaning, it is necessary to check not only for simple temporary splashing but also the equipment conditions and site conditions.

Checking the surrounding environment is also useful when deciding cleaning frequency. In locations prone to soiling, it is more efficient to intensify fixed-point monitoring around the period when contaminants are dispersed than to respond after power output has dropped significantly. Conversely, at sites that recover easily with natural rainfall and where the impact on power generation is small, a decision may be made to avoid excessive cleaning and focus on data monitoring. Because cleaning also involves work safety and equipment protection considerations, you should not automatically wash as soon as dirt is visible; it is necessary to consider both the impact on generation and the risks associated with the work.

To determine that soiling is the cause of low power generation, it is important to confirm whether the environment is prone to soiling. Along with the timing of airborne dispersal, the on-site appearance, and recovery after rainfall, if the surrounding environment favors soiling, you should more realistically consider the impact of yellow sand (Asian dust) and pollen. Conversely, when the environment is unlikely to produce soiling and the decline is concentrated in only part of the system, it is safer to prioritize checking the equipment.

Verification procedures to avoid mistaking dirt for faults

Yellow sand and pollen soiling can cause reduced power output, but they should not be used as a reason to overlook faults or anomalies. In practice, what matters is to suspect soiling while at the same time isolating the possibility of an anomaly. When you receive a complaint of low power output, an effective workflow is to first look at the overall trend, then examine differences at the system level, check the generation curve on sunny days, and finally compare that with the onsite soiling patterns.

First, confirm whether the plant’s overall power generation is low or only a portion is low. If the decrease is overall, consider weather, irradiance, output control, and widespread soiling. If only a portion is decreased, prioritize checking shading, connections, fuses, string abnormalities, and equipment shutdowns. At this stage, it is important not to narrow the focus to soiling alone.

Next, check not only the daily generation totals but also the generation curve by time of day. If the panels are dirty from yellow sand or pollen, the entire curve tends to be gently lower. If it drops only at certain times, it’s likely shading; if it suddenly goes to zero, it indicates a shutdown; if it plateaus at a constant value, control or equipment settings may be responsible—these are other possible causes. The shape of the curve is an important clue for distinguishing the type of cause.

In addition, we cross-check the timing of yellow sand and pollen, the presence of strong winds, whether it rained, and on-site dust deposition. Even if the data look like soiling, if clear shadows, physical damage, or equipment abnormality indications are observed on site, prioritize those. Conversely, if no equipment abnormalities are found, the decline is similar across multiple sections, and dust is widely deposited on panels and surrounding equipment, strongly suspect the influence of soiling.

Don't forget to check error displays and alarm histories. On days with low power generation, if there are records of power conditioner shutdowns, communication losses, protective operations, or grid abnormalities, it is risky to explain the drop by soiling alone. Even if the values on the monitoring screen are low, you need to distinguish whether that is an actual decrease in generation or a loss of communication.

To avoid confusing soiling with a fault, it’s important not to make a judgment based on a single observation. Track changes over time — for example, the day you notice lower power output, after rainfall, after cleaning, and on the next sunny day. If it’s soiling, performance tends to vary with rain, cleaning, and dispersal conditions. If it’s a fault, the same system will tend to remain low, alarms will persist, and it will not recover.

Points to consider when deciding on cleaning or inspection

Even if contamination from yellow dust or pollen is suspected, whether to carry out cleaning immediately should be considered carefully. Cleaning may help restore power output, but if done incorrectly it can scratch the panel surface or lead to accidents such as electric shock or falls. Especially for live photovoltaic systems, the use of water, scaffolding, the work area, and ensuring worker safety are important.

Before considering cleaning, verify that the drop in power output is actually caused by soiling. Even if soiling is visible, if an equipment fault is the primary cause of the generation decline, cleaning will not improve it. Conversely, if the low output is caused by widespread soiling, does not recover after rainfall, and the power-generation curve on sunny days remains low, it is worth considering the potential effect of cleaning.

When making cleaning decisions, it is also important to define the target scope. Whether you clean the entire power plant at once, only inspect the most heavily soiled sections, or evaluate the effect in representative sections before expanding the scope will change the work plan. When verifying effectiveness, compare days with similar solar irradiance conditions before and after cleaning to check whether power output and the power curve have improved. Do not judge that cleaning was effective based solely on the fact that it was done; it is important to confirm the effect with numerical data.

Care should also be taken with cleaning methods. Tasks such as scrubbing with hard tools, using inappropriate chemicals, washing forcefully with high-pressure water, or suddenly applying cold water to a hot panel can place strain on the equipment. In actual work, follow the equipment specifications, management standards, and on-site safety rules, and request professional inspection or cleaning as necessary. Operations personnel should avoid carrying out dangerous tasks based on their own judgment.

Sometimes inspection should be prioritized before cleaning. When the drop in power generation is sudden, there is an abnormal display, burn marks or damage are visible, only a specific circuit is not generating power, there are abnormal noises or odors, or there is damage around the equipment, prioritize safety checks over cleaning. Even during yellow sand or pollen seasons, treating every decline as dirt can lead to serious abnormalities being overlooked.

Decisions on cleaning and inspection should consider not only the magnitude of power generation loss but also worker safety, equipment protection, the risk of recurrence, and seasonal factors. If soiling is minor, its impact on generation is small, and substantial rain is expected soon, continuing to monitor can be an option. Conversely, if the decline persists, the soiling has become firmly adhered, and recovery does not occur even after rain, it makes sense to plan an inspection and cleaning.

For continuous monitoring of low power generation

A decline in power generation due to soiling from yellow sand and pollen can be difficult to determine from a single inspection. Rather than rushing to inspect the site only after you notice low output, it is important to keep routine records that are easy to compare. Having regular generation data, clear-day curves, rainfall records, on-site photographs, cleaning history, and inspection records makes it easier to assess the impact of soiling.

Especially from early spring through early summer, yellow dust, pollen, strong winds, rain, and rising temperatures tend to coincide, increasing the number of factors that cause power generation to fluctuate. During this period, it is useful to pay attention not only to monthly generation but also to comparisons between clear-sky days. Because monthly generation is affected by the number of days with particular weather, the effects of soiling can be masked. Comparing the area around the peak and the overall generation curve on clear-sky days makes it easier to notice declines caused by surface soiling.

It is also important to have site-specific reference values for each power plant. Even with the same equipment, susceptibility to soiling differs depending on location, tilt, and the surrounding environment. At one plant, rain may be sufficient to restore performance, while at another, pollen can adhere and remain. By accumulating past generation performance and the local soiling patterns, you can create judgment criteria suited to that site.

To detect low power generation quickly, you need to combine multiple perspectives rather than viewing an anomaly as a single number. By linking total generation, generation by section, string-level differences, generation curves, post-rain recovery, and on-site photos, it becomes easier to determine whether the issue is soiling from yellow sand or pollen, equipment malfunction, or differences in weather conditions. If these pieces of information are managed separately, isolating the cause takes longer and can lead to unnecessary site visits and delayed responses.

The six indicators for judging whether a decline in power generation is caused by contamination from yellow sand or pollen are: whether the decrease is system-wide, the power-generation curve on sunny days, alignment with dispersal timing, recovery after rainfall, the on-site condition of deposits, and the tendency for recurrence in the surrounding environment. Rather than deciding based on any single indicator, it is safer and more realistic in practice to assume a high likelihood of contamination when multiple indicators are present.

To avoid overlooking low power generation and to distinguish soiling from yellow sand and pollen from equipment abnormalities, it is essential to manage on-site inspections and generation data together. Organizing daily generation status, inspection records, site photos, and cleaning history and keeping them in a state that allows chronological comparison makes it easier to prioritize cleaning and inspections. Although yellow sand and pollen are seasonal factors, the way generation declines appear can overlap with faults, shading, and communication failures, so it is important to make careful judgments by combining the data with on-site conditions.