6 Cleaning and Inspection Timings to Prevent Power Generation Decline

When managing a solar power system, if you feel that power generation is low, before immediately suspecting equipment failure you should check the timing of cleaning and inspections. Power generation fluctuates due to multiple factors such as weather, season, solar irradiance, temperature, shading, dirt, and the condition of the equipment. Therefore, even if there are days when generation appears to have decreased, it is not necessarily an abnormality. On the other hand, leaving dirt or minor faults unaddressed can cause you to miss out on the power generation you should be getting. The important thing is not to judge changes in generation by feeling alone, but to determine the timing of cleaning and inspections according to the season and site conditions.

Table of Contents

• To prevent a decline in power generation, managing the timing of cleaning and inspections is important.

• Check after early spring pollen and yellow sand have settled.

• Check the condition of drainage, dirt, and shading before the rainy season.

• Check the exterior and surrounding environment after typhoons, strong winds, or heavy rain.

• Organize the factors causing power generation losses before the high solar radiation period in summer.

• Check for localized soiling during the season when fallen leaves and bird damage increase.

• Reconcile annual data with on-site conditions at year-end and at the end of the fiscal year

• If consecutive days of low power generation occur, perform an unscheduled inspection.

• Record cleaning and inspections to inform the next decision.

Managing the Timing of Cleaning and Inspections Is Crucial to Prevent Power Generation Decline

The power output of solar photovoltaic systems fluctuates daily even when the equipment is operating normally. Even on clear days, if thin clouds spread out the output tends not to increase much, and on hot days the output can be slightly lower even when solar irradiance is sufficient. If rainy or cloudy weather continues, power generation will be low, and in winter the generation pattern changes from summer due to shorter sunshine hours and lower solar elevation. For operational personnel, it is important to distinguish changes caused by natural conditions from decreases in output caused by soiling or equipment faults.

Cleaning and inspections are not something that can be improved simply by increasing their frequency. Performing work indiscriminately raises safety risks on roofs and at heights and can also damage equipment. Especially for residential solar systems and building-mounted equipment, the basic approach is to combine checks that can be done from the ground, verification of generation data, and requesting a specialist when necessary rather than forcibly climbing up to inspect. Even for power plants and large installations, whether cleaning is required depends on how each site becomes soiled, the surrounding environment, the inspection regime, and past generation data.

What matters in preventing a drop in power generation is not "scrambling to deal with dirt after it appears," but "identifying in advance the periods when soiling or faults are likely to occur." If you decide ahead of time when to check—such as periods with heavy pollen and yellow sand, before the rainy season, after typhoons, before the summer high-solar-radiation period, times of heavy leaf fall, and year-end or fiscal-year-end review periods—you will find it easier to trace the causes of low power generation.

Also, when you notice a drop in power generation, you should avoid immediately trying to fix it with cleaning alone. While dirt on the panel surface can be the cause, other factors may be hiding behind the problem, such as shading, growth of nearby trees, abnormalities around wiring, equipment shutdowns, overlooked measurement values, or communication failures. Treating cleaning and inspection as a combined process makes it easier to detect a wide range of causes of reduced power generation, not just surface soiling.

Things to check after early-spring pollen and yellow dust have settled

Early spring is an important time for cleaning and inspection to prevent a drop in power generation. During this period, solar irradiance gradually increases and power generation tends to rise, but at the same time pollen, yellow sand, and dust readily adhere to the surface of the panels. The extent varies depending on the region and installation environment, but factors such as the angle of the roof or racking, dust from nearby roads, and the influence of nearby fields or development sites can make surface soiling noticeable.

Pollen and yellow sand can be washed away to some extent by rain, but depending on the way the rain falls, streak-like dirt can remain on the surface. Dirt tends to collect especially at the lower edge of panels and around the frame, and when it dries it can remain as a thin film. If days of low power generation continue and output remains lower than expected compared with past data and weather conditions for the same period, it is worth checking for the presence of dirt.

However, it is premature to attribute all declines in power output in early spring to soiling. Spring brings large changes in weather, and even when it appears sunny, solar irradiance can be weakened by haze or thin clouds in the air. Power generation can also be affected by temperature and wind conditions. Therefore, before considering cleaning, it is important to first compare with days that have similar weather and determine whether the decline is a sudden drop or a seasonal fluctuation.

During early-spring inspections, you should check not only for widespread dirt on panel surfaces but also for bird droppings and stubborn deposits that are hard to remove. Localized dirt, even if small in area, can adversely affect power generation. If you visually confirm obvious dirt within the area you can access, do not attempt to clean it yourself; address it using safe methods appropriate to the type of equipment and the installation location.

By conducting an inspection at this time, you can enter the spring-to-early-summer period—when power generation increases—in better condition. Early spring is not only the time to determine whether cleaning is necessary, but also an opportunity to assess dirt that accumulated during winter and changes in the surrounding environment. Checking before you notice a drop in power output reduces the effort required to trace the cause later.

Check drainage, soiling, and shading conditions before the rainy season

A pre-rainy-season inspection is important as preparation before the period of continuous rain. Because solar power generation drops on rainy days, the rainy season tends to be a time when looking only at generation data can be worrying. However, recording the system’s condition before the rainy season helps distinguish the natural decrease caused by rain and clouds from declines due to equipment or environmental issues.

Before the rainy season, what you should check is not limited to dirt on the panel surface. You should also inspect the drainage flow, surrounding vegetation, the condition around the mounting structures, spots where rainwater tends to accumulate, and any visible abnormalities around the wiring. When rain continues, some dirt will be washed away, but other soiling can occur from splashing mud, puddles, and accumulated fallen leaves. In areas with poor drainage, dirt tends to remain after the rain, which can cause a decrease in power generation.

Also, the period just before the rainy season is a good time to check the growth of surrounding trees and grasses. Because plant growth progresses from spring to early summer, places that were not a problem before may become shaded. Shadows cast on solar panels are a factor that is easily overlooked as a cause of reduced power generation. In particular, shadows that occur during the low sun angles of morning and evening, or only in certain seasons, can be hard to notice if you look only once during the daytime.

When checking shadow conditions before the rainy season, it is important to compare the times when power generation drops with the on-site conditions. If there is a pattern such as low generation only in the morning, only in the afternoon, or output fluctuations during certain time periods despite clear weather, that can be a clue to suspect shadows from surrounding objects. Rather than looking only at the daily total generation, checking changes by time period makes it easier to identify the cause.

In inspections before the rainy season, it is important not only to decide whether to carry out cleaning but also to be conscious of establishing baseline criteria that can be compared after the rainy season ends. If you record site conditions with photos and notes, it will be easier to determine when changes began if you later feel that power generation is low. The more unstable the weather, the more advance condition checks improve the accuracy of power generation management.

Inspect the exterior and surrounding environment after typhoons, strong winds, and heavy rain

After a typhoon, strong winds, or heavy rain, it is the right time to carry out an ad-hoc inspection to prevent decreases in power generation. Strong winds and rain can cause dirt containing mud and salt to adhere to the surfaces of the panels, and debris from the surrounding area can be blown in. Even if the power generation equipment itself has not sustained major damage, fallen leaves, branches, pieces of plastic, sediment, and bird damage can locally obstruct power generation.

Particular attention should be paid to cases where power generation remains low after a typhoon. If, after the weather clears, output is clearly lower than on past sunny days, a visual inspection is necessary. Within visible range, check whether there is any debris on the panel surfaces, whether nearby trees have fallen or tilted and are creating new shading, and whether there are any changes around the mounting racks or on the roof. Since inspections at height or on roofs involve danger, avoid approaching or climbing in an unsafe manner and carry out checks only within a safe range.

After heavy rain, dirt can be washed away and power generation performance may improve. However, in areas where soil and sediment wash in or where there are nearby unpaved surfaces, splashing rainwater can leave dirt on the lower parts. If the rain is too heavy, fallen leaves and small branches from the surroundings can also collect and accumulate around drainage areas and near the lower edges of panels. Such dirt is hard to notice from the overall appearance, so it is important to check it together with changes in power output.

After strong winds, you may be concerned about surface scratches or damage caused by flying debris. However, fine scratches and internal conditions can be difficult to judge by visual inspection alone. Even if there are no visible abnormalities, if low power output persists, if abnormal readings appear on equipment displays, or if there are sudden changes in generation data, you should arrange for a professional inspection. Because touching equipment based on your own judgment can be dangerous, it is important to clearly define the scope of your checks.

After inspections following typhoons, strong winds, or heavy rain, it is necessary to check not only the power generation equipment but also changes in the surrounding environment. Temporary structures on adjacent land, signs, trees, and changes around buildings can create shadows or soiling that differ from before. After natural disasters, confirming both abnormalities in the equipment itself and changes in the surrounding environment can reduce the likelihood of overlooking a decrease in power generation.

Summarize the causes of power generation losses before the high solar radiation period in summer

Summer is a season with high solar irradiance, so higher power generation is often expected. Therefore, it is important to perform cleaning and inspections before the peak of summer and identify factors that can lead to generation losses. Even if you feel the generated power is low, in summer output can be reduced by rising temperatures, so clear skies do not necessarily mean higher generation. Still, when factors such as dirt, shading, and equipment downtime overlap, the gap from expected values can become large.

In pre-summer inspections, check for dirt accumulated from early spring through the rainy season, shading caused by plant growth, signs of poor drainage, and fallen leaves or foreign objects that have collected under and around the panels. In particular, for ground-mounted or low-mounted installations, weed growth can affect power generation. Conditions such as grass shading the panels, obstructing ventilation, or blocking inspection walkways not only reduce power output but also make maintenance operations more difficult.

Summer is also a period when power generation data are easier to compare. During stretches of clear weather, comparing generation on days with similar conditions makes it easier to spot abnormalities. Of course, temperature, cloud cover, and solar irradiance vary from day to day, so conditions are never exactly the same. Even so, if you see a clearly lower trend compared with the same period in past years, or if only certain equipment shows reduced output, that provides grounds for prioritizing inspections.

When planning cleaning before summer, it's important to pay attention to safety during the work. Working in high temperatures carries a risk of heatstroke, and panel surfaces and roofing materials can become hot. Improper handling of the water and tools used for cleaning can also place stress on the equipment. Therefore, it's important to determine whether cleaning is necessary and, if so, carry it out using methods that match the equipment specifications and installation conditions. Rather than attempting to clean it yourself unnecessarily, it's safer to use power generation data and visual inspections to determine whether professional intervention is necessary.

If you identify the factors that cause generation losses before the period of high solar radiation in summer, it will be easier to investigate the causes when you feel the power output is low. By checking for dirt and shading in advance and preparing the surrounding environment, you are less likely to miss generation opportunities during the height of summer. Especially when managing multiple systems or power plants, regularizing pre-summer inspections makes it easier to grasp each site's weaknesses.

Check for localized soiling during seasons when leaf fall and bird damage increase

From autumn through winter, it's a season to be mindful of localized soiling caused by fallen leaves and bird activity. As causes of reduced power generation, not only a thin film of dirt across the entire panel but also fallen leaves or bird droppings attached to certain areas can have an impact. Localized soiling is easy to overlook from a distance, and even if the whole looks clean, specific sections may be adversely affecting power generation.

In areas with many fallen leaves, debris tends to accumulate not only on the panel surface but also at the lower edges of panels, around the frames, around the mounting structures, and in drainage paths. When fallen leaves repeatedly get wet and dry, they can leave deposits that are difficult to remove. Also, if nearby trees are growing, not only the fallen leaves but shadows from branches can reduce power output. In autumn the sun angle changes, and places that were not shaded in summer can become shaded, so season-specific inspections are necessary.

Regarding bird damage, items to check include bird droppings and nesting materials. For residential solar systems, problems can be hidden in parts of the roof that are not visible, making assessment from the ground difficult. If low power generation persists, bird activity around the roof is noticeable, or soiling repeatedly occurs in specific locations, these can prompt an inspection. However, because appropriate bird damage countermeasures vary depending on the condition of the building and equipment, it is important not to make changes unnecessarily.

During inspections at this time of year, changes in power generation data are also checked. After autumn, daylight hours shorten and power generation naturally declines, which makes it easy to confuse seasonal variation with an abnormality. Expecting the same output as in summer can cause unnecessary anxiety. Therefore, it is necessary to compare with past data from the same month and with days of similar weather to determine whether the decline is natural or due to soiling or shading.

The advantage of carrying out inspections during the season when fallen leaves and bird damage increase is that you can detect localized power losses earlier. Even if it’s not a major failure, small dirt and shadows can accumulate and affect annual power generation. In particular, if you find that "power output is low" every year in the same season, suspect and check for season-specific dirt or shading, as this can make it easier to find clues for improvement.

Reconcile annual data with on-site conditions at year-end and fiscal year-end

Year-end and the end of the fiscal year are ideal times to conduct review inspections to prevent declines in power generation. If you only look at daily generation, you tend to be distracted by temporary bad weather or short-term fluctuations. However, by examining data over the course of a year, you can identify when generation was low, which seasons are prone to drops, and whether any anomalies occur only during specific periods.

When reviewing annual data, it is important not to judge based solely on the simple total generation. A year with poor weather can have lower overall generation, and seasonal generation trends also vary depending on installation conditions. What matters is comparing with the same period in the past or with periods that had similar weather to see if there are any unusual changes. Trends such as a particularly large drop in a specific month compared with the previous year, periods when output does not rise even on sunny days, or weaker generation in the morning and evening than before can serve as a basis for arranging on-site inspections.

At year-end or fiscal-year-end on-site inspections, we organize the need for cleaning, changes in shading, changes in the surrounding environment, inspection history, and whether previously occurring malfunctions have recurred. We check, one by one, elements that might affect power generation—such as tree growth, changes to nearby buildings or equipment, the condition of the ground and around the roof, and changes in the visible range of wiring and equipment. Small changes that are often missed during routine inspections become easier to notice when viewed as part of an annual review.

At this stage, it is also useful to cross-check the dates when cleaning and inspections were carried out with changes in power generation. Checking whether generation trends improved after cleaning, whether there was no change after inspections, or whether specific work was linked to improvements in power output will make it easier to draw up next year’s management plan. If power generation does not improve even after cleaning, you should suspect factors other than dirt. Conversely, if improvement is seen after cleaning during a particular period, you can make that period a focus for management in future cycles.

Year-end and fiscal-year-end inspections are not merely the closing of tasks but preparation to prevent a decline in power generation over the coming year. By compiling daily data, on-site photos, inspection notes, and cleaning histories, management becomes easier without relying on individual judgment. At sites where multiple personnel are involved, keeping records makes handovers smoother and speeds up the initial response when power generation is low.

Conduct an unscheduled inspection when consecutive days of low power generation occur

In addition to regular cleaning and inspections, if several consecutive days of low power generation occur, you should consider an unscheduled inspection. What’s important is not to assume there is a fault just because generation was low for a single day. There are many reasons output can drop: cloudy skies, rain, thin clouds, temperature, snowfall, haze, temporary shadows in the surroundings, and so on. Begin by checking the weather and solar irradiance conditions to determine whether the variation falls within natural fluctuations.

Guidelines for considering an unscheduled inspection include situations such as when it is sunny but generation remains clearly lower than under the same past conditions, when only certain equipment has low output, when output suddenly drops and does not recover, or when there are abnormalities in device displays or notifications. Whether residential or commercial, if the reduction in generation continues, you need to check multiple possible causes in sequence, such as soiling, shading, equipment shutdown, measurement errors, and communication failures.

In an ad-hoc inspection, we first clarify how to view the power generation data. By looking not only at daily totals but also at generation curves by time of day, it becomes easier to see the likely direction of the cause. If generation is low only in the morning, there is a dip only at midday, it suddenly drops in the afternoon, or it is low throughout the day, the suspected factors differ depending on the pattern of the drop in generation. A decrease during specific time periods may suggest the influence of shading, while a decrease throughout the day can prompt checks for dirt or equipment-related causes.

Next, carry out a check of the areas visible on site. Inspect the panel surfaces for any noticeable dirt or foreign objects, check whether nearby trees or buildings are casting shadows, whether fallen leaves or branches have accumulated, and whether any equipment displays show abnormalities. If you need to approach heights or electrical equipment, do not take unnecessary risks. If you cannot determine the cause by visual inspection or if a decline in power generation persists, it is safer and more reliable to arrange for a professional inspection.

The purpose of an unscheduled inspection is not to immediately determine the cause. It is to distinguish whether a low power output is due to the weather, soiling or shading, or whether an equipment inspection is necessary. If you perform cleaning only without gathering information, it will not lead to improvement if the cause lies elsewhere. Conversely, by combining data with on-site conditions, it becomes easier to narrow down the necessary actions.

Record cleaning and inspections to inform the next decision

Record-keeping is as important as the cleaning and inspections themselves for preventing declines in power output. By recording when, where, and under what conditions inspections were carried out, you can make quicker decisions the next time you notice low power generation. Without records, it becomes difficult to review when soiling began, whether performance improved after cleaning, or when shading first occurred.

It is helpful to record power generation data, weather, site photos, the details of cleaning and inspections, any observed changes, and points to check next time. What matters is not producing a tidy report but leaving information that can be compared later. For example, even basic records—taking photos from the same position, checking shadows at the same time of day, and comparing power generation trends before and after cleaning—will improve the accuracy of root-cause analysis.

Especially when multiple people are responsible for operations, whether records are kept has a major impact on management quality. Relying solely on staff experience makes judgment criteria prone to vary from person to person. If records are kept, insights such as "power generation was low at this time last year," "it improved after cleaning," "this spot tends to accumulate fallen leaves," and "there are locations that must always be checked after a typhoon" can be shared. This makes it less likely that the task of identifying the causes of low power generation will become dependent on individual personnel.

Cleaning and inspection records also inform the next work plans. For sites where dirt becomes noticeable every early spring, you can plan checks after pollen and yellow sand events. For sites prone to debris entering after typhoons, you can standardize temporary inspections following strong winds. For sites with heavy leaf fall, focusing inspections from autumn through winter makes it easier to prevent a decline in power generation. Records are not just a log of past work but a resource for reducing future generation losses.

When you suspect power generation is low, deciding whether to clean, inspect, or simply monitor requires both data and the on-site condition. Daily generation figures alone cannot determine the cause, and site photos alone cannot fully assess the impact on generation. By recording both together, it becomes easier to get closer to the cause of the reduced power generation.

In managing solar power generation, it is essential to set the timing for cleaning and inspections and to continuously monitor generation data and site conditions. By checking at milestones—early spring, before the rainy season, after typhoons, before summer, during the leaf-fall period, and at year-end or fiscal-year-end—before you feel the output is low, you can reduce unnecessary anxiety and make timely decisions about required actions. To grasp changes in generation more accurately, it is important to continuously keep records of cleaning and inspections, generation data, and site photographs, and use them as reference for the next decision.