5 Cleaning Decisions to Increase Power Generation｜Effects and Precautions

Table of Contents

• Don't decide whether to clean solely based on the presence of dirt

• Cleaning decision 1: Use power generation data to confirm whether the cause is soiling

• Cleaning decision 2: Check the pattern and extent of soiling on the panel surface

• Cleaning decision 3: Differentiate between shading, weeds, fallen leaves, and soiling

• Cleaning decision 4: Ensure the ability to compare power generation before and after cleaning

• Cleaning decision 5: Confirm safety and potential recurrence factors before carrying out cleaning

• Inspection items to check when cleaning alone does not lead to improvement

• Record-keeping to sustain power generation improvements

• Summary

Don't base cleaning decisions aimed at increasing power generation solely on the presence of dirt

When you want to increase the power output of a solar power system, the first measure that often comes to mind is cleaning the panels. Solar panels generate electricity by receiving sunlight at their surface, so if dirt or deposits are present on the surface, the light reaching the cells is reduced, leading to decreased power output. The types of soiling vary depending on the site environment: soil dust, pollen, yellow sand, bird droppings, fallen leaves, tree sap, dust from nearby construction, road-derived dust, and salt-containing dirt that easily adheres in coastal areas. If such soiling is the main cause of reduced power output, cleaning is an effective improvement measure.

However, decisions to carry out cleaning to increase power generation should not be based solely on whether it “looks dirty.” The cause of reduced power generation is not necessarily dirt alone. Shadows from weeds or trees, faults at connection points, cable damage, shutdown of conversion equipment, output curtailment, temperature increases, poor drainage, lack of inspection records, and other factors can combine to reduce power generation. Even if dirt is visible, if the primary cause of the power drop lies elsewhere, cleaning alone will not improve performance as much as expected.

For operations personnel searching "how to increase power generation," what matters is not deciding whether to clean based on intuition, but making a judgment by linking power generation data with on-site conditions. Check when generation is low, which equipment is underperforming, whether there is a difference compared with equipment under the same conditions, whether output is low even on sunny days, and whether dirt tends to remain after rain. Then see whether the dirt corresponds to the range of power output decline. Only after these checks will it be easier to determine cleaning priorities and the scope of work.

A common pitfall when deciding on cleaning is making the decision based only on the on-site appearance. Judgments such as cleaning the entire area because the panel surface looks slightly whitish, cleaning a wide area because there are fallen leaves, or immediately working on whole rows because there is dirt at the lower edge may seem straightforward at first glance. However, if you perform the same operations over areas where a decline has not been confirmed in power generation data, you increase the number of actions whose effects are hard to see. Conversely, even if there is no significant visible dirt, a band of dirt along the lower edge or bird droppings may be reducing the power generation of specific equipment.

To enhance the effectiveness of cleaning, it is necessary to record the condition before cleaning and verify how power generation changed afterward. Cleaning should not be the end; it is important to confirm whether it led to improved power generation and use that information for future decisions. By keeping before-and-after photos, the scope of the work, the work date, weather conditions, and power generation data, it becomes easier to determine to what extent soiling at the site affected power generation. A cleaning decision is not simply a judgment of whether to carry out the work, but a verification process to establish the basis for power generation improvement.

Cleaning Decision 1: Verify if soiling is the cause using power generation data

Before considering cleaning, first check the power generation data. If you only look at monthly or annual generation, it becomes difficult to determine whether dirt, weather or shading, or equipment-related factors are the cause. Even if the result is the same—low generation—the suspected cause changes depending on whether it is only low in the morning, the midday peak fails to rise, it drops only in the evening, or there is a sudden dip during the day. When deciding on cleaning, it is important to first look at the generation curve by time of day.

If soiling is causing reduced power output, the overall increase in output can be muted even on sunny days. If only certain rows or pieces of equipment are heavily soiled, those areas may show lower output than their surroundings. On the other hand, if output is low only in the morning it may indicate eastern shading, if only in the evening western shading, if the generation curve suddenly drops it may indicate equipment shutdowns or connection failures, and if the midday peak is flattened and capped it may indicate output curtailment or equipment capacity limits—so causes other than soiling should also be considered. Just because soiling is visible does not necessarily mean immediate cleaning should be the top priority.

Comparisons at the equipment/unit level are also necessary. Compare installations with the same orientation, the same tilt, a similar number of panels, and the same shading conditions to check whether only a specific area is underperforming. If you simply compare installations with different conditions, you may mistake a normal difference for an abnormality. If the area of low power generation corresponds to the area where soiling is noticeable on-site, improvements from cleaning are more likely. Conversely, if soiling is visible but does not correspond to the area of reduced power output, you should prioritize checking causes other than cleaning.

When examining power generation data, compare sunny days with other sunny days whenever possible. On cloudy or rainy days, output fluctuates greatly due to cloud movement, making it difficult to determine whether a decline is caused by soiling or by weather-related variability. Using a sunny day’s generation curve as a baseline makes it easier to see an overall loss in growth from soiling, time-of-day dips caused by shading, and sudden drops due to equipment shutdowns. When deciding whether to clean, it is important to choose and compare days with similar solar irradiance conditions.

Also check whether the drop in power output occurred suddenly or progressed gradually. A decline caused by the buildup of dirt can emerge gradually over several weeks to several months. Localized deposits such as bird droppings or fallen leaves can sometimes cause a sudden impact at a specific time. Conversely, equipment shutdowns or connection faults can appear as a sudden plunge. Reading the pattern of the decline makes it easier to decide whether to clean or to check the equipment and connections first.

Cleaning Assessment 2: Check the type and extent of soiling on the panel surface

Once you have determined the extent of the decline from the power generation data, inspect the condition of the panel surfaces on site. Dirt can mean thinly spread dust, banded lower-edge grime, localized deposits like bird droppings, accumulation of leaves and small branches, or adhesion of tree sap and particulate dust—each type affects power generation and cleaning priority differently. To increase power generation, it is important not just to see that the panels are “dirty,” but to confirm which types of dirt are present, over what area, and to what degree.

Particularly noteworthy are the band-like stains that remain along the lower edge of the panel and near the frame. It is often assumed that rain will naturally wash them away, but in reality the flow of rainwater can collect dirt at the bottom edge and leave it there. On panels with a shallow tilt, water does not drain well and dirt tends to accumulate. Even stains that are not noticeable from a distance can affect power generation if they cover part of a cell. When deciding whether to clean, carefully check not only the overall color of the panel but also the lower edge, the corners, and the areas around the frame.

Localized deposits such as bird droppings and fallen leaves should not be overlooked. Unlike dirt that spreads thinly over the entire surface, these cover specific areas more heavily and act as partial shading that impedes power generation. Rows near trees, around structures where birds tend to perch, rows that are often downwind, and areas near unpaved paths are more prone to dirt and deposits. If only some systems have lower power output, focus on inspecting the panel surfaces around those systems.

When checking the extent of soiling, it is also important not to broaden the cleaning scope too much. Rather than cleaning all panels the same way, prioritize equipment that has shown a drop in power generation, rows where soiling is concentrated, areas where lower-edge soiling is noticeable, and locations with heavy bird damage or leaf accumulation. If you decide the cleaning scope by linking it to power generation data, it will be easier to verify the effect after the work. If it is unclear which areas were cleaned, it will be difficult to determine the cause even if power generation improves.

When deciding on cleaning, we also check for conditions that make soiling likely to recur. We look at whether dust easily rises from unpaved walkways, whether leaves or sap are likely to fall from surrounding trees, whether there are places where birds tend to gather, and whether the panel tilt makes it difficult for rainwater to drain. Even if soiling is removed once, it will recur in the same place if the conditions that cause it do not change. To increase power generation, it is important not only to remove soiling but also to record locations prone to soiling as management targets.

Cleaning Decision 3: Distinguish shadows, weeds, and fallen leaves from dirt

When considering cleaning to increase power generation, it is important to distinguish soiling from shading. Even if the panel surface appears dirty, if the primary cause of reduced generation is shading from weeds or trees, cleaning alone will not lead to sufficient improvement. Conversely, even if shading seems present, the actual cause of reduced generation may be soiling at the lower edge or bird droppings. On-site inspections should assess soiling, shading, fallen leaves, and weeds separately and correlate these findings with the power generation data.

Weeds are a common factor that readily leads to reduced power generation. Even if there is no problem in winter or immediately after installation, they can grow rapidly from spring to summer and cast shadows on the lower edge of panels or on the front row. Even if the vegetation does not touch the panels, shadows lengthen when the sun is low in the morning and evening. Furthermore, when weeds become overgrown they reduce ventilation, block inspection walkways, and make it difficult to check around equipment. When deciding on cleaning, check not only the panel surface but also the condition of the vegetation in front of and beneath the panels.

It is also necessary to check shadows from trees and surrounding structures. Trees may have little impact when first installed, but they can grow over several years and reduce power generation. Trees located especially on the south, east, and west sides can cast shadows on the panels depending on the time of day. Surrounding structures, fences, signs, and monitoring poles can also cast shadows at certain times. Even if no shadow is visible at noon, large shadows may appear in the morning and evening.

Fallen leaves have both soiling and shading characteristics. Dry leaves may simply be resting temporarily on the panels, or they may stick after rain and remain at the lower edge, causing soiling and water pooling. In rows close to trees, leaf debris and tree shade can occur simultaneously. In such cases, it is necessary not only to clean, but also to check the source of the leaves, overhanging branches, wind direction, and drainage flow.

When checking for shadows, it is important to inspect the site at the times indicated by the power generation data when output is reduced. If generation is low in the morning, check for morning shadows; if it is low in the evening, check for evening shadows. If the midday peak does not reach expected levels, also consider candidates such as soiling, temperature, and equipment-side limitations. To avoid making a mistaken cleaning decision, do not judge by soiling alone—confirm shadows and the surrounding environment together.

Cleaning Decision 4: Ensure that power generation can be compared before and after cleaning

If you carry out cleaning, it is important to set things up so you can compare power generation before and after the cleaning. Cleaning is not an end in itself. Since it is performed as a measure to increase power generation, you need to confirm how power generation changed as a result of the cleaning. If you have not recorded the pre-work condition, even if power generation improves after cleaning it will be difficult to determine whether the improvement was due to the cleaning or to differences in weather or solar irradiance conditions.

Before cleaning, record power generation data, the equipment in question, the soiling condition, on-site photos, and weather conditions. Clearly documenting which row was cleaned, the extent of the soiling, and which equipment had a confirmed drop in power generation makes post-work comparisons easier. This is especially helpful on sites with multiple pieces of equipment, where being able to compare cleaned and uncleaned units under the same conditions makes it easier to assess cleaning effectiveness.

When making comparisons, choose days with as similar solar irradiance conditions as possible. If the weather differs greatly before and after cleaning, a simple comparison of power generation is not possible. Comparing clear-sky days with other clear-sky days, or days with similar weather conditions, makes it easier to see changes due to cleaning. Also, compare installations under the same conditions to check whether only the cleaned installation shows an improving trend. If power generation improved, it may indicate that soiling had a significant effect at that site.

There are cases where power generation does not improve significantly after cleaning. In such cases, instead of concluding that the cleaning was pointless, verify whether other causes remain. Shadows, poor connections, shutdowns of power conversion equipment (inverters), output curtailment, temperature increases, poor drainage, and so on may be related to the reduction in power generation. The comparison results after cleaning are important material for narrowing down which causes should be checked next.

By continuing to compare before-and-after cleaning results, site-specific trends become apparent. At some sites, soiling has a large impact on power generation, while at others factors such as shading or equipment-side issues are more significant. If you record the rows that tend to get dirty, the seasons when cleaning is most effective, and the locations prone to recurrence, it will be easier to decide on cleaning for future occasions. To increase power generation, it is important not to treat cleaning as a one-off task but to incorporate it into operations including verification of effectiveness.

Cleaning Decision 5: Confirm Safety and Causes of Recurrence Before Implementation

When deciding on cleaning, always confirm not only the effect on power output but also safety. Solar power generation equipment is electrical equipment installed outdoors, and even cleaning the surface of panels can, if the work environment or methods are incorrect, lead to equipment damage or accidents. You should avoid, in pursuit of increasing power output, forcing work in locations with unstable footing, omitting precautions required for electrical equipment, or cleaning in ways that will damage the panels.

When cleaning, it is important to choose a method that will not damage the panel surface. Rubbing hard with stiff tools or rubbing while sand or small stones remain can damage the surface. You should also avoid performing sudden work during times when the panels are hot. Cleaning is not a task to make the appearance tidy, but a maintenance operation to restore the light-receiving condition and enable long-term, stable use of the equipment. Decisions need to be made with safety and equipment protection as the premise.

Confirming the safety of the work area is also essential. If walkways are muddy, grass makes footing hard to see, slopes or steps are nearby, cables are exposed, or there are obstacles around equipment, an environmental check is necessary before cleaning work. Rather than looking only at the panel to be cleaned, confirm that workers can move safely, will not come into contact with equipment or wiring, and that there are no hazards in the surrounding area.

Checking for causes of recurrence is also important. If the same areas get dirty again shortly after cleaning, factors such as drainage, wind direction, surrounding trees, unpaved pathways, or a tendency for birds to gather may be involved. In locations where soil and sediment wash in after rain, dirt tends to accumulate at the lower edges of panels and around cables. Where puddles remain, weeds are more likely to grow, which can cause shading and poor ventilation. If you only remove the dirt without examining why it keeps recurring, improvements in power generation are likely to be temporary.

When performing cleaning, keep records after the work. Recording the cleaned area, the date of work, the type of soiling, after-work photos, and any locations that require reinspection will help inform decisions next time. If you can identify locations that are prone to recurrence, you can check them before power generation drops significantly. What is required at the end of the cleaning decision is not only to complete the work safely but also to leave information that will help improve power generation going forward.

Inspection Items to Check When Cleaning Alone Does Not Improve the Situation

If cleaning does not significantly improve power output, you need to check for causes other than dirt. Causes of reduced power output extend beyond the panel surface to include shading, connection points, cables, conversion equipment, temperature conditions, drainage conditions, and so on. If power output does not recover after cleaning, the cleaning scope may have been insufficient, but it is also possible that dirt was not the primary cause of the power loss in the first place.

The first thing to check is shadows. If power output is low only in the morning or evening, shadows from weeds, trees, or surrounding structures may be affecting it. Even if an on-site inspection at noon shows no problems, long shadows can appear in the morning and evening. If power output drops at the same times even after cleaning, suspect shadows rather than dirt. Record the times when shadows occur, their sources, and the equipment that is shaded, and use that information to guide weeding, pruning, and layout checks.

Next, check for power generation differences at the string level. If only certain strings remain low even after cleaning, possible causes include faults in connections or cables, localized shading, or panel-level abnormalities. If a string is continuously lower compared with equipment under the same conditions, it is necessary to check the electrical path. However, inspections of connections, cables, and the inside of equipment must prioritize safety and, if necessary, be escalated to professional inspections. On-site personnel should organize information on the affected equipment, time of occurrence, changes in power generation, photographs, and the surrounding environment.

Check for shutdowns of the conversion equipment and for output curtailment. If the time when generation drops matches the shutdown logs and alarm history, it becomes easier to narrow down the possible causes. If the generation curve on sunny days plateaus, factors such as output curtailment, equipment capacity limits, temperature rise, or measurement anomalies may be involved. If the midday peak does not increase even after cleaning, it is important to check equipment-related causes and the temperature environment.

Drainage and terrain must not be overlooked. If the same spots become dirty soon after cleaning, the flow of rainwater, sediment inflow, puddles, or muddy access paths may be involved. If poor drainage is left unaddressed, dirt and weeds will recur and the effects of cleaning will not last. To increase power generation, it is necessary not only to remove dirt by cleaning but also to create a site environment where the same dirt is less likely to occur.

Record Keeping to Sustain Power Generation Improvements

To continuously improve cleaning decisions aimed at increasing power output, record keeping is indispensable. It is important to record not only whether cleaning was performed, but also why it was done, where it was cleaned, and how power output changed before and after cleaning. Without records, when the same issue occurs again it will be difficult to determine whether the cause was soiling, shading, or an equipment problem.

What should be recorded are the locations of equipment with low power output, rows prone to soiling, the scope of cleaning, locations where shadows occur, places where water pools, spots suspected of poor connections, locations that were repaired, before-and-after photos of the work, the work performed, and the need for reinspection. If you add too many items to the records it becomes difficult to maintain them in the field, so first aim for a state where, when you review later, you can see where something happened and how it was addressed.

Linking power generation data with on-site photographs improves the accuracy of cleaning decisions. If the locations of equipment with low output overlap with areas where soiling occurs, the justification for cleaning becomes clear. If generation improves after cleaning, it can be concluded that soiling had a significant impact at that site. If it does not improve, this provides material to investigate other causes such as shading, connections, equipment, or drainage. Having records speeds up decision-making in future instances.

When multiple people are responsible for management, it is also important that they can share the exact same location. In a large power plant, rows of similar equipment are often arranged, so photos alone can make it difficult to identify the location. If equipment numbers, location information, photos, and work histories are recorded together, on-site staff, managers, inspectors, and repair personnel will find it easier to confirm the same location. Cleaning decisions become more reproducible when they are recorded and shared, rather than relying solely on one person's experience.

Improving power generation is not something that can be completed with a single cleaning. Even if you clean, dirt will reaccumulate; even if you remove weeds, grass will grow back; trees will grow; drainage paths will change. That is why it is necessary to repeatedly make cleaning decisions, carry them out, verify their effectiveness, and update records. If you can identify locations where dirt is likely to appear, places where cleaning has the greatest effect, and periods when recurrence is likely, it becomes easier to take action before power output drops significantly. Record management is the foundation for steadily increasing power output.

Summary

When deciding on cleaning to increase power output, it is important not only to determine whether panels are dirty but also to assess the causes of the power decline and the likely effect of cleaning. In solar power generation, it is not possible on-site to increase the amount of solar irradiance itself. However, you can improve power output by bringing the system closer to a state where the received solar irradiance is converted to electricity without waste. To do so, you need to check in order: power generation data, the type and extent of soiling, shading and weeds, comparisons before and after cleaning, safety, and causes of recurrence.

When considering cleaning, first check the power generation data by time of day and by equipment. Determine whether output is low even on sunny days, whether only specific equipment is underperforming, or whether there are differences compared with equipment under the same conditions. Then inspect for soiling at the lower edges of panel surfaces, bird droppings, fallen leaves, and dust. If the soiling corresponds to the areas of reduced output, cleaning should be given a high priority. Conversely, if shading, equipment shutdowns, or poor connections are the main causes, cleaning alone is unlikely to restore generation.

Also, cleaning must be carried out with safety as a prerequisite. Avoid operations that could damage the panels, work performed with unstable footing, and any work that omits electrical-safety precautions. By recording power generation before and after cleaning, on-site photos, the cleaning area, and weather conditions, you can verify the effectiveness of the cleaning. If the effect is evident, it can inform decisions about future cleanings; if the effect is small, it provides an opportunity to investigate other causes.

In large-scale power plants, a system for accurately sharing problem locations is especially important. Recording rows prone to soiling, areas where shadows occur, places where water accumulates, the scope of cleaning, abnormal strings, repair locations, and inspection photos together with location information makes it easier for stakeholders to confirm the exact same spots. By combining power generation data with on-site location information, it becomes easier to explain priorities for cleaning, weeding, and repairs, and it also streamlines follow-up checks for recurrence.

If you want to continue making cleaning decisions to increase power generation based on field data, leveraging LRTK is also effective. LRTK, an iPhone-mounted GNSS high-precision positioning device, is useful for recording inspection locations within a solar power plant, areas prone to soiling, shadow occurrence points, poor drainage locations, cleaning extents, abnormal equipment, repair locations, and on-site photos together with high-precision positional information. By keeping the rationale and results of cleaning decisions with location information, it becomes easier to pursue power generation improvements based on field data rather than on intuition.