Four Revisions to Solar Power Generation Calculations to Account for Panel Soiling

When calculating solar power generation, factors such as solar irradiance, panel capacity, tilt angle, azimuth (orientation), and conversion efficiency tend to be relatively well considered. On the other hand, the way panel soiling is handled often causes discrepancies in real-world operation. Even if calculations predict sufficient generation, surface deposits — sand and dust, pollen, bird droppings, fallen leaves, exhaust-related grime, salt, residual dirt after snowfall, and so on — can lead to significant deviations from measured values. Especially when generation calculations are used for financial assessments, maintenance planning, budget vs. actual management, or decisions on equipment improvements, it is important not to dismiss soiling as mere error but to clarify how it will be accounted for in the calculation assumptions.

In this article, we explain four items that practitioners should review to incorporate panel soiling into solar power generation calculations. The aim is to move toward an assessment of generation that matches site conditions, without overestimating or underestimating the impact of soiling.

Table of Contents

• Include panel soiling as a loss factor in the calculation conditions

• Examine the types of dirt and the times when they are likely to occur separately

• Reassess the impact of contamination based on differences from measured data.

• Update calculation conditions before and after cleaning and inspection.

• Summary

Include panel soiling as a loss factor in the calculation conditions

In calculating solar power generation, the basic approach is to first estimate theoretically how much electricity can be generated and then subtract various losses to arrive at a realistic figure. Even with the same solar irradiance, if the panel surface is dirty, light has a harder time reaching the cells and power output tends to decrease. Therefore, it is natural to treat panel soiling as one of the loss factors in power generation calculations.

However, losses due to soiling are not uniform. They vary depending on installation location, surrounding environment, tilt angle, frequency of rainfall, wind direction, ground conditions, traffic volume on nearby roads, and the presence of farmland or factories. Therefore, simply deducting a fixed percentage for soiling can lead to calculations that do not reflect reality. What is important is not to treat soiling vaguely as part of the deviation in power generation, but to set it up in advance as an assumption in the calculations and make it possible to revise it later based on measured data.

For example, when estimating power generation you derive the expected output from annual insolation and system capacity, and then account for temperature losses, wiring losses, conversion losses, and degradation over time. At this stage, organizing soiling as a separate loss item makes later root-cause analysis easier. If generation is lower than expected, rather than immediately concluding equipment failure or panel degradation, this provides clues to separate out soiling, shading, weather, downtime, and differences in measurement conditions.

When including panel soiling in calculations, it is important not to treat the impact of soiling as overly fixed. Some regions are partially washed by rain, while others are prone to accumulation during dry periods. On roofs with sufficient slope, dirt tends to wash away with water, whereas under low-slope installation conditions it is more likely to remain. Also, if mud or pollen remain in bands along the bottom edge of the panels, this can cause localized reductions in power generation. Ignoring these differences makes it difficult to explain deviations from actual energy production.

In practice, it is easier to handle losses from soiling by first dividing them into assumptions for "normal conditions" and assumptions for "periods when soiling is noticeable." Consider "normal conditions" to be when there is no significant soiling, and "periods when soiling is noticeable" to be when pollen, yellow sand, dust, bird droppings, fallen leaves, etc. can be observed, and adjust the correction range in power generation calculations accordingly. You do not need to fix precise numbers from the outset, but recording in writing what conditions are considered normal and under which conditions you would strengthen the correction will make decisions less likely to vary even if personnel change.

Also, when using power generation calculations for internal briefings or customer presentations, you must avoid confusing "soiling-adjusted generation" with "theory-oriented generation that does not account for soiling." Calculations that do not account for soiling can be useful for understanding the potential of the facility, but using them directly for actual financial or operational decisions risks being overly optimistic. Conversely, overestimating soiling can lead to underestimating the facility's performance more than warranted. It is important to vary how soiling is treated depending on whether the calculation's purpose is an initial assessment, operational improvement, or verification of cleaning effects.

What to be especially careful about is not attributing every drop in power generation to soiling. Soiling certainly affects generation, but other causes of decline include weather variations, shading from the surroundings, power conditioner (inverter) control, output curtailment, equipment shutdowns, temperature increases, faults in wiring or connections, and missing measurement data. The purpose of including soiling in the calculations is not to assign blame, but to align the conditions for comparison. Treating soiling independently within power generation calculations makes it easier to separate it from other factors and helps improve the accuracy of practical decision-making.

When including panel soiling as a loss factor, it is useful to record in the calculation sheets and management tables the assumed soiling conditions, the inspection date, the inspection method, and the rationale for any adjustments. If you simply enter numbers, you may not understand later why that adjustment was made. Managing these together with site photographs, inspection notes, cleaning records, rainfall conditions, and changes in the surrounding environment makes it easier to explain discrepancies between expected and actual power generation.

The first step in reflecting panel soiling in solar power generation calculations is to treat soiling not as a subjective issue but as part of the calculation conditions. Simply understanding that generation decreases because panels are dirty is insufficient for practical operations. By clearly specifying how much soiling was assumed, over what period, and which calculated values were adjusted, generation calculations become management information that can be used for operational improvement rather than mere forecasts.

Separate types of dirt and the periods when they are likely to occur

To reflect panel soiling in power generation calculations, it is important not to lump all soiling into a single term, but to classify it by type and by the periods when it is likely to occur. The contaminants that accumulate on panel surfaces include sand and dust, yellow sand, pollen, bird droppings, fallen leaves, sap, volcanic ash, salt, oil from exhaust, dust from agricultural land, and dust from construction sites, among others. These differ in how they adhere, how easily they can be removed, and how they affect power generation.

For example, when a light layer of dust or pollen is widely deposited, it tends to cause a slight shading effect across the entire panel. Even if it does not look like a large amount of dirt, when it adheres over a wide area like a film, power output on sunny days may not reach expected levels. On the other hand, localized soiling such as bird droppings or fallen leaves, even if small in area, can strongly shade part of a cell. Because partial shading can affect power output depending on the panel’s wiring configuration and the location of the soiling, it is safer not to judge solely by the area of the dirt.

Also, the tendency for soiling changes with the seasons. In early spring, some areas are prone to pollen and yellow dust. During dry periods, soil dust tends to become airborne, and when there are prolonged periods of little rain, dirt is more likely to accumulate. After typhoons or strong winds, salt-laden winds, soil and sand, fallen leaves, and small branches can remain on panel surfaces and around the mounting structures. In snowy regions, not only the snow itself but also the mud and dust left after snowmelt can affect power generation once operation resumes. In coastal areas, salt deposition; around farmland, soil dust and dust from agricultural operations; and along roads, exhaust and road dust are also factors to consider.

When you separate the types and timing of soiling in this way, the approach to revising power generation calculations also changes. If you handle soiling solely as a uniform annual loss, it becomes difficult to explain why generation is lower in certain months. When calculating monthly generation, consider months that are prone to soiling and months when rain is likely to wash it away, and evaluate month-by-month adjustments to better reflect actual conditions. In particular, when using this for monthly forecasting or budget‑versus‑actual management, it is important to look not only at the annual average but also at monthly trends.

In practical power generation calculations, you compare the assumed monthly generation with the actual monthly generation to identify periods when discrepancies tend to occur even after accounting for weather. For example, if generation is consistently lower than expected at the same time each year, inspect for types of soiling that commonly occur during that period. If the discrepancy appears in spring, consider pollen or yellow sand; during dry seasons, consider dust; in areas with high bird activity, consider bird droppings; and where trees are nearby, consider fallen leaves or sap — organize these candidate causes in light of the site environment. The important point here is not to judge by the numbers alone, but to combine them with on-site inspections and photographic records.

The type of soiling changes the priorities for cleaning and inspection. Widespread, light soiling often causes a gradual decline in power output and tends to be detected late. Localized heavy soiling is easier to spot visually, but can be overlooked in high places or on large sites. Some dirt can be removed easily with water, while other types become harder to remove over time. When calculating power output, it is not necessary to mathematically model these properties in detail, but at minimum distinguishing between "widespread, light soiling" and "localized, heavy soiling" makes it easier to organize approaches to correction and inspection.

These are points to check for locations where dirt is likely to accumulate. Dirt tends to remain at the lower edges of panels, near frames, on shallow slopes, in areas where water does not flow easily, and on edges affected by surrounding objects. For ground-mounted installations, panels can be affected by mowing, airborne dust from soil, vehicle traffic, and nearby construction. For roof-mounted installations, check places where birds are likely to perch, areas near exhaust vents, close to trees, and the way rainwater flows. When incorporating this into power generation calculations, rather than treating the entire system uniformly, it's better to distinguish and account for any sections that are prone to soiling.

When there are multiple systems or sections, it is important not to judge the impact of soiling solely by the overall average for the entire facility. If a particular section is close to a roadside, a specific row is affected by trees, or a surface has a different tilt or orientation, soiling patterns can vary. If you only look at total generation, what appears to be a minor anomaly may actually be a large drop in a particular section when viewed by system. When revising generation calculations, it is desirable to check section-, orientation-, tilt-, and system-level data as much as possible and decide at which unit to reflect the effects of soiling.

Furthermore, the effects of soiling are not necessarily eliminated by rain. Rain can wash surface dirt away, but if the rainfall is light, the dirt is adhered, or mud remains near the frames, the washing may be insufficient. In some cases power generation temporarily recovers after rain, while in others it changes little. Therefore, it is safer to avoid assuming in calculations that "if it rains, the soiling will be gone." It is important to determine how much recovery natural cleaning achieves based on post‑rain measured values and visual inspections.

Distinguishing between the types of soiling and their timing not only improves the accuracy of power generation calculations but also helps streamline maintenance planning. Knowing when and what kinds of soiling are likely to occur makes it easier to link inspection timing and cleaning decisions to power generation data. Rather than just applying the same monthly correction, reflecting site-specific seasonality of soiling in the calculation conditions enables more realistic solar power generation calculations.

Reassess the impact of soiling based on discrepancies with measured data

An important aspect of reflecting panel soiling in calculations is comparison with measured data. Power generation estimates are, after all, calculations made under assumed conditions. Unless you continuously check how actual generation differs from the calculated values, you cannot properly reassess the impact of soiling. Especially for systems in operation, it is important not to stop at producing predicted generation values but to compare them with daily or monthly measured values and analyze the causes of any discrepancies.

When reviewing measured data, do not simply conclude "it's soiled because it's lower than the calculated value." Power generation is strongly affected by weather. In months with many cloudy or rainy days, power generation will decrease even if there is no soiling. Also, during periods of high temperatures, output can be suppressed due to increased panel temperature. Output curtailment, equipment shutdowns, missing data due to communication failures, differences in measurement units, and discrepancies in metering periods can also cause variances. To reassess the impact of soiling, it is necessary to isolate these factors as much as possible.

First, you should confirm that the periods and units being compared are aligned. If you are comparing monthly generation, both the calculated and measured values need to be looked at for the same number of days and the same period. Comparing meter-reading-date-based data with calendar-month-based calculations will produce differences due to the number of days. Even when looking at daily data, averaging while including outage days or communication-dropout days can mix in declines unrelated to soiling. Before reflecting soiling in the generation calculation, it is essential to confirm that the basis for comparison is aligned.

Next, check the relationship with solar irradiance. Even in months when power generation is low, if solar irradiance is similarly low, the cause is not necessarily soiling. Conversely, if irradiance is sufficient but generation does not increase, factors such as soiling, shading, temperature, or equipment control may be suspected. In practice, looking not only at generation itself but at how much is generated relative to irradiance makes it easier to assess the impact of soiling. If you look only at generation while ignoring irradiance conditions, you are likely to confuse weather variability with soiling effects.

It is also useful to look at the generation curve on clear days. If soiling is widespread, overall output may be suppressed even on clear days. If there is localized soiling or shading, the generation curve can vary by time of day or by system. Of course, you cannot determine the cause from the generation curve alone, but it can provide a starting point for analyzing discrepancies from calculated values. Changes that are not visible in monthly totals may become apparent in daily or hourly data.

If there are multiple power conditioners or circuits, compare the generation for each system under the same weather conditions. If only one system has lower output, you need to check for soiling of the panels connected to that system, shading, poor connections, or equipment-side problems. Even if the total generation appears only slightly low, looking by system can reveal pronounced drops in specific locations. When reflecting panel soiling in generation calculations, it is important not to apply the same correction across the entire facility but to consider the possibility that soiling is unevenly distributed.

When analyzing discrepancies in measured data, cross-checking with visual records is indispensable. Taking photos of the panel surface during periods of low power generation makes it easier to later confirm any relationship with dirt. When photographing, it is useful to capture not only the overall condition but also areas where dirt is noticeable, the lower edges, near the frame, spots with bird droppings or fallen leaves, and angles that show the surrounding environment. Recording the date of the photo, weather, recent rainfall, and whether cleaning has been performed will also make it easier to trace correlations with the power generation data.

What you should be careful about here is not to try to quantify the effect of soiling too precisely. In practice, it is not easy to completely separate the effect of soiling alone, because solar irradiance, temperature, wind, shading, equipment condition, and so on vary simultaneously. Therefore, in power generation calculations you should first grasp the trend in the differences and check whether soiling is likely to be a major factor. On that basis, it is realistic to revise the correction conditions step by step using changes before and after cleaning, changes before and after rainfall, comparisons with systems under the same conditions, and so on.

Comparing power generation before and after cleaning provides an easy-to-understand basis for re-evaluating the impact of soiling. However, if the weather differs between the before and after measurements, the difference in generation cannot be directly attributed to the cleaning. If possible, make efforts to align conditions—choose days with similar solar irradiance for comparison, look at power generation efficiency relative to solar irradiance, or compare output during the same time periods. Even if generation increases after cleaning, not all of that increase will necessarily be due to soiling removal, so it is important to allow some margin in your judgment.

When reviewing power generation calculations, it is important not to judge differences between predicted and measured values on a single occurrence, but to monitor them continuously. If a discrepancy appears in only one month, it may be due to weather or a temporary shutdown. If the same trend continues over several months, or if a discrepancy appears in the same season each year, it is worth considering seasonal soiling or the influence of site conditions. By continuing to manage forecast versus actual power generation, soiling trends that were not initially visible will gradually become apparent.

When updating calculation conditions, instead of deleting past correction values, it is useful to keep a record of when and why they were revised. For example: noting that monthly corrections were reviewed because pollen-related soiling became noticeable in spring; that normal corrections were adjusted after confirming a recovery in power generation following cleaning; or that corrections were reverted because the dust impact diminished after nearby construction finished. Having such a history makes it possible to use it in power generation calculations and maintenance planning in subsequent years.

Reassessing the impact of soiling from the differences with measured data is a process of adapting power generation calculations to the site. An initial discrepancy in calculated values does not mean failure. Rather, by updating correction parameters based on measured values and reflecting site-specific characteristics, the calculations become usable in practice. Panel soiling is a visible phenomenon, but to incorporate its effects into generation calculations it is essential to combine three elements: data, on-site inspection, and records.

Update calculation conditions before and after cleaning and inspection

When reflecting panel soiling in power generation calculations, it is important to link cleaning and inspection histories to the calculation conditions. If cleaning has been performed but the calculation conditions remain unchanged, the projected power generation may deviate from reality. Conversely, if soiling has accumulated but calculations are still performed using the same conditions as immediately after cleaning, the discrepancy with measured values tends to widen. In other words, calculations that account for soiling should not be treated as a one-time setup; they need to be updated at the times of cleaning and inspection.

Before cleaning, it is important to record the condition of soiling. Check which panels have what degree of dirt: whether it is thinly spread over a wide area, whether there are localized heavy deposits, whether mud remains on the lower edges, or whether there are bird droppings or fallen leaves. If possible, also compile the pre-cleaning power generation data, weather, solar irradiance conditions, recent rainfall, and whether there was any shutdown. This will make it easier to compare changes after cleaning.

After cleaning, check not only the surface condition but also changes in power output. Even if the appearance looks clean, the difference in power generation may be small, or power output may recover more than the appearance suggests. What is important is determining to what extent cleaning can restore the calculation conditions, and when in the future you should strengthen soiling corrections. By treating cleaning not merely as maintenance but as material for reviewing power generation calculations, you can more easily improve the accuracy of future forecasts.

However, cleaning must take into account safety and equipment protection. Work at height or on roofs carries a risk of falling. Rubbing the panel surface in the wrong way, or using inappropriate tools or cleaning methods, can adversely affect the surface, wiring, mounting structures, and surrounding components. Because power generation equipment is electrical equipment, inspections and cleaning must be carried out in accordance with safety procedures, the equipment's handling conditions, and the rules set by the administrator. Even if the aim is to increase power output, safety and equipment preservation must not be neglected.

When reflecting cleaning history in generation calculations, it becomes easier to manage if you separate calculation conditions at the cleaning date. For the period before cleaning, apply corrections assuming dirt has accumulated, and for the period after cleaning, revise the corrections assuming dirt has been reduced. If cleaning occurs mid-month, treating the entire month under the same conditions can diverge from the actual situation. Even if strict day-by-day calculations are difficult, simply separating generation data into before-and-after cleaning for review will make it easier to incorporate into the next monthly calculation.

Inspection history is equally important. If panel soiling is confirmed during an inspection, the effect of the soiling should be reflected in the power generation calculation at that time. If the inspection determines the soiling is minor, this provides a basis for investigating other causes of reduced power generation. If inspection results are separated from the calculation conditions, the valuable information obtained on site will not be used for forecasting or management. Inspection records should include not only the presence or absence of soiling, but also the type, extent, location, photographs, the inspector, and the date of inspection.

Also, power generation calculations are useful when deciding how often to clean and inspect. Increasing cleaning excessively at sites where power loss from soiling is small may not be justified given the effort and safety risks. Conversely, at sites where power output declines are repeatedly confirmed due to soiling buildup, it is worth reviewing the timing of inspections and cleanings. By incorporating soiling into the power generation calculations, cleaning can be evaluated based on power output data and site conditions rather than on intuition.

When evaluating the effectiveness of cleaning, it is important to look at trends over a given period rather than just short-term increases or decreases in power output. If the weather is poor immediately after cleaning, generation may not increase as much as expected. Conversely, if clear weather continues after cleaning, the increase in power output may appear larger than the cleaning effect alone. Therefore, when comparing before and after cleaning, you need to take solar irradiance and weather into account and compare under as similar conditions as possible. Rather than making a simple comparison using only the cleaning day as the dividing line, it is easier to make a judgment by looking at trends for several days to several weeks before and after cleaning.

Depending on the site, dirt may reattach soon after cleaning. Along roads, on reclaimed land, on farmland, around construction sites, in areas with many birds, or near the sea, the post-cleaning condition may not last long. At such sites, consider factoring into calculations not only the correction value immediately after cleaning but also the assumption that dirt will accumulate again over time. For example, one approach is to assume three states—immediately after cleaning, normal conditions, and when dirt has accumulated—and to use them by month or season.

Calculation conditions for panel soiling should also be reviewed according to the facility’s years of operation. Immediately after installation, the surrounding environment may not yet be stable. There can be first-year-specific conditions such as freshly developed land with a lot of dust, ongoing nearby construction, undecided weed-management methods, or unpredictable bird visitation patterns. After a few years of operation, the times and places where soiling tends to occur become clear. At that point, it is important to update the initial calculation conditions based on actual measured data.

Updating calculation conditions before and after cleaning or inspections links power generation calculations with maintenance management. Even when calculation staff, inspection staff, cleaning staff, and facility managers operate separately, standardizing the format of records makes it easier to share information. When power generation drops, being able to review calculation assumptions, inspection results, cleaning history, and measured data in the same workflow speeds up root-cause analysis and decisions on improvements.

If you want to reflect panel soiling in solar power generation calculations, it's important not to keep the calculated values fixed. Cleaning and inspections are opportunities not only to reset or verify the soiling condition but also to update calculation parameters. By combining measured data with on-site records and revising the correction approach before and after cleaning, generation estimates will more closely match reality.

Summary

Panel soiling is a factor that is easily overlooked in solar power generation calculations, yet it can readily cause discrepancies with measured values. Calculations based only on solar irradiance and system capacity may not sufficiently reflect on-site accumulation of soiling, seasonal variations, and the influence of the surrounding environment. If generation calculations are to be used in practice, it is important not to treat soiling as an ambiguous error but to organize it in a form that can be handled within the calculation conditions.

First, by treating panel soiling as an independent loss factor, it becomes easier to organize the causes of reduced power generation. Rather than handling soiling as a single fixed value, it is important to change the perspective according to the condition — normal times, periods when soiling is noticeable, after cleaning, and so on. Next, by separating the types of soiling and the periods when they are likely to occur, it becomes easier to reflect them in monthly generation calculations and plan‑vs‑actual management. Pollen, yellow dust, soil dust, bird droppings, fallen leaves, salt, and so on each have different conditions for occurrence and different ways their impacts appear, so organization tailored to the site environment is necessary.

Furthermore, by checking the differences with measured data, you can reassess the impact of soiling more realistically. When calculated values and measured values diverge, it is important to check not only soiling but also weather, solar irradiance, temperature, curtailment, equipment shutdowns, and measurement conditions, and to isolate the causes. Linking photo records, inspection notes, and cleaning histories with power generation data makes it easier to explain the impact of soiling. By updating calculation conditions before and after cleaning or inspections, you can bring generation calculations closer to actual operating conditions.

Calculations of solar power generation are not something where you keep using the same numbers once created. They should be revised gradually while reflecting on-site soiling, seasonal changes, the surrounding environment, maintenance history, and measured data. Once panel soiling can be reflected in calculations, the accuracy of generation forecasts, plan-vs-actual management, cleaning decisions, and maintenance planning tends to improve. For operational staff in particular, this provides an important perspective for sorting out causes when generation is lower than expected and informing subsequent countermeasures.

When advancing power generation calculations that account for panel soiling, it is important to treat on-site inspections and data management together rather than separately. Record the soiling conditions observed on site, cross-check them with power generation data, and update calculation conditions as necessary. By continuing this process, desk-based calculations and on-site reality will converge. Even when efficiently visualizing power generation and keeping on-site records, it is important not to rely solely on specific product or tool names; instead, organize equipment conditions, recording methods, safety management, and the scope of data verification, and then choose a management approach that fits the site.