4 Ways to Read Soiling Loss in PVSyst｜Basics of Soiling Loss

Table of Contents

• What is Soiling Loss in PVSyst?

• Basics to grasp before reading Soiling Loss

• Reading method 1: Look at the impact of soiling loss on energy production

• Reading method 2: Examine monthly settings and seasonal variations

• Reading method 3: Avoid overlap with other loss items

• Reading method 4: Cross-check with measured data, cleaning records, and site conditions

• Approach when explaining Soiling Loss in practice

• Common misinterpretations of Soiling Loss

• Positioning of Soiling Loss within the overall PVSyst report

• Utilizing a combination of site verification and high-precision positioning

What is Soiling Loss in PVSyst?

In PVSyst, Soiling Loss is a loss term that represents the impact whereby dirt adhering to the surface of photovoltaic modules reduces the amount of solar irradiance they receive, causing a decrease in power generation. In Japanese, it is sometimes called dirt loss, contamination loss, or loss due to module surface soiling.

In solar power plants, modules are installed outdoors for long periods. Outdoors, their surfaces become soiled by various factors such as sand and dust, pollen, yellow sand, fallen leaves, bird droppings, dust from farmland and roads, coastal salt, and mud splashes after snowfall. When the module surface becomes dirty, some of the sunlight is blocked before it reaches the cells. As a result, even under the same irradiance conditions, power generation is reduced.

In PVSyst, the effects of such soiling are entered or checked as Soiling Loss. Soiling Loss may appear small in power generation estimates, but it directly affects annual energy production, Performance Ratio, Specific Yield, and the projected energy sales and self-consumption. In particular, for large-scale plants or when used for financing, performance guarantees, energy output guarantees, or business profitability assessments, even a difference of just a few percentage points can be significant.

When interpreting Soiling Loss in PVSyst, simply looking at the percentage is not enough. You need to check whether that value is applied uniformly throughout the year or varies by month, how rainfall and snowfall are accounted for, whether there is a cleaning plan, and whether it suits the site environment.

In practice, Soiling Loss is often treated as “an item where you just enter a few percent as appropriate.” However, the appropriate value actually varies depending on weather, topography, the surrounding environment, maintenance plan, module tilt angle, and whether the site is in a snow-prone area. Those responsible for reading PVSyst reports should interpret Soiling Loss not as a standalone number but as an assumption that reflects site conditions and operational conditions.

Fundamentals to Know Before Reading Soiling Loss

Before reading the Soiling Loss, first confirm what the PVSyst report was intended for. Whether it is a preliminary design estimate, an EPC bid estimate, an energy-yield assessment for financial institutions, or a post‑commissioning performance verification, the level of rigor required for the Soiling Loss will vary.

In the early design stage, because the surrounding environment has not yet been finalized, values may be provisionally set using typical figures. On the other hand, in documents related to power generation guarantees or contractual conditions, the basis for Soiling Loss becomes more important. Those reviewing the report need to verify not only the numerical values themselves but also which stage of the assessment those values were used in.

Soiling Loss is, in many cases, treated as a loss occurring on the path from solar irradiance to energy production. Solar irradiance reaching the module surface is partially lost due to soiling, and then temperature loss, mismatch loss, wiring loss, inverter loss, and so on compound to produce the final energy output. Therefore, changing the value of Soiling Loss also affects the downstream energy quantities.

What's important here is that Soiling Loss is not an assessment that "the power plant is at fault," but an assumption representing the natural decline expected in outdoor installations. It is unrealistic to maintain a completely dirt-free state every day. Therefore, setting an appropriate Soiling Loss is not intended to underestimate power generation, but to estimate a power output that is closer to reality.

Also, Soiling Loss is a loss that can be recovered by cleaning or rain. Unlike losses that depend strongly on the physical characteristics of the equipment, such as temperature losses or conversion losses, there is room for improvement through operations. Therefore, when planning O&M or cleaning frequency, the interpretation of Soiling Loss is important.

When interpreting PVSyst, a practical sequence for examining Soiling Loss is: first check the annual value, then the monthly values, next review the Loss Diagram and the impact on monthly energy production, and finally verify that the results are consistent with the site conditions.

How to Read 1: Examine the Impact of Soiling Loss on Power Generation

One way to read Soiling Loss is to see how much soiling losses affect annual energy production. In PVSyst reports, Soiling Loss may be shown in the loss diagram or in the detailed losses. What you want to check here is not simply what percentage the loss rate is, but how much that loss actually impacts the final energy production.

For example, if the Soiling Loss is 2%, the simple interpretation is that the effective irradiance received by the module surface is reduced by about 2% due to soiling. However, in PVSyst’s energy flow, the impact on the final system output is not a simple sum, because the location where a loss is applied and its relationship with other losses matter. Since loss items are applied in stages, when interpreting the overall losses you need to look at how the energy is reduced throughout the flow rather than summing each loss rate.

What a practitioner reviewing PVSyst should check first is whether the Soiling Loss value is reasonable for the energy yield assessment. If the value is extremely low, verify that the site soiling is not being underestimated. If the value is extremely high, check whether it reflects the surrounding environment and cleaning conditions, or if it is being overly conservative.

In general, soiling losses vary depending on ground conditions and the surrounding environment. In paved areas or where grassland is stable, dust generation may be lower. Conversely, soiling can be greater on unpaved roads, recently disturbed bare land, farmland, quarries, roads with heavy traffic, coastal areas, and regions affected by volcanic ash or yellow sand.

The module tilt angle also affects Soiling Loss. If the tilt is sufficient, rainwater more easily washes surface dirt away. Conversely, when the tilt is small, dirt tends to remain, and puddle marks and mud accumulation are more likely to occur. Therefore, even in the same area, the validity of Soiling Loss varies depending on the installation angle.

When assessing the impact on annual energy production, being aware of the sensitivity to changes in Soiling Loss is also useful in practice. For example, checking how much annual production would change if Soiling Loss were to change by 1% makes it easier to compare cleaning schedules and maintenance costs. Soiling Loss provides an important starting point for evaluating how much generation is likely to improve from cleaning and whether that improvement justifies the cleaning costs.

When reading a PVSyst report, after you find the Soiling Loss value, it is natural to next look at changes in annual energy production, Specific Yield, and Performance Ratio together. If Soiling Loss is large but PR is high, other losses may have been set to small values. Conversely, if Soiling Loss is small but PR is low, other items such as temperature loss, shading loss, wiring loss, inverter loss, or output curtailment may be affecting the results.

In other words, Soiling Loss is not an item that can be judged as good or bad on its own. It is a parameter to interpret within the overall PVSyst context to assess how much of the reduction in power generation it explains.

How to Read 2: Viewing Monthly Settings and Seasonal Variations

The second way to interpret Soiling Loss is to look at monthly settings and seasonal variations. Soiling loss is not necessarily constant throughout the year. Rather, at many sites it often changes with the seasons.

In spring, some regions are susceptible to pollen and yellow sand. When dry periods persist, dust tends to accumulate on module surfaces. In summer, in regions with heavy rainfall, dirt is easily washed away, while near farmland or construction sites, soil dust may occur. In autumn, falling leaves, bird droppings, and dust caused by dry conditions may have an impact. In winter, snowfall, snowmelt, mud splashing, freezing, and dirt from snow removal operations can be involved.

When monthly Soiling Loss values are set in PVSyst, check whether each month’s values reflect the site’s seasonality. A uniform annual setting is not necessarily wrong, but in regions with large seasonal differences, monthly settings can more closely match the actual conditions.

For example, in regions that receive heavy rainfall during the rainy season and typhoon periods, a certain degree of natural cleaning effect from rain can be expected. On the other hand, in regions with long dry seasons, dirt tends to accumulate and Soiling Loss is more likely to increase. In snowy regions, it is important to distinguish between the impact of module surfaces being covered by snow during the snow-covered period and soiling loss.

One thing to be careful about here is not to confuse generation loss caused by snowfall with Soiling Loss. A condition in which modules are covered by snow and do not generate power should, in some cases, be treated as a factor separate from soiling. There may be room to consider the effects of mud and dirt left behind after the snow melts as Soiling Loss, but including the snowfall itself in soiling losses can cause overlap with other loss items.

When reviewing monthly settings, you should look not only at the Soiling Loss value but also at the monthly generation and the monthly PR trend. If generation drops significantly in a particular month, you need to distinguish whether it is caused by a decrease in irradiance, a rise in temperature, shading effects, or soiling losses.

In particular, when Soiling Loss is set high on a monthly basis, one would want to verify the basis for that. Some explanation is required, such as the presence of unpaved roads nearby, heavy dust during the dry season, increased dust during periods of agricultural work, abundant yellow sand or pollen, low cleaning frequency, or little rainfall.

Conversely, when Soiling Loss is set low throughout the year, verify whether this aligns with the site conditions. A lower setting can be justified if the site has sufficient slope, expected rainfall, few nearby dust sources, and regular cleaning. However, if it is set too low for reclaimed land, areas under construction, or sites adjacent to farmland, the projected energy output may be overly optimistic.

The important thing when interpreting PVSyst is not to dismiss the monthly figures as "fine adjustments." Monthly Soiling Loss is important information that reflects the site's seasonality. When explaining seasonal variations in power generation, knowing which months have higher Soiling Loss settings makes it easier to explain to the project owner and internal stakeholders.

How to Read 3: Avoid Overlap with Other Loss Terms

The third way to read Soiling Loss is to avoid overlap with other loss terms. In PVSyst, there are many losses that affect energy production. Typical examples include shading losses, incidence-angle losses due to the IAM, temperature losses, low-irradiance losses, mismatch losses, wiring losses, inverter losses, transformer losses, auxiliary losses, and output curtailment. Soiling Loss is just one of these.

The effect of solar radiation being blocked by soiling can visually appear similar to shading or angle-of-incidence effects. However, in PVSyst they have different meanings. Shading loss is the effect of sunlight being blocked by surrounding terrain, structures, spaces between racking rows, trees, etc. IAM loss is the effect whereby sunlight striking the module at an oblique angle increases reflection and reduces the light that is effectively absorbed. Soiling Loss is the effect of light being blocked by dirt adhered to the module surface.

If you do not understand this difference, you may end up counting the same phenomenon across multiple loss items. For example, if the effect of snow covering the module surface is accounted for in another item but is also heavily included in Soiling Loss, you may end up underestimating the energy production. Likewise, if shadows from trees and surrounding structures are reflected in 3D shading but you also assign a large value to it as soiling loss, the explanation becomes unclear.

On the other hand, caution is also necessary when losses appear too small. Even if shading losses and IAM losses are properly included, that does not mean Soiling Loss is unnecessary. The effects of shading and incidence angle are separate from the effects of surface soiling. If there are soiling factors at the site, they need to be verified separately as Soiling Loss.

When reading a PVSyst report, first check where Soiling Loss appears in the Loss Diagram. Next, see which loss items appear before and after it. Understanding what is included as losses on the irradiance side, on the array side, and on the system side will make the meaning of Soiling Loss clear.

Also, when comparing Performance Ratio, how Soiling Loss is handled is important. When comparing multiple reports, one may set Soiling Loss uniformly for the year while another sets it on a monthly basis. Likewise, one may conservatively assume snow and dirt, while the other minimizes them. If you compare PR alone in this situation, you may end up comparing differences in assumptions rather than which design is superior.

In practice, when interpreting Soiling Loss, it is important to consider “which phenomenon this loss represents.” Clarify whether it is sand or dust, pollen, bird droppings, mud from snowmelt, or accumulation due to lack of cleaning. A Soiling Loss that cannot be put into words as a phenomenon will be weak when explained.

Especially in internal reviews and explanations to clients, simply describing the Soiling Loss as "general dirt accumulation" can appear unsubstantiated. If you can tie the explanation to site conditions—for example, "we expect dust during the dry season because unpaved areas remain," "the surrounding area is grassland with few dust sources, so we expect natural cleaning by rainfall," or "we keep the annual value low on the assumption of regular cleaning"—it becomes more persuasive.

Reading Method 4: Cross-check with actual measurements, cleaning, and on-site conditions

The fourth way to interpret Soiling Loss is to read it by cross‑checking with measured data, cleaning records, and site conditions. PVSyst’s Soiling Loss is a simulation assumption, but once a plant is in operation its validity can be verified by comparing it with measured data.

If power generation after the start of operation is lower than expected, the cause is not necessarily a single one. It is necessary to distinguish whether solar irradiance was lower than expected, the temperature was higher, output curtailment occurred, the PCS experienced stoppages, shading had a large effect, or soiling had accumulated. Among these, Soiling Loss is easier to assess by combining on-site inspections with data analysis.

For example, if you compare days with similar solar irradiance conditions and observe that energy output and PR improved after cleaning, it can be considered that there was a certain degree of loss due to soiling. If only a particular string or area shows low output, suspect localized soiling, bird droppings, fallen leaves, mud splatter, shading from weeds, equipment malfunction, or similar issues. If the decrease is uniform across a wide area, consider overall soiling or the influence of weather conditions.

It is also important to cross-check with the cleaning plan. If you set the Soiling Loss low in PVSyst but in reality there is no cleaning plan and the surrounding environment is prone to soiling, the assumptions may be optimistic. Conversely, if there is regular cleaning, frequent rain, sufficient slope, and little on-site soiling, applying an excessively large Soiling Loss may make the predicted energy yield overly conservative.

When assessing site conditions, check not only the module surface but the entire surrounding environment. Confirm whether the access roads within the plant are unpaved, whether there are nearby roads used by soil-hauling trucks, whether farmland or livestock barns are close by, how far the site is from the sea, whether there are many trees or birds, the condition of drainage, and whether mud splashing beneath the modules is likely.

Also, attention is needed for cases where dirt remains only on the lower edge of a module row. Dirt washed down by rain can collect at the lower edge and remain in a band. Although such soiling may appear partial to the eye, depending on the cell layout it can have a large impact on power generation. Because it cannot always be represented by a simple uniform whole-surface Soiling Loss, assessments should be made together with site photographs and inspection records.

When interpreting PVSyst, Soiling Loss is not something that ends at the design stage. By cross-checking with measured data after commissioning, cleaning records, inspection photos, and weather data, it can be used to inform the next simulation and improve O&M. As performance records accumulate for each plant, it becomes easier to set more realistic Soiling Loss values for projects in the same region or similar environments.

In this way, Soiling Loss is a factor that connects simulations and on-site conditions. Rather than leaving it to the numbers in PVSyst alone, interpreting it while looking at how the site becomes soiled, the status of cleaning, and the relationship with measured power generation will provide information useful for practical work.

How to approach explaining Soiling Loss in practice

When explaining Soiling Loss in practice, simply saying "it's the amount by which power generation decreases due to dirt" can be insufficient. In particular, different stakeholders—such as the project owner, internal managers, financial institutions, O&M personnel, and design engineers—will want different information.

When explaining to the project owner, the impact on project economics and generation forecasts is important. You should convey that by accounting for Soiling Loss, you are estimating a realistic annual energy yield. It should be explained that including soiling loss does not mean the design is poor, but rather reflects the typical decline expected for an outdoor power plant.

When explaining to internal superiors, the validity of the assumptions is emphasized. You must be able to concisely explain why you chose those values, whether they are higher or lower compared with other projects, whether they match the site conditions, and how much they affect power generation.

When explaining to O&M personnel, the relationship with cleaning and inspections is important. At sites with large Soiling Loss, it may be necessary to consider cleaning and inspection frequencies. Conversely, if the improvement in power generation is small relative to cleaning costs, it may be appropriate to reconsider the priority of cleaning.

When explaining to the design team, module tilt, racking height, ground surface treatment, drainage planning, access path design, and the relationship to nearby dust sources are important. Dirt can be mitigated not only during operation but also to some extent at the design stage. For example, panel heights that reduce mud splash, layouts with good drainage, ground surface management that suppresses dust generation, and access routes that allow cleaning vehicles to enter easily all contribute to suppressing Soiling Loss.

Those responsible for reviewing PVSyst reports should explain Soiling Loss not only as a "simulation value" but also as an "assumption for design and operation." Soiling is related to the assumptions made during design, the condition of the ground surface during construction, maintenance after commissioning, and the prevailing weather conditions.

When explaining, it is easier to be understood if you discuss not only annual values but also monthly trends, assumptions about cleaning, and site conditions. For example, if you expect increased soiling during the dry season, you can explain that generation and PR may be slightly lower during that period. If you expect natural cleaning from rainfall, you can also note that in years with little rain more soiling may remain than anticipated.

Soiling Loss can make the reported energy generation appear either excessively good or excessively poor. That is precisely why evidence and consistency are important when interpreting it. Verifying whether the value is reasonable given site conditions, whether it does not overlap with other loss items, and whether it does not conflict with the operation plan will increase the overall reliability of the report.

Common Misinterpretations of Soiling Loss

One common misinterpretation of Soiling Loss is judging whether it is large or small based solely on the loss rate. For example, concluding that 2% is reasonable, 5% is excessive, or 1% is too small is overly simplistic. The appropriate value varies depending on the site environment, monthly conditions, cleaning frequency, rainfall, tilt angle, and nearby dust sources.

Another misreading is treating Soiling Loss as if it were a fixed equipment performance. Unlike the rated performance of modules or PCS, soiling loss varies with operation and environmental conditions. Even for the same installation, actual soiling loss changes depending on whether cleaning is performed, whether there is nearby construction, and whether it is a wet year or a dry year.

Also, Soiling Loss is sometimes confused with snow loss. A condition in which snow covers a module and it does not generate electricity is different in nature from soiling. Shading caused by snow, soiling after snowmelt, and mud splatter after snow-removal work should each be considered separately. If you put everything into Soiling Loss, the basis will become unclear when explaining the report later.

Furthermore, Soiling Loss and shading loss are sometimes confused. Objects that locally block light, such as bird droppings or fallen leaves, may visually appear like shadows, but in PVSyst it is necessary to clarify whether they should generally be treated as surface soiling or as anomalies identified during on-site inspections. Whether they are handled as widespread, uniform soiling or managed separately as localized reductions in power generation changes how they should be interpreted.

When comparing PVSyst results, it's common to end up comparing PR and annual energy yield while the assumptions for Soiling Loss differ. If Report A assumes a low Soiling Loss and Report B assumes a high one, B's generation may look lower not because its design is worse but because its assumptions are more conservative. When making comparisons, it's important to either align the loss conditions including Soiling Loss or clearly state the differences.

Also, if the measured power generation is lower than the simulation, it is dangerous to immediately attribute it to Soiling Loss. Measured values include various factors such as pyranometer accuracy, measurement gaps, PCS outages, output curtailment, grid constraints, temperature conditions, shading, and string abnormalities. Whether soiling is the cause must be determined by examining comparisons before and after cleaning, site photographs, comparisons with days under the same conditions, and area-by-area generation differences.

Soiling Loss is a useful parameter, but it should not be used as a catch-all adjustment that can absorb everything. If you attribute all unexplained generation declines to Soiling Loss, its meaning as a design parameter becomes weakened. To interpret PVSyst correctly, it is important to return Soiling Loss to its original meaning—“reduction in irradiance due to dirt”—and check it accordingly.

Positioning within the overall PVSyst report

Within the overall PVSyst report, Soiling Loss is part of the assumptions for energy yield assessment. When reading the report, do not view Soiling Loss in isolation; understand it within the flow of solar irradiation, temperature, shading, wiring, conversion, output limitations, and so on.

First, horizontal-plane irradiance and tilted-plane irradiance are determined by the meteorological data. From there, effects such as angle of incidence, shading, and soiling determine the irradiance that the module effectively receives. Then, after losses due to the module’s temperature characteristics and low-irradiance characteristics, mismatch, wiring, inverter, transformer, and auxiliary equipment, the final power generation is obtained.

In this flow, Soiling Loss is a relatively upstream loss. When energy is reduced upstream, the energy entering subsequent conversion processes is also reduced. Therefore, even a small Soiling Loss will certainly affect the forecast of annual power generation.

When reading PVSyst, it is useful to check the Soiling Loss and then examine the sequence of losses in the Loss Diagram. By confirming at which stage the soiling loss is applied and which losses follow, you can describe the factors leading to reduced power generation in order.

Also, when explaining the figures in the report to the client or to internal stakeholders, positioning Soiling Loss as "an item that affects power generation but may be improved through operations" makes it easier to understand. Temperature losses and regional solar radiation conditions cannot be easily changed, but soiling can sometimes be improved through cleaning, ground surface management, drainage improvements, and measures to mitigate surrounding dust.

However, cleaning will not necessarily be economically advantageous. You need to compare the increase in power generation from cleaning with the labor, costs, and operational risks involved. PVSyst's Soiling Loss is a starting point for that assessment.

When comparing multiple projects, how Soiling Loss is handled is important. Even for projects in the same region, the appropriate value for Soiling Loss will change if site development conditions, nearby roads, farmland, distance to the coast, racking angle, or cleaning plans differ. Conversely, if projects with similar conditions show substantially different values, you should check the reasons.

For readers of a PVSyst report, Soiling Loss is not a "small loss item" but a checkpoint to see whether site conditions are being reflected. Especially practitioners who are just starting to learn how to read PVSyst tend to focus on major indicators such as PR and annual energy production, but reading the underlying assumptions like Soiling Loss deepens their understanding of the report.

Utilization Combining On-site Verification and High-Precision Positioning

To handle Soiling Loss more practically, it is important not only to check the numerical values in PVSyst but also to accurately record the conditions at the site. Being able to identify which areas show noticeable soiling, where mud splashing occurs, where drainage is inadequate, and the proximity to unpaved roads and dust sources will make it easier to explain the basis for the Soiling Loss.

During power plant inspections, simply taking photographs can leave the location ambiguous. This is especially true at large-scale solar power plants, where similar rows of mounting racks continue, making it difficult when reviewing photos later to tell which row and which position they showed. To record locations of dirt, weeds, drainage, damage, subsidence, and where shadows occur, combining location information with on-site photographs is effective.

LRTK is a high-precision GNSS positioning device that can be attached to and used with an iPhone. Because it lets you obtain high-precision location information on site while recording inspection points and verification results, it is a convenient method for managing photovoltaic power plant sites. By checking Soiling Loss in PVSyst and using LRTK on site to record dirt and mud splashes, poor drainage, and areas requiring cleaning with location data, you can more easily link the assumptions used in simulations with actual site conditions.

Soiling Loss appears in reports as a percentage—typically a few percent. However, behind that figure are site topography, the surrounding environment, cleaning schedules, seasonal variability, and inspection history. To put PVSyst readings to practical use requires both the skill to interpret the numbers and the ability to accurately document site conditions. Leveraging high-precision positioning such as LRTK makes it easier to integrate energy yield assessment, inspections, cleaning planning, and O&M improvements.