5 Items to Consider for Soiling Loss in Solar Power Generation Simulations

In solar power generation simulations, irradiance, system capacity, orientation, tilt angle, and shading effects are commonly checked. However, when considering actual generation, losses caused by dirt on the panel surface must not be overlooked. When sand and dust, pollen, fallen leaves, bird droppings, exhaust-related grime, particulate matter, or residues remaining after snow adhere to the panel surface, the amount of sunlight reaching the panel is reduced and generation may decline. This article explains five practical items for considering soiling loss for practitioners who search for "solar power generation simulation."

Table of contents

• The importance of considering soiling loss in solar power generation simulations

• Item 1: Organize soiling sources as site-specific conditions

• Item 2: Check roof pitch/tilt angle and the effectiveness of rain washing

• Item 3: Read soiling loss from monthly generation and seasonal factors

• Item 4: Reflect ease of cleaning and inspection in the loss rate

• Item 5: Check the impact of soiling loss on self-consumption and surplus electricity

• Points to confirm so you do not underestimate soiling loss

• How to compare soiling loss in vendor proposals

• Site information accuracy affects judgment of soiling loss

• Summary

The importance of considering soiling loss in solar power generation simulations

The reason to check soiling loss in solar power generation simulations is that the condition of the panel surface directly affects generation. Solar panels generate power by receiving sunlight, but when dirt adheres to the surface, sunlight reaches the panel less effectively. Dirt may spread thinly over the surface or accumulate in concentrated areas. In either case, it must be considered as a factor that reduces generation.

Simulations typically show annual and monthly generation, but those figures generally include some generation losses. Alongside temperature, wiring, power conversion, shading, snow, and aging, soiling may also be treated as part of the loss rate. However, proposals sometimes do not show soiling loss separately and instead include it within an overall loss rate. If you do not confirm the extent to which soiling is anticipated, you may overestimate generation.

Soiling behavior varies significantly with site environment. Areas with many surrounding trees may experience fallen leaves, sap, and bird impacts. Unpaved areas or sites prone to wind-blown soil make sand and dust adhesion more likely. If factories, roads, farmland, material yards, or land under development are nearby, dust and exhaust-related grime should be considered. Shallow roof pitch can make rain less effective at washing away dirt, allowing soiling to persist.

Correctly accounting for soiling loss affects not only simulation accuracy but also maintenance planning. If cleaning and inspection are easy due to a good layout, generation declines from dirt can be detected early. Conversely, roofs lacking inspection walkways or sites without maintenance access are harder to manage even when soiling occurs. As a result, simulation estimates and post-installation actual performance can differ.

The aim of considering soiling loss in solar power generation simulations is not to be unduly pessimistic about generation. It is to anticipate possible generation declines at the site in advance and reduce gaps after installation. By identifying areas prone to soiling and planning with cleaning and inspection in mind, it becomes easier to maintain generation over the long term.

Item 1: Organize soiling sources as site-specific conditions

The first item in considering soiling loss is to organize sources of soiling as site-specific conditions. In solar power generation simulations, beyond setting general loss rates, you need to check how prone the site is to soiling. Sites with many sources of dirt may produce lower generation than standard assumptions.

For roof-mounted projects, check surrounding trees, places where birds congregate, rooftop equipment, exhaust vents, ventilation equipment, and the positional relationship to roads and parking lots. If trees are nearby, fallen leaves, pollen, or sap can adhere to panels. Roofs or nearby structures that attract birds can cause localized soiling from droppings. Near exhaust vents or ventilation equipment, airborne contaminants are more likely to adhere to panel surfaces.

For ground-mounted projects, check for nearby unpaved areas, farmland, gravel paths, material yards, development sites, factories, and high-traffic roads. In environments where wind easily stirs up soil and dust, dirt accumulates on panel surfaces. Especially for ground-mounted installations, changes in surrounding land use can alter soiling conditions, so you should consider not only the current state but also potential future changes.

Sources of soiling can overlap with sources of shading. Trees not only cast shade but can produce fallen leaves and attract birds. Rooftop equipment can cast shade and also limit inspection routes, creating places that are hard to clean. Therefore, when considering soiling loss, it is important to organize not only soiling itself but also shading, maintainability, and layout together.

Before site surveys, preliminarily identify soiling sources from drawings, aerial photos, site photos, and information on the surrounding environment. After a site survey, confirm tree locations, exhaust vent orientations, distances to roads and unpaved areas, wind patterns, and rooftop equipment layout, and reflect these in the simulation loss rates. By concretely organizing sources of soiling, you can move from generic values to generation forecasts tailored to the site.

Organizing soiling sources also helps post-installation maintenance planning. You can anticipate which areas are prone to soiling, which faces should be checked preferentially, and which seasons see increased soiling. Understanding soiling sources at the simulation stage contributes to maintaining generation in the long term.

Item 2: Check roof pitch/tilt angle and the effectiveness of rain washing

The second item is to examine the relationship between roof pitch or panel tilt angle and the effectiveness of rain washing. Panel surface dirt can be partially washed away by rain. However, how easily dirt is removed depends on roof pitch, panel angle, installation method, type of dirt, and rainfall conditions. It is important not to overlook this point when considering soiling loss in simulations.

On sloped roofs, rainwater runs off more easily and light surface dirt may be washed away. Conversely, on shallow-pitched roofs or installations close to flat roofs, rainwater may not flow well and dirt can remain. Pollen, fine dust, bird droppings, and small leaf fragments in particular are not always completely removed by rain.

On flat roofs and ground-mounted installations, the way dirt remains can vary with the mounting angle. A small tilt can hinder runoff, causing dirt to accumulate near the lower edge of panels or in certain surface areas. A larger tilt can improve runoff but also affects wind exposure, row-to-row shading, spacing, and maintainability. Decide angles considering not only soiling mitigation but also generation, system capacity, and constructability.

Also check whether the panel lower edges or borders are prone to trapping dirt. If rain-washed dirt remains in some places, it can cause localized generation declines. In simulations, soiling loss is often treated as part of a comprehensive loss rate, so whether the layout is likely to produce localized soiling should be confirmed through site conditions and maintenance planning.

The effectiveness of rain washing is also influenced by regional conditions. In regions with regular rainfall versus long dry seasons, accumulation tendencies differ. Even in rainy areas, rainfall intensity, wind direction, and roof shape affect how dirt is washed away. Do not assume that rain will always eliminate soiling; combine local rainfall patterns with installation conditions to judge.

During site surveys, confirm roof pitch, drainage direction, rainwater flow, tilt of the planned panel surfaces, and locations where leaves and dust tend to collect. Reflecting these findings will help you decide whether to treat soiling loss as standard or to assume a somewhat larger loss. Considering whether soiling will naturally wash away or require cleaning in advance reduces post-installation generation gaps.

Item 3: Read soiling loss from monthly generation and seasonal factors

The third item is to interpret soiling loss from monthly generation and seasonal factors. Soiling does not occur uniformly throughout the year. Depending on the season, pollen, yellow dust, fallen leaves, bird activity, residues after snow melt, and dust during dry periods change in type and frequency. Therefore, when considering soiling loss, you should be aware not only of annual average loss but also of impacts on monthly generation.

In spring, pollen and fine dust may adhere more readily. In some regions, windborne sand or fine particles may remain on panel surfaces. Extended dry periods with little rain can lead to accumulation and gradual generation declines. Even if simulations forecast high spring generation, check for soiling loss depending on site environment.

In summer, generation tends to rise with higher irradiance, but typhoons, strong winds, nearby dust, and bird impacts may occur. High temperatures also cause generation declines, so to distinguish whether a drop in generation is due to temperature loss, soiling, or weather you need to consider monthly generation and separate loss factors.

In autumn, fallen leaves and dead branches may affect panels. In areas with many trees, leaves can accumulate on panels or nearby areas, causing soiling and localized shading. Roof pitch and wind direction can cause leaves to collect in the same places. For tree-adjacent projects, confirm autumn soiling and shading risks as a site condition.

In winter, snow, freezing, and post-snowmelt residues can occur. Dirt can remain after snow melts, and mud or particles can collect on roof surfaces or panel lower edges. With lower irradiance and a lower sun angle in winter, soiling and shading effects can be more apparent in generation.

When reviewing monthly generation in solar power generation simulations, confirm whether seasonal soiling factors are reflected. Often soiling loss is treated as a uniform annual loss, but at some sites particular seasons may see stronger soiling effects. Comparing monthly generation with seasonal site factors helps realistically assess generation risk.

Monthly data are also useful for post-installation performance monitoring. If generation falls below expectations in a particular season, you can use that as a clue to determine whether soiling, pollen, fallen leaves, snow, weather, or temperature is the cause. Anticipating seasonal soiling risks before installation makes it easier to plan cleaning and inspection timing.

Item 4: Reflect ease of cleaning and inspection in the loss rate

The fourth item is to reflect the ease of cleaning and inspection in the loss rate. Soiling loss depends not only on whether dirt occurs but also on whether it can be detected and managed when it does. Systems that are easy to inspect and clean allow quicker detection of generation declines from soiling. Conversely, equipment installed in hard-to-access locations may delay response, prolonging generation loss.

For roof projects, it is important whether inspection walkways and working space are secured. Filling a roof with panels may make capacity appear large, but it can reduce space for cleaning and inspection walkways. In particular, overconcentrating panels around rooftop equipment, drains, or inspection hatches can impede building management. Even if simulation shows large capacity, low maintainability can make long-term generation maintenance difficult.

For ground-mounted projects, management walkways, weed control, drainage, and working space around equipment matter. Ground-mounted installations are vulnerable to grass growth, dust, and surrounding environmental impacts. Narrow maintenance paths, poor drainage, or difficult access to equipment and panels makes inspection and cleaning harder. When considering soiling loss, assess not only panel surface dirt but whether maintenance work is realistically feasible.

Ease of cleaning and inspection may not be directly reflected in simulation loss rates, but it is an important practical premise. Equipment that is easy to inspect allows quicker awareness of soiling and generation decline and makes necessary responses easier. Hard-to-inspect installations may delay cause identification even when generation falls.

Whether cleaning is necessary depends on the type and occurrence of dirt. Light dust may wash away with rain, but bird droppings, leaf residues, exhaust-related grime, and adhered dust may not fall off naturally. At the simulation stage, decide whether to take a conservative view of soiling loss, taking local conditions and cleanability into account.

Reflecting ease of cleaning and inspection in the loss rate does not simply mean setting a larger loss rate. It means confirming whether the site is prone to soiling, whether inspections can detect issues early, and whether cleaning is feasible, thereby planning to maintain generation in the long term. Solar power generation simulations should consider not only maximizing generation but also effective generation that includes maintainability.

Item 5: Check the impact of soiling loss on self-consumption and surplus electricity

The fifth item is to check how soiling loss affects self-consumption and surplus electricity. When generation falls due to soiling, the practical impact depends on whether that reduction affects the facility’s planned self-consumption or merely reduces surplus power.

Self-consumption refers to the portion of electricity generated by solar that is used within the facility. Self-consumption is crucial when considering electricity cost savings and the benefit of installation. If soiling-related generation reductions occur during daytime hours when the facility demand is high, the reduction in purchased electricity may be smaller than expected.

On the other hand, if soiling-related reductions occur during times when generation already exceeds demand, the effect may be limited to a decrease in surplus electricity, with little impact on self-consumption. Therefore, you must confirm when and how much soiling loss occurs to correctly assess installation benefits.

Overlaying hourly generation with hourly electricity usage is effective for this check. During hours when generation falls below demand, generated power is almost entirely self-consumed. Soiling loss in those hours directly reduces the benefit of reduced purchased electricity. During hours when generation greatly exceeds demand, generation reductions due to soiling appear as reductions in surplus.

Also check monthly impacts. If facility demand is high in summer, soiling loss in summer is more likely to affect self-consumption. Confirm whether spring pollen and dust, autumn leaves, or post-snow soiling coincide with high-demand periods. It is important to assess not only the generation decline from soiling but also the impact on how electricity is used.

When batteries are combined, the impact of soiling loss changes. For systems that store daytime surplus in batteries, soiling that reduces surplus also reduces energy available for charging the battery. As a result, discharge energy available in the evening or night may decrease. For simulations including batteries, check post-soiling charge amounts, discharge amounts, and state-of-charge transitions.

If you view soiling loss solely as a decline in generation, you may misjudge its practical effects. By separating impacts on self-consumption, surplus electricity, and battery usage, you can more realistically assess how soiling loss affects installation benefits.

Points to confirm so you do not underestimate soiling loss

Soiling loss is an item that tends to be underestimated in generation simulations. Compared with conspicuous factors like shading or system capacity, soiling can be overlooked in early stages. However, to maintain generation after installation over the long term, it is important not to take soiling lightly.

First, confirm where soiling loss is included in the simulation. Some proposals do not show soiling loss as an individual item. In such cases, verify whether it is included in the overall loss rate or considered separately. If the loss rate is set low in a proposal, it may not have sufficiently accounted for soiling.

Next, confirm whether the site environment fits standard soiling assumptions. If there are many trees, nearby unpaved areas, dust-prone environments, places where birds gather, nearby exhaust equipment, or abundant fallen leaves, standard loss rates may be insufficient. Consider whether you should adopt a slightly conservative view of soiling loss depending on site conditions.

Check roof pitch and tilt angle as well. Determine whether dirt is likely to run off with rain or remain on shallow slopes. For flat roofs or low-angle installations, pay attention to runoff and residue patterns. Do not assume rain will always resolve soiling; judge based on dirt type and installation conditions.

Ease of cleaning and inspection is also important. To reduce soiling loss you must be able to detect and address dirt when needed. If inspection walkways are inadequate, roof work is difficult, maintenance paths on the ground are insufficient, or weed control and cleaning are hard, soiling-induced generation decline may persist.

Plan for post-installation performance monitoring. Recording monthly generation, hourly generation, and generation by installation face helps detect soiling declines early. To avoid underestimating soiling loss, think through not only pre-installation simulation but also post-installation verification methods.

Soiling loss can be a major cause of large generation declines or may have little effect; what matters is checking according to site conditions. In practice, do not adopt standard assumptions uncritically; assess the site’s susceptibility to soiling and its maintainability.

How to compare soiling loss in vendor proposals

When you receive solar power generation simulations from multiple vendors, it is important to compare how they handle soiling loss. Even for the same capacity and installation site, differences in loss rate assumptions change the appearance of annual generation and self-consumption. For proposals that show large generation, verify how much soiling loss they assume.

First, check whether soiling loss is shown separately or included in an overall loss rate. If temperature, wiring, conversion, shading, soiling, snow, and aging are all lumped together, it is harder to isolate the effect of soiling. In that case, confirm whether the overall loss rate is appropriate for the site environment.

Next, see whether site survey findings are reflected. Proposals that confirm rooftop exhaust equipment, surrounding trees, unpaved areas, bird impacts, dusty environments, and leaf conditions will have different reliability than those that rely on drawings and generic loss rates. Proposals that carefully reflect site conditions may forecast lower generation, but they might better match post-installation performance.

Also compare maintenance assumptions. Confirm whether maintenance routes are provided, whether those panels susceptible to soiling are accessible, and whether inspection targets are clear. Good generation figures alone are not sufficient; layouts that are difficult to clean may undermine long-term generation.

Viewing monthly generation is also useful. Check how generation is forecast in seasons prone to soiling, during low-rainfall periods, or when leaves and pollen are common. Even if soiling loss is treated uniformly month to month, site conditions can raise seasonal risk.

When comparing vendor proposals, do not simply choose the one with the largest projected generation. Prioritize proposals with clear loss assumptions that reflect site conditions and maintenance. How vendors treat soiling loss is one way to gauge the robustness of a proposal.

Site information accuracy affects judgment of soiling loss

Accurate site information is essential for correctly judging soiling loss. Solar power generation simulations rely not only on regional irradiance and system capacity but also on the surrounding environment of the installation. Accurately identifying where trees are, where dust is likely to originate, where exhaust equipment is located, and which areas are difficult to clean allows you to make soiling loss estimates closer to reality.

For roof projects, record the positional relationships of rooftop equipment, exhaust vents, piping, railings, inspection hatches, drains, surrounding buildings, and trees. When panels are placed near exhaust vents or ventilation equipment, check how prone they are to soiling. On roofs near trees, consider fallen leaves and bird impacts. Drains and inspection hatch areas are also important from cleaning and building-management perspectives.

For ground-mounted projects, confirm site boundaries, surrounding roads, unpaved areas, farmland, material yards, trees, utility poles, drainage channels, and maintenance paths. Note prevailing wind directions that stir up dust, directions of rainwater flow, areas prone to vegetation growth, and routes accessible to cleaning vehicles or inspection personnel. Accurate site information makes it easier to determine which areas are soiling-prone and which areas should be prioritized for inspection.

If site information is vague, soiling loss must rely on general loss rates. But because site-to-site environmental differences are large for this item, general values alone are often insufficient. Keeping concrete site information helps when comparing vendor proposals or making final layout adjustments before construction.

Also, if generation later falls short of expectations, recorded site information makes it easier to determine the cause. Knowing which areas are prone to soiling, which are hard to clean, which collect leaves, and which are susceptible to birds allows you to prioritize inspections. Site information affects not only the accuracy of simulations but also the quality of operation and maintenance.

Soiling loss is difficult to judge precisely by desk calculations alone. By accurately capturing the site and reflecting that information in simulations and maintenance plans, you can better predict post-installation generation declines.

Summary

To consider soiling loss in solar power generation simulations, you need to comprehensively check soiling sources, roof pitch and tilt angle, seasonal factors, ease of cleaning and inspection, and impacts on self-consumption and surplus electricity. Soiling may appear to be an inconspicuous loss factor, but depending on site conditions it can affect generation and cause gaps between simulation and actual performance after installation.

Item 1 organizes soiling sources as site conditions. By checking trees, unpaved areas, dust, exhaust equipment, bird impacts, fallen leaves, roads, and nearby facilities, you can grasp each site’s susceptibility to soiling. Item 2 checks roof pitch and tilt angle and the effectiveness of rain washing to see whether dirt will wash away or remain on low-angle installations.

Item 3 reads soiling loss from monthly generation and seasonal factors. Pollen, dust, fallen leaves, and post-snow soiling vary by season. Item 4 reflects the ease of cleaning and inspection in the loss rate. Lack of inspection or maintenance walkways makes it difficult to respond to soiling, affecting long-term generation.

Item 5 checks how soiling loss affects self-consumption and surplus electricity. If soiling-related generation declines occur during the facility’s demand periods, they affect reductions in purchased electricity. If they occur during times of large surplus, the practical impact may be limited. It is important to assess not only generation decline but also the impact on usable electricity.

To avoid underestimating soiling loss, confirm what the loss rate includes, whether site conditions fit standard assumptions, and whether cleaning and inspection are feasible. When comparing vendor proposals, check not only projected generation but also whether soiling assumptions are clear and whether site surveys are reflected.

Accurate site information forms the foundation for improving soiling loss judgment. Precisely identifying candidate installation ranges, rooftop equipment, exhaust vents, obstacles, trees, site boundaries, maintenance routes, and surrounding structures allows you to bring simulation loss rates closer to reality.

If you want to improve the accuracy of soiling-loss considerations by precisely recording installation candidate ranges, rooftop equipment, exhaust vents, obstacles, trees, site boundaries, maintenance routes, and management walkways on site, using LRTK, an iPhone-mounted GNSS high-precision positioning device, is effective. High-precision local positioning makes it easier to organize soiling-prone locations, hard-to-clean areas, positions of shading and obstacles, and maintenance routes, facilitating consistent comparison of vendor proposals, pre-construction checks, and post-installation maintenance. To correctly consider soiling loss in solar power generation simulations, it is important to establish a system for accurately capturing the site, not just rely on desk-based loss rates.