The 7 Basic Loss Settings to Learn from the PVSyst Manual

Table of Contents

• Why are loss settings in PVSyst important?

• Basic 1: Consider soiling losses based on the local environment and the cleaning plan

• Basic 2: IAM losses represent the power reduction caused by the angle of incidence.

• Basic 3: Temperature loss varies greatly depending on installation conditions and ventilation

• Basic 4: LID and module quality loss should anticipate initial performance differences

• Basic 5: Organize mismatch losses for string configuration and the effects of shading

• Basic 6: Check wiring losses separately for the DC side and the AC side.

• Basic 7: Reflect auxiliary equipment, transformers, and operational losses without overlooking them

• Practical approach to reviewing loss settings

• Points to keep in mind when reading the PVSyst manual

• Summary

Why Are the Loss Settings in PVSyst Important?

When running a generation simulation in PVSyst, what many people first pay attention to are solar irradiance data, panel capacity, selection of the power conditioner, azimuth, and tilt angle. These are of course important conditions that affect energy production, but in practical work, understanding the loss settings is indispensable for enhancing the persuasiveness of the results. Even with the same system capacity, the same installation angles, and the same meteorological data, changing the assumptions about losses can greatly alter the annual energy yield, performance ratio, monthly generation trends, and the way the economic evaluation appears.

When consulting the PVSyst manual, you will find that multiple loss items are handled as array and system losses, including IAM loss, soiling loss, temperature loss, LID loss, module quality loss, mismatch loss, wiring loss, transformer loss, auxiliary consumption, and others. In other words, PVSyst’s loss settings are not merely a collection of input fields but a framework for organizing how a photovoltaic plant actually generates power on site and where energy is lost.

A common pitfall in loss settings is to view each item as an isolated percentage. For example, if attention is limited to deciding what percentage to assign to soiling losses, wiring losses, or how much to expect for temperature losses, the connection to on-site conditions and design parameters becomes tenuous. Because PVSyst’s output is presented as numerical values it may appear objective at first glance, but if the input assumptions do not match the site, the results will also diverge from reality. Loss settings should be seen not as a safety factor to be conservative about energy yield, but as the process of translating the assumptions about design, construction, environment, and maintenance into an energy yield assessment.

When using PVSyst results in project feasibility assessments, documents for financial institutions, internal approval processes, customer explanations, EPC proposals, or O&M plans, it is important to be able to explain the rationale for the loss settings. If you cannot explain why a particular soiling loss, why those temperature conditions, or why that wiring loss were chosen, the reliability of the energy yield simulation will decrease. Conversely, if the loss settings are understood systematically, it becomes easier to compare design options, perform sensitivity analyses, and identify opportunities for improvement.

This article organizes the loss settings that those from beginners reading the PVSyst manual to field practitioners should grasp into seven fundamentals. Rather than memorizing detailed on-screen procedures, it is important first to understand what each loss means, which site conditions affect it, and in what order it should be reviewed.

Principle 1: Consider soiling losses based on the local environment and the cleaning plan

Dirt loss refers to the reduction of light reaching the module caused by substances adhering to the surface of solar panels, such as sand and dust, pollen, yellow dust, fine particles from exhaust gases, bird droppings, fallen leaves, salt, volcanic ash, and soil dust originating from agricultural land. Among the loss settings in PVSyst, it is a relatively easy-to-image item, but in reality there is a large variation depending on site conditions, and entering a uniform value carelessly can cause simulation results to diverge from on-site reality.

PVSyst allows soiling losses to be set on a monthly basis, so you can reflect seasonal variations in soiling rather than entering a single annual value. The official documentation also explains that soiling losses are strongly dependent on rainfall and that monthly loss coefficients can be defined. This concept is very important in practice. For example, in regions with a rainy season or frequent typhoons, natural washing can be expected, whereas in seasons of prolonged dryness or periods with a lot of yellow sand, soiling tends to accumulate. In snowy regions, not only does snow make power generation difficult in winter, but soiling after snowmelt and partial residual snow must also be taken into account.

When setting soiling losses, first check the surroundings of the power plant. The way panel surfaces become soiled varies depending on whether there are unpaved roads nearby, a quarry or factory, farmland or livestock barns, proximity to the coast with salt damage, or structures that tend to attract birds. In addition, the tilt angle of the panels is important. On low-pitch roofs and flat roofs with a small tilt, rainwater does not drain well and dust tends to remain. Conversely, if there is a certain amount of tilt, natural rainfall is more likely to provide a cleaning effect.

Cleaning plans should also be included in the assumptions for soiling losses. Projects that carry out regular cleaning and projects that basically rely on natural rainfall do not have the same annual soiling losses. After determining cleaning frequency, cleaning timing, cleaning scope, and the frequency of visual inspections, you need to set loss values that match that operation. However, it is risky to estimate losses too low simply because cleaning is planned. In practice there are constraints such as cleaning costs, worker safety, securing water in areas with water shortages, potential scratches to panel surfaces, and weather conditions suitable for work.

When reading the PVSyst manual, it is important to regard soiling losses not merely as an input item but as a setting that links the local environment and the O&M plan. If generation falls short of expectations, soiling is a factor that is relatively easy to investigate. If you organize a month-by-month approach during the simulation stage, it will be easier to verify which seasons are likely to show the effects of soiling when comparing with measured data after commissioning.

Basic 2: Read IAM losses as the reduction in power output due to the angle of incidence

IAM loss represents the effect whereby the amount of sunlight that can be effectively utilized decreases due to reflections and similar phenomena when sunlight strikes the module surface at an oblique angle. In solar power generation, not only the irradiance itself but also the angle at which sunlight strikes the panel surface is important. In the morning and evening, during winter, or under installation conditions where the azimuth or tilt are not optimal, the influence of the angle of incidence tends to become greater.

The term IAM can be a little confusing for beginners, but the idea itself is simple. When light hits at an angle close to perpendicular it is more easily captured by the panel, and when it strikes at a shallow angle reflection increases and the amount of light available for power generation decreases. The effect also varies with the properties of the glass surface, anti-reflective treatments, and module construction. In PVSyst, IAM loss is treated as one of the main components of array and system losses, and it is reflected in the energy yield assessment along with other loss items.

What is important for understanding IAM losses is their relationship with the design conditions. For ground-mounted systems facing south with an appropriate tilt angle, incident conditions tend to be relatively balanced throughout the year; however, for east–west roofs, low‑slope roofs, installations on near‑vertical walls, special racking for agrivoltaics, and designs using bifacial modules, the influence of incidence angle needs to be examined more carefully. The implications of IAM losses change depending on how you regard not only generation during peak hours but also the contributions in the morning and evening.

Also, IAM losses should be considered not in isolation but together with azimuth, tilt angle, nearby shading, and array spacing. For example, reducing the mounting angle can be advantageous for racking cost and wind load, but it may change the tendency for soiling to remain and the impact of the angle of incidence. An east-west installation can broaden generation in the morning and evening, while its angle-of-incidence conditions differ from those of a south-facing installation. When comparing multiple options in PVSyst, checking not only annual energy production but also the differences in IAM losses on the loss diagram makes the characteristics of each design option easier to see.

When setting IAM losses, you need to check manufacturer specifications and module characteristics and avoid relying too heavily on standard values. However, before getting into unnecessarily complex settings, first grasp during which times of day, which seasons, and under which installation conditions the IAM losses will take effect. When reading the PVSyst manual, understanding the physical reasons why the losses occur—not just the numbers on the screen—will improve your ability to explain the simulation results.

Basic 3: Temperature loss varies greatly with installation conditions and ventilation

Solar modules produce more electricity the more sunlight they receive, but their output decreases as module temperature rises. This reduction in output is assessed as temperature loss. Among the loss settings in PVSyst, temperature loss is a parameter that can significantly affect the annual energy yield. It is particularly important in regions like Japan, where summer temperatures and humidity are high, to realistically account for temperature loss.

One point to understand first about temperature losses is that ambient temperature and module temperature are not the same. Even when the ambient temperature is 30 °C, a module surface exposed to strong solar radiation will become hotter than that. If the mounting structure has good ventilation, heat can escape more easily, but when installed close to the roof or when rear-side airflow is poor, the module temperature tends to rise. In PVSyst's temperature loss settings, factors such as the heat transfer coefficient are involved, and the thermal behavior can be adjusted in the detailed losses and in the module file settings.

Ground-mounted, corrugated-metal-roof, flat-roof, pitched-roof, BIPV-like, and agrivoltaic installations have different airflow behind the modules. While ground-mounted systems with sufficient space under the racking dissipate heat more easily, installations mounted close to the roof surface tend to trap heat. In hot regions, this difference affects annual energy yield. Therefore, when evaluating temperature losses in PVSyst, you should check the installation method and ventilation together, not just the local ambient temperature data.

Temperature losses also affect panel selection. Modules have a temperature coefficient that specifies how much their output decreases with a rise in temperature. Modules with better temperature coefficients can be relatively advantageous under high-temperature conditions. However, in simulations, module performance, installation conditions, irradiance conditions, and wind conditions interact, so judging based on a single specification value should be avoided. When comparing multiple modules and racking options in PVSyst, checking how much temperature loss changes can reveal differences that are not apparent from a simple installed-capacity comparison.

In practice, temperature losses tend to be underestimated. In regions with high solar irradiance in summer, it may appear that power output will be large, but module temperatures also rise, so the output may not increase as expected. Especially for rooftop installations, the type of roofing material, the spacing from the roof surface, surrounding parapets, and the placement of outdoor air-conditioning units and other equipment also have an impact. When reading the PVSyst manual, it is important not to regard temperature loss solely as a matter of meteorological conditions, but to understand it in connection with the design drawings and mounting/racking details.

Basic 4: Anticipate initial performance differences for LID and module quality loss

LID stands for Light Induced Degradation, and refers to the phenomenon in which module output decreases during the initial period after beginning to receive sunlight. It is particularly considered for crystalline modules, and LID loss is included among the loss items in PVSyst. When evaluating a solar power plant, it is important to account not only for the catalog values immediately after installation but also for the initial degradation that occurs after actual operation begins.

Module quality loss is a parameter that reflects the difference between the nominal output stated in the manufacturer's specifications and the average performance of the modules actually delivered. Modules have output tolerances, and not all modules will generate power exactly at their nominal value. For products with a positive tolerance, the average output may exceed the nominal value, but how to handle quality variability and differences in measurement conditions must be established as assumptions for the simulation.

LID and module quality loss both relate to how much a panel’s performance deviates from its specified value, but they are not the same. LID refers to the initial degradation expected from light exposure, while module quality loss deals with the difference between the average performance at delivery and the manufacturer’s specifications. Confusing these can lead to double-counting the same effect or, conversely, overlooking losses that need to be accounted for.

When reading the PVSyst manual, it is important not to judge solely by the names of the loss items but to verify which stage of performance degradation each represents. If you can refer to equipment certifications, contract documents, manufacturer warranties, output inspection reports, flash test results, etc., you should reconcile those sources with the information in the manual. For large-scale projects in particular, variations between module lots and differences in delivery timing cannot be ignored. When using energy-yield simulations for project feasibility assessments, it is also important to distinguish between first-year generation and long-term average generation.

Also, LID and module quality losses need to be distinguished from the aging degradation and mismatch losses described below. Aging degradation is the performance decline that occurs with the passage of operating years, and mismatch loss is the loss caused by differences in characteristics between modules and by the string configuration. If you lump everything together as "panel losses," you will lose the ability to analyze the root causes. When setting loss parameters in PVSyst, the basic practice is to separate initial degradation, quality differences, variability, and aging degradation, and to appropriately reflect each in the necessary fields.

Basic 5: Organize mismatch losses by string configuration and the effects of shading

Mismatch losses are losses that occur when multiple modules or strings do not generate power equally. In photovoltaic systems, modules are connected in series and parallel to adjust voltage and current. Therefore, if some modules have lower performance, if some are shaded, or if modules with different orientations or tilts are combined in the same circuit, the overall output can be limited.

In the PVSyst manual, mismatch loss is described as the difference between the sum of the maximum outputs of each independent submodule and the maximum output obtained from the combined I-V characteristic. In other words, the mismatch is the difference between the output that individual modules would be able to produce on their own and the output that can actually be extracted from the array as connected.

Understanding this concept shows that mismatch loss is not merely an empirical value but is deeply related to the electrical connection configuration.

When setting mismatch losses, first check the string configuration. It is important to verify the number of modules in series, the number of parallel strings, how input circuits are divided, the connections per MPPT, whether surfaces with different azimuths or tilts are assigned to the same MPPT, and whether shaded and unshaded areas are placed in the same string. On roof installations, roof surfaces are often divided into multiple sections with different azimuths and tilts. For ground-mounted installations as well, terrain undulation, shading between racking rows, nearby trees, utility poles, fences, and building shadows can cause only certain strings to experience worse conditions.

Mismatch losses vary not only because of module-to-module differences but also depending on how the design is laid out. For example, even for a system of the same capacity, the way you split strings and which inputs you connect them to changes the impact of shading and orientation differences. When handling detailed electrical losses in PVSyst, the positions of modules and their assignment to strings and inverter inputs become important. Calculating detailed electrical losses requires the exact position of each PV module and a definition of which string and inverter input it belongs to.

What beginners should be careful about is not to treat shading loss and mismatch loss as completely separate things. Shading itself is a factor that blocks solar irradiance, but the impact on the entire array depends on how the shaded modules are electrically connected. In other words, the same shading can produce different losses depending on the string design. When dealing with nearby shading or module layout in PVSyst, it is important to check not only the shape of the shading but also its relationship to the electrical connections.

If mismatch losses are underestimated, actual energy production may be lower than the simulation predicts. Conversely, if they are overestimated, the project's viability may be judged too conservatively, potentially disadvantaging even good design proposals. When reading the PVSyst manual, it is more practical to understand mismatch losses not as "what percentage to enter" but as "which non-uniformities are represented by which settings."

Basic 6: Check wiring losses separately for the DC side and the AC side

Wiring losses are power losses that occur when electric current flows through cables and connection points. In solar power generation systems, there is DC-side wiring from the modules to the power conditioner, and AC-side wiring from the power conditioner to the substation/transformer equipment and the interconnection point, each of which incurs losses. When setting loss parameters in PVSyst, the basic approach is to consider the DC side and the AC side separately.

Wiring losses vary depending on distance, cable cross-sectional area, current, voltage, and circuit configuration. As the size of the power plant increases, cable routes become longer and the impact of wiring losses becomes harder to ignore. Even for rooftop installations, losses change depending on the location of the power conditioner, the distance to the cubicle, and the wiring routes within the existing building. Rather than simply entering standard values, it is important to align them with the design drawings, single-line diagrams, and cable selection.

PVSyst's documentation explains that, while it is common to define wiring losses as a loss rate, in practice the loss rate is proportional to power, so the specified nominal loss rate does not directly equal the actual annual loss. At each time step of the simulation, losses are evaluated according to the instantaneous power and current, so the wiring energy loss that appears in the annual results will differ from the nominal loss rate. This is a point that beginners often misunderstand.

For example, setting the wiring loss to 2 percent does not necessarily mean that annual energy production will simply decrease by 2 percent. Generated power varies over time, and the current is lower in the mornings, evenings, and on cloudy days. Because wiring losses are strongly affected by current, the loss rate at rated output and the loss rate observed in annual totals do not match. When reading the PVSyst manual, you need to understand the difference in meaning between input values and annual results.

To reduce wiring losses, you can use thicker cables, shorten distances, appropriately raise the voltage, or review the collector design. However, using thicker cables affects material costs and installability. Changing the placement of the power conditioner can cause issues with maintainability, waterproofing, dustproofing, noise, and installation space. Therefore, wiring loss is not simply something that should be minimized; it must be evaluated in balance with cost, installability, safety, and maintainability.

When comparing multiple design options in PVSyst, do not treat wiring loss as a fixed value; instead, verify it as an item that varies with each design. In particular, when comparing centralized and distributed placements of power conditioners, the way losses occur changes depending on whether the DC-side or AC-side wiring is longer. While confirming the position of wiring losses in the loss diagram, it is important to consider whether there is room for design improvement.

Basic 7: Do not overlook auxiliary equipment, transformers, or operational losses — make sure they are accounted for

In PVSyst’s loss settings, you need to check not only losses related to modules and wiring but also auxiliary consumption, transformer losses, and losses associated with external equipment. A solar power plant is not composed solely of panels and power conditioners. There are devices such as monitoring equipment, communications equipment, cooling fans, control equipment, substation and transformer equipment, and, in some cases, tracker mounts, snow-melting systems, and security equipment that consume part of the generated power or cause losses during conversion.

The Array and System losses items in PVSyst also include external transformer losses and auxiliary power consumption. These may seem modest compared with losses occurring at the module level, but they are important when evaluating the net energy production of the entire plant. In particular, for projects connected to high-voltage or extra-high-voltage systems, transformer and substation equipment losses cannot be ignored. Even in small-scale projects, if monitoring devices and communication equipment run continuously, their power consumption will have a certain impact over the long term.

Transformer losses include losses that vary with load and losses that occur simply from being energized. It is also necessary to confirm how to treat losses that occur not only during daytime generation but also during nighttime standby. When configuring PVSyst, it is desirable to check the no-load losses, load losses, rated capacity, and other values listed in the equipment specifications and ensure they are consistent with the simulation assumptions. Transformer efficiency is often high and therefore easily overlooked, but its impact on annual energy production grows as the system size increases.

The same applies to auxiliary equipment consumption. The items covered vary by project, such as the power plant’s monitoring system, network equipment, meteorological measuring instruments, standby power consumption of power conditioners, and power for cooling and control. In cases like battery integration, self-consumption systems, agrivoltaic setups, or remote power plants, auxiliary equipment different from standard ground-mounted projects may be added. If these are not reflected in the simulation, there is a risk of overestimating the amount of electricity the plant can supply externally.

From the perspective of operational losses, equipment shutdowns, output curtailment, maintenance inspections, fault response, communication outages, power conditioner replacement, and temporary generation reductions caused by weeds or snow should also be considered. If PVSyst’s standard loss settings cannot fully capture these, you need to separately organize the assumptions and reflect them in the generation assessment and business plan. Rather than treating PVSyst results directly as the amounts for power sales or self-consumption, it is important to clarify which aspects are forecasted within PVSyst and which are handled by external commercial evaluations.

Auxiliary equipment, transformer, and operational losses tend to be deferred in the early design stages. However, because they affect the final energy yield guarantees and financial projections, adding them hastily in later stages can substantially change simulation results. If you read the PVSyst manual and organize which losses correspond to which equipment or operations within the plant, it becomes easier to avoid omissions.

Practical steps for reviewing loss settings

When setting loss parameters in PVSyst, trying to fine-tune every item at once leads to confusion. First enter standard project conditions and check the overall energy production and the loss diagram. Then, review in order the soiling loss, which strongly depends on local site conditions; the temperature loss, influenced by the installation method; the wiring loss, related to the design drawings; and the mismatch loss, associated with string configuration and shading. This approach makes the work easier to organize.

The first thing I want to check is not the magnitude of the losses but whether there is a justification. Even large losses are acceptable if there are reasonable grounds based on site conditions or design parameters. Conversely, small losses without justification are risky. For example, setting a low soiling loss but having no cleaning plan, assuming a small temperature loss despite a small clearance from the roof surface, or using standard values for wiring loss while the cable routes are long — such situations need to be reviewed.

Next, we will check the relationship between the input values and the output results. In PVSyst, you can understand where and to what extent losses occur through the loss diagram. The documentation also explains that the loss diagram is useful for identifying weaknesses in system design. 年間発電量だけを見ていると、どの損失が効いているのかがわかりません。 By reviewing the loss diagram, the items that should be improved become easier to see.

When comparing multiple options, it is also important not to change loss settings excessively. When comparing module capacity, tilt angle, orientation, power conditioner capacity, wiring design, and so on, changing loss conditions that are unrelated will make it unclear what is causing the differences in energy production. Depending on the purpose of the comparison, you need to separate assumptions that should be fixed from those that should be changed.

In practice, it is also important to keep notes documenting the rationale behind PVSyst settings. Recording why a particular soiling loss was adopted, which installation method the temperature conditions assume, which stage of the drawings the wiring losses are based on, and how fully the mismatch losses reflect shading analysis and string configuration will make later review easier. Because drawings and equipment specifications often change as the design progresses, be careful not to continue using simulations based on outdated assumptions.

Points to keep in mind when reading the PVSyst manual

The PVSyst manual contains a large amount of information, and beginners may find it difficult if they try to understand everything from the start. Regarding loss settings, it is most efficient to read in the order of: first grasp the meanings of the terms, then understand their relationship to site conditions, and finally organize the rationale behind the input values. Rather than merely following the field names on the screen, it is important to be aware of which physical phenomena, design conditions, and operational conditions each loss corresponds to.

Be careful not to assume PVSyst’s default or commonly used values are correct as-is. Initial values can serve as a starting point for input, but they are not universal values that fit every site. Coastal areas, mountainous regions, snowy regions, urban areas, factory roofs, farmland, and newly developed sites with a lot of dust all exhibit different loss behaviors. The purpose of reading the PVSyst manual is not to find standard values, but to be able to judge the assumptions that are appropriate for your specific project.

Be cautious about the notion that it is acceptable to be overly conservative when assuming losses. Estimating generation conservatively can be effective as risk management, but increasing assumed losses without justification can lead to incorrect evaluation of design proposals and investment decisions. Conversely, reducing assumed losses too much to make the project's viability look better can create large gaps with actual performance after commissioning and undermine credibility. When setting loss assumptions, explainability is more important than conservatism.

When explaining PVSyst results internally or externally, make sure you can explain not only the annual energy production but also the meaning of the main loss items. Items such as soiling, temperature, IAM, mismatch, wiring, auxiliary equipment, and transformers can also lead to improvements in design and maintenance. For example, if temperature losses are large, consider improving ventilation; if wiring losses are large, review cable routing and conductor cross-sectional area; if soiling losses are large, reconsider the cleaning schedule. Loss settings are not adjustments to reduce energy production, but an analysis to identify improvement opportunities for the power plant.

Finally, the PVSyst manual is not something you read once and then finish. The points that need to be checked differ between the design phase, the detailed design phase, before construction, after completion, and after the start of operation. In initial studies, rough loss settings may be sufficient, but at the stage when it is used for contracts or investment decisions, detailed settings supported by evidence are required. After operations begin, it is also important to compare measured generation with the simulation and verify which losses differed from the assumptions.

Summary

When learning loss settings in the PVSyst manual, it is important to understand each item not merely as a percentage input but as a way of reflecting the plant’s realities in the simulation. Soiling loss is related to the local environment and the cleaning schedule, while IAM loss is related to the angle of incidence and installation conditions. Temperature loss is influenced not only by ambient air temperature but also by ventilation and racking/mounting conditions, and LID and module quality losses are items for clarifying assumptions about initial performance. Mismatch loss is closely linked to string configuration and the effects of shading, and wiring losses need to be separated into DC and AC sides and aligned with the design drawings. Furthermore, auxiliary consumption, transformer losses, and operational downtime factors cannot be overlooked when considering net energy production.

In PVSyst loss settings, the important thing is not to fine-tune every value but to provide a justification for each setting. By checking site conditions, design drawings, equipment specifications, maintenance plans, and operating conditions and mapping them to the loss items, the reliability of the energy yield simulation is improved. Reviewing the loss diagram also makes it easier to see where the weaknesses of the plant are.

Beginners should start by mastering seven basics: soiling, IAM, temperature, LID/quality, mismatch, wiring, and auxiliary equipment/transformers. Understanding these seven items will make the detailed explanations in the PVSyst manual easier to follow and will make it easier to apply simulation results to design improvements and client explanations. PVSyst is not only software for calculating energy yield but also a practical tool for organizing the design assumptions of a solar power plant. Correctly understanding loss settings and carefully reviewing them according to the conditions of each project is the first step toward a reliable energy yield assessment.