5-Item Manual for Loss Settings in PVSyst That Won't Fail

In solar power plant energy-yield simulations, not only module capacity and irradiance but also the approach to loss settings affect the reliability of the results. In practical work using PVSyst, it is important not just to enter the on-screen fields in order, but to verify what physical phenomenon each loss represents, which design documents are used as the basis, and whether the values can be explained and justified later.

In particular, the granularity of information available differs between preliminary studies, basic design, detailed design, and post-construction verification. While many assumed values are used in the early stages, as the project advances to detailed design, settings that reflect layout drawings, single-line wiring diagrams, cable routes, equipment specifications, and site conditions are required. Presenting only the power generation figures while leaving loss settings vague makes it difficult to explain the rationale during internal reviews, client briefings, documents for financial institutions, and comparisons with post-construction performance.

This article organizes five aspects of loss settings that are prone to mistakes for practitioners working with PVSyst while referring to the manual. Rather than a mere operational guide, it explains in a practical, work-ready way what information to check, how to decide input values, and how to verify the results.

Table of Contents

• Align the assumptions before starting loss settings

• Do not underestimate losses from solar radiation, nearby shading, and terrain shading

• Adjust losses from temperature, soiling, and reflection to local conditions

• Confirm electrical losses and equipment-side conditions in the design information

• Verify the consistency of losses in the results report and feed them into operations

Align prerequisites before starting loss configuration

Before setting losses in PVSyst, the first thing you should confirm is the purpose of the simulation. Even for the same solar power plant, the required level of accuracy and what needs to be documented will differ depending on whether it is being used for an initial feasibility study, for comparing before-and-after design changes, for forecasting expected generation prior to construction, or for evaluating the performance of an existing plant. If you enter losses while the purpose is unclear, it will be difficult later to explain “why this value was chosen” or “which documents were used as the basis.”

In initial studies, site development plans and detailed cable routes may not yet be finalized. At this stage, it is not uncommon to use general assumed values or in-house standards. However, it is important to clearly indicate that they are assumptions and to manage them as items to be reviewed in subsequent stages. Conversely, if the assumed values from the initial study are continued to be used unchanged in detailed design or in simulations immediately before construction, the projected power generation can diverge from actual equipment conditions. In particular, when array layout, azimuth, tilt angle, spacing, PCS capacity, cable length, or transformer equipment configuration change, loss settings must also be reviewed.

Next, it is important to align the definitions of the installed capacity you input. In photovoltaic power generation, multiple capacity representations are used, such as the modules' DC capacity, the PCS's AC capacity, the output at the grid connection point, and the contractual output. If the capacity set in PVSyst is mixed with the capacity used in internal documents or proposals, the perceived loss rates and generation figures will change. For example, if the DC-side capacity is designed larger than the AC-side capacity, there will be periods when the PCS limits output depending on conditions. This loss is not a simple malfunction or drop in efficiency, but arises from the design balance between DC and AC capacities.

The assumptions regarding meteorological data cannot be separated from loss settings. In power output simulations, solar irradiance, ambient temperature, and wind speed affect generation. Even if you finely adjust the loss settings, if the meteorological data’s location or period differs significantly from actual conditions, the overall reliability of the results will not improve. In mountainous, coastal, snowy, or fog-prone areas, general meteorological data from nearby sites may not adequately represent local characteristics. Before fine-tuning loss settings, it is important to clarify the rationale for selecting the meteorological data.

Consistency with the design drawings is also essential. Array layout, tilt angle, azimuth, row spacing, and ground slope in PVSyst affect assumptions such as shading, temperature, reflections, and wiring lengths. When comparing a simulation made from a preliminary layout before site development with a simulation made from the detailed construction design after development, you need to record which set of drawings was used. This makes it easier to determine whether differences in energy yield are due to differences in loss settings or to differences in design assumptions.

Also, loss settings are not something you enter once and finish. As the project progresses, information such as equipment specifications, cable sizes, racking specifications, construction scope, and surrounding obstructions will be updated. Each time, it is necessary to review the loss items in the simulation. Even slight changes in layout during the construction phase can affect inter-row shading, cable length, azimuth, and tilt angle. To treat energy yield assessment as part of design management, it is important to keep a history of updates to loss settings and be able to explain the reasons for any changes.

To avoid failures in practice, it is effective to manage not only the loss setting values but also the supporting documents, the date set, the person in charge, the relevant drawings, and the planned review schedule together. On the PVSyst screen the focus is on numeric input, but in actual work you will be asked which documents those numbers are based on. Even when operating while referring to the manual, you should not only follow the field names on the screen but also make a point of assembling the design rationale and explanatory materials at the same time.

Do not underestimate losses from solar radiation, nearby shading, and terrain shading

One aspect to pay particular attention to in the loss settings of a solar power plant is shading losses. Shading losses are caused by various factors such as surrounding buildings, trees, utility poles, slopes, mountains, and adjacent array rows. In PVSyst you can set near-shading and horizon conditions to assess the impact of shading, but if you oversimplify the input assumptions you risk overestimating actual energy production. Because shading varies with time of day, season, and solar altitude, it is important not to conclude simply that “there are few obstacles so it’s not a problem.”

First, what should be confirmed is to consider near-field shading and terrain shading separately. Near-field shading is shading caused by relatively nearby obstacles around or within the power plant or site. This includes adjacent buildings, trees, fences, utility poles, electrical cubicles, slopes, and shading between arrays. On the other hand, terrain shading is shading in which the sun is blocked by distant mountains, hills, or other large-scale terrain features. Because the times of occurrence and the areas affected differ, treating them collectively as the same type of shading makes explanations of power generation assessments ambiguous.

One common pitfall when assessing near-field shading is judging obstacles based only on the layout plan. Even if the plan view appears to show no issues, shadows can actually be cast due to elevation differences, slopes, or the heights of surrounding trees. In particular, on sloped terrain or graded/developed sites, ground surface elevations can vary by location, which can increase shading between array rows and from surrounding terrain more than expected. During the design phase, it is necessary to confirm shading factors not just from plan positions but also including height information.

Care must also be taken regarding inter-row shading. While installing more arrays increases the installed capacity, narrowing the spacing between rows makes it more likely that shadows from the front rows will fall on the rear rows during winter and at dawn and dusk when the solar elevation is low. Over-prioritizing installed capacity can affect annual energy yield and output during specific time periods. When checking inter-row shading in PVSyst, it is important not just to look at the annual loss rate, but to understand which months and times of day are most likely to be affected. Losses that look small on an annual basis can still be relevant to project planning in cases where winter generation is important.

In terrain shading, the effects of distant mountains and hills are easy to overlook. At power plants in mountainous or valley terrain, the sun can be hidden by the terrain just after sunrise and just before sunset, reducing the time available for power generation. Even if land appears open on a map, solar irradiation may actually be limited by the surrounding terrain. Under such conditions, it is advisable to verify the horizon obstruction using on-site photos, topographic data, and survey results. When handling terrain shading in PVSyst, it is also necessary to confirm that the horizon conditions entered reflect the on-site situation.

When setting shading losses, attention is required not only to underestimation but also to overestimation. It may seem prudent to assume larger shading losses for safety, but an unsubstantiated excessive loss assumption can make the project feasibility assessment look worse than it actually is. Furthermore, when making later design changes or comparing actual performance, it becomes unclear which effects were assumed and to what extent. What is important is not to enter conservative values, but to set values that can be explained based on site conditions and design information.

This is an item you should also document concerning future changes in the surrounding environment. Even if there is little shading now, trees on neighboring plots may grow, structures may be built nearby, or the layout of equipment on the site may change. However, if all future uncertainties are incorporated into losses, the simulation will become excessively conservative. Therefore, it is important to separate and organize conditions that are confirmed at present from those that should be monitored for future management. For example, one approach is to reflect current conditions in the power generation simulation and separately record tree management and surrounding-area checks as maintenance notes.

After configuring the shading losses, it is also important to check how the results appear. Because PVSyst reports allow you to review the breakdown of losses, if the shading loss is unreasonably small or large compared with other losses, review the input conditions. If there are clearly obstacles nearby but the shading loss is almost nonexistent, the obstacles’ heights, positions, or terrain conditions may not have been reflected. Conversely, if shading losses are prominent on an open, flat site, check for errors in the array layout or the obstacle model inputs.

Adjust temperature, soiling, and reflection losses to local site conditions

Temperature losses are an important factor affecting the power output of photovoltaic systems. PV modules heat up when exposed to sunlight, and in general the higher the cell temperature, the lower the output. Therefore, conditions such as ambient air temperature, ventilation, mounting structure, installation height, and whether the installation is roof-mounted or ground-mounted affect temperature losses. When setting temperature conditions in PVSyst, you should not look only at the local ambient temperature but consider the actual heat-dissipation conditions the modules will experience.

In ground-mounted power plants, because space beneath the racking is secured, ventilation is often easier to achieve than with roof-mounted installations. However, the local thermal environment changes depending on the surrounding topography, weed-control sheets, condition of the ground surface, slopes, enclosures, and so on. In regions that tend to become hot in summer in particular, expected temperature losses affect power generation. Rather than using standard input values as-is, it is necessary to verify their validity according to the installation method and site conditions.

Soiling loss is also an item that is easily overlooked in practical work. Sand and dust, pollen, bird droppings, fallen leaves, soil dust, and post-snowmelt grime that adhere to module surfaces reduce the amount of incident light. The degree of soiling varies depending on region, season, surrounding environment, rainfall frequency, condition of the ground surface, distance to farmland or factories, and dust from roads. PVSyst allows setting soiling losses, but simply entering generic values may not sufficiently represent site-specific risks.

For example, if there are unpaved roads nearby that readily generate dust, or if the site has a lot of bare ground immediately after development, soiling may increase for a certain period. In locations near farmland, seasonal dust and plant-derived deposits can also accumulate. In coastal areas, deposits containing salt can become problematic. These conditions relate not only to reductions in power generation but also to cleaning and inspection plans. It is important to consider simulated soiling losses together with maintenance policy, rather than treating them separately.

If soiling loss is set excessively low, it becomes difficult to explain when actual power generation falls below the simulation. Conversely, if it is set excessively high, it will make the project's economics look worse than necessary. In practice, it is desirable to organize the local environment, cleaning frequency, expectations for natural washing by rainfall, and surrounding dust factors, and set a defensible value. If a cleaning plan has not yet been decided, treat it as an assumed condition and review it once the operation and maintenance plan is finalized.

Regarding reflection losses, the angle at which light strikes the module surface is also important. Sunlight is more effectively utilized when it hits the module surface at an angle close to perpendicular, but when it strikes at an oblique angle some is reflected and the amount of light available for power generation is reduced. This effect is more pronounced at times when the solar altitude is low, such as in the early morning and late afternoon or during winter. Because the impact also varies with the module’s surface characteristics and mounting angle, reflection losses should not be viewed as a fixed value but understood as losses related to the angle of incidence of solar radiation.

The settings for tilt angle and azimuth affect not only temperature and reflection but also the overall power generation. When entering azimuth and tilt angles in PVSyst, you must verify that the notation on the drawings matches the angle definitions used by the software. If the sign convention or reference direction for azimuth is mistaken, the simulation assumptions will be incorrect even before loss settings are applied. In projects that feature east–west layouts, low tilt, or mixed arrays with multiple orientations in particular, you need to organize the conditions for each array before entering them.

Temperature, soiling, and reflection losses are treated as separate items, but in practice they are connected as site conditions. For example, in dry, windy regions dust may increase, but ventilation can sometimes suppress temperature rises. In regions with heavy rainfall, soiling is more easily washed away, while it is also necessary to check generation trends due to cloud cover and humidity. Not only entering loss items individually but also verifying that the overall site environment contains no contradictions leads to settings that avoid failure.

Confirm electrical losses and equipment-side conditions in the design information

In PVSyst's loss settings, the treatment of electrical losses is also important. Electrical losses include multiple elements such as module variability, mismatch between strings, wiring resistance, PCS conversion, transformer losses, auxiliary consumption, output limiting, and clipping. These change not only with equipment performance but also with design configuration, wiring routes, equipment capacity, and connection methods. Simply entering standard values into the on-screen fields may not sufficiently represent the actual plant conditions.

The first thing to check is the DC-side configuration. The number of modules in series, the number of strings, the configuration of the combiner/junction boxes, and the assignment of PCS inputs affect the voltage and current ranges and mismatch losses. If string lengths are not uniform, or if strings with different azimuths, tilt angles, or shading conditions are combined into the same input, losses due to differences in generation characteristics are more likely to occur. When setting up PVSyst, you need to verify not only the total capacity but also that the string configuration matches the design drawings.

Losses due to variability among modules should not be overlooked. Photovoltaic modules exhibit manufacturing-related output variations, so even units of the same model do not produce exactly the same output. Within a string, the overall output can be reduced by the influence of lower-output modules. In practice, considerations are based on module datasheets and shipping conditions, but when detailed information is not available, general assumptions may be used. Even in those cases, it is necessary to review the assumptions once the specifications are finalized.

Wiring losses are an item particularly closely tied to design information. Resistance losses vary depending on DC cables, AC cables, collector lines, the distance to transformer equipment, cable size, installation route, and current. In preliminary studies you may only know an approximate cable length, but in detailed design you should review this based on the actual route. If wiring losses are underestimated, the estimated power generation may be overstated. Conversely, if they are overestimated, you cannot properly assess the validity of the design.

Regarding PCS-side losses, not only conversion efficiency but also the input voltage range, MPPT configuration, DC oversizing, temperature conditions, and output limitations are involved. A PCS is a device that converts DC power to AC power, and certain losses occur during that process. Also, during periods when DC-side generation exceeds the PCS’s maximum output, output may be curtailed. It is important to understand this not as a mere equipment fault but as a design decision for system capacity. Increasing DC capacity can raise generation during low irradiance, while at high irradiance some output may not be fully utilized.

Losses in transformer equipment and up to the point of interconnection can be non-negligible depending on the project. When AC voltage is transformed within the power plant and transmitted to the point of interconnection, losses occur in transformers and cables. If the simulation scope—whether it covers up to the PCS output or up to the point of interconnection—is not clearly defined, the definition of generated energy will be inconsistent. It is important to specify to the owner and stakeholders which point the reported generation values refer to, as this is crucial for later comparisons and acceptance.

Don't forget auxiliary power consumption. At a power plant, monitoring devices, communication equipment, air conditioning, ventilation, control systems, and so on may consume electricity. Whether to deduct these consumptions from generated energy depends on how the evaluation target is defined. When assessing the amount of electricity sold or the output at the point of interconnection, the treatment of auxiliary consumption needs to be clarified. In PVSyst settings, it is important to clearly define which auxiliary loads are considered and to what extent, and to avoid double counting or failing to account for them.

Output curtailment and grid-side constraints may be treated either as losses in the simulation or separately as risks in the business plan. Depending on the plant’s region and interconnection conditions, there may be periods when output is restricted. However, it is not easy to accurately predict future amounts of curtailment. Therefore, if you enter them as losses in PVSyst, you must make the underlying assumptions clear and avoid conflating them with normal equipment losses. Being able to explain equipment-originated losses separately from constraints caused by external conditions will help reduce misunderstandings among stakeholders.

One thing to avoid when setting electrical losses is a mismatch between the design drawings and the simulation. For example, if PVSyst shows an ideal string configuration while the actual drawings include circuits with different orientations or shading conditions, the simulation results can appear better than reality. The same problem can occur if the simulation is not updated after the drawings are revised. To set losses correctly, you need an operational process that reflects the latest electrical design, layout design, and construction plans.

Verify the consistency of losses in result reports and integrate them into operations

After entering loss settings in PVSyst, it is important not to stop at simply checking the results report. You should interpret the energy production, performance ratio, and loss breakdown shown in the report and verify that the input conditions and the results are consistent. Just because numbers are produced does not mean they are reasonable. In practice, while reviewing the results you must check for input errors, misinterpreted assumptions, or loss settings that are excessively high or low.

The first thing to check is the balance of the loss breakdown. Verify whether shadow losses, temperature losses, soiling losses, wiring losses, conversion losses, etc., are reasonable compared with the site conditions and design assumptions. If a plant surrounded by many potential shading sources shows almost no shadow loss, it may indicate insufficient input for nearby shading or terrain shading. If temperature losses are extremely small in a region prone to high temperatures, the temperature conditions or mounting configuration settings should be reviewed. Rather than looking at loss rates in isolation, it is important to judge them against site photos, layout drawings, and specification sheets.

Next, what we want to check is the monthly power generation trends. Annual generation alone cannot capture seasonal effects. In projects prone to winter shading, losses that are not noticeable in the annual total can affect monthly generation during the winter months. In projects prone to temperature-related losses in summer, power generation may not increase as expected despite high solar irradiance. By examining monthly trends, you can gain a more concrete understanding of the causes of losses.

The performance ratio is also an important verification metric. The performance ratio is used as an indicator to see how efficiently generation equipment produces electricity relative to solar irradiance. However, it is not appropriate to judge the quality of a power plant solely by the performance ratio. Differences in meteorological conditions, installation angle, temperature, shading, and equipment configuration will change how the performance ratio appears. Do not simplistically conclude that a high performance ratio necessarily means a good design or that a low one necessarily means a poor design; it is necessary to check it together with the breakdown of losses.

When submitting a report to stakeholders, it is desirable to include an explanation of the input conditions and loss settings. If you present only the power generation results, the recipient cannot verify the assumptions behind the numbers. At a minimum, you should organize the assumptions about the meteorological data, system capacity, azimuth, tilt angle, major loss items, shading conditions, and the handling of output limits. If there have been design changes, prepare to explain the differences from the previous simulation to facilitate internal approval and customer explanations.

Simulation results can also be used for post-construction performance comparisons. However, when comparing with actual generation, you should not simply compare annual generation alone; it is necessary to take into account actual weather conditions, downtime, power curtailment, failures, cleaning status, snow accumulation, measurement scope, and so on. Simulations are projections based on standard or assumed conditions, while actual performance reflects that year’s weather and operational circumstances. If differences appear, analyzing which loss items differed from the assumptions will help improve the next design and operations.

In the operational phase, it is also effective to map the loss items set in PVSyst to inspection items. If soiling losses are anticipated, check the actual soiling conditions and cleaning history. If shading losses are being considered, check tree growth and changes in the surrounding environment. For wiring losses and equipment losses, monitor for abnormal heating, output decline, and differences between strings. By linking simulation and operation, the power generation forecast can be used not merely as design documentation but as a standard for asset management.

Managing updates to loss settings is also important. When multiple simulations exist—such as preliminary study, basic design, detailed design, and post-construction versions—if it is not made clear which is the latest and which conditions each one is based on, there is a risk of referencing incorrect documents. Record the file name, creation date, affected drawings, main changes, and the person who made the settings so that older materials can be distinguished from the latest. Especially when multiple people are working, do not leave management to individuals; instead, establish rules at the project level.

Establishing loss settings as an internal standard also contributes to quality improvement. If each person in charge sets values freely for each project, results can vary even under similar conditions. Of course, adjustments to match site conditions are necessary, but standardizing the basic approach, verification documents, review timing, and items to watch for can reduce configuration errors. In practice, it is effective not only to refer to the PVSyst manual but also to create operating rules that incorporate the insights gained from your own projects.

Finally, loss settings are not intended to conveniently adjust generation output. The objective is to appropriately reflect site conditions and design conditions and to produce a power generation forecast that stakeholders can accept. If you reduce losses to make the numbers look better, or increase losses without justification to avoid concerns, it will become difficult to explain later in the process. A reliable simulation requires the validity of input values, supporting evidence, interpretation of results, and update management.

Summary

When configuring losses in PVSyst, it is important not just to fill in the on‑screen fields in sequence, but to review design assumptions, site conditions, equipment specifications, and operational policies as a whole. Losses are not simply coefficients to reduce energy output; they are important settings to reflect effects that actually occur at a PV plant in the simulation. Therefore, you should not focus only on the magnitude of the values, but be able to explain why each value was chosen.

What is particularly important is to align the simulation objectives and assumptions before starting the loss settings. The level of precision of the information to be used changes depending on whether it is an initial study, detailed design, or post-construction comparison. When using assumed values, clearly state that they are assumptions and manage them so they are reviewed in later stages. Continuing to use outdated loss settings despite updates to drawings or equipment specifications will reduce the reliability of the energy yield assessment.

Regarding shading losses, it is important to treat near shading, inter-row shading, and terrain shading separately, and to verify not only the plan view but also elevation information and the surrounding environment. For losses from temperature, soiling, and reflection, site-appropriate settings must be made taking into account regional characteristics, installation method, cleaning plan, and meteorological conditions. For electrical losses, the organization of string configuration, wiring routes, PCS capacity, transformer equipment, auxiliary consumption, and the handling of output limitations should be arranged, and the simulation scope and the definition of power generation must be clearly specified.

In the results report, it is important to check not only the annual power generation but also the breakdown of losses, monthly trends, the performance ratio, and the differences from the previous simulation. By verifying that the loss breakdown does not contradict on-site conditions, that it does not contain extreme values, and that there are no input errors, you can produce a power generation forecast that is easy to explain. Additionally, by linking it to post-construction performance comparisons and maintenance management, the simulation can be used not only as a design-stage document but also as a basis for operational improvements.

To avoid failures in PVSyst loss settings, in addition to reading the manual, it is essential to establish verification rules that can be used for your company’s projects. By deciding which documents to check, which values to assume, and at what stage to review them, you can reduce variation among staff and produce simulations that are easier to explain both internally and externally.

In the design of solar power plants and in energy-yield assessments, not only the precision of loss settings but also how accurately site information can be captured is important. If surveying, layout verification, terrain assessment, shading checks, and post-construction records are managed consistently, the rationale for inputs in PVSyst becomes clearer. Combining the PVSyst manual with your company's verification rules and continuously reviewing design assumptions, site conditions, and operational records is fundamental to improving the reliability of energy-yield simulations.