How to Use PVSyst Loss Settings | 7 Checks to Prevent Energy Yield Differences

In solar PV energy-yield estimates, not only irradiance and panel capacity but also how loss settings are configured affects how the results appear. PVSyst allows you to set and check in detail factors that impact generation—temperature, wiring, soiling, mismatch, incidence angle, shading, shutdowns, degradation, and so on. On the other hand, if you proceed with default values or enter figures with unclear justification, the generation projected at the design stage and the generation after operation are likely to differ.

Note that the official product name is written as PVsyst, but it may also appear as PVSyst in searches and internal documents. In this article, we will use PVSyst to match search terms. For external documents and submission materials, it is important to use a consistent notation.

This article compiles seven points that practitioners who are learning how to use PVSyst should check in the loss settings.

Table of Contents

• PVSyst loss settings are a critical step to prevent discrepancies in energy output

• Check 1 Align temperature loss assumptions with site conditions

• Check 2 Review wiring losses based on distance and voltage drop

• Check 3 Set soiling losses according to region and maintenance practices

• Check 4 Do not underestimate mismatch losses

• Check 5 Verify incident-angle losses and near-field shading impacts separately

• Check 6 Account for losses from equipment downtime and degradation

• Check 7 Confirm consistency using the loss diagram before report output

• Summary for mastering PVSyst loss settings

PVSyst's loss settings are a crucial step in preventing discrepancies in energy production.

When estimating PV generation with PVSyst, the items that many responsible engineers first focus on are installed capacity, tilt angle, azimuth angle, and solar irradiance data. These form the foundation of the simulation results. However, confirming loss settings is just as important in practice to prevent discrepancies in predicted generation. In a photovoltaic system, the amount of sunlight striking the modules does not directly become AC output or usable electrical energy. Output is reduced by various factors such as module temperature rise, electrical resistance in wiring, surface soiling, equipment conversion losses, shading effects, downtime, and aging degradation.

PVSyst's loss settings are the process for incorporating on-site performance degradation factors that are difficult to avoid into the simulation. By configuring these carefully, you can reduce the gap between the projected energy generation at the design stage and the actual operational results after completion. Conversely, if you proceed with estimates without thoroughly checking the loss items, a generation report that looks tidy may still present figures that do not adequately reflect real site conditions.

In practice, even with the same installed capacity, losses manifest differently depending on site conditions. Whether the installation is rooftop or ground-mounted, whether the area experiences heavy snowfall or high dust levels, whether it is coastal and prone to salt-laden soiling, whether cable runs are long, and whether the power conditioners are appropriately sited all affect the expected energy production.

Therefore, PVSyst loss settings should be treated not merely as a data-entry task but as a process of quantifying site conditions.

When setting loss parameters, the important thing is not to make every figure more detailed than necessary. What matters is identifying which losses strongly affect power generation and which items are assumptions. Enter values that are supported by evidence, treat values with weak justification conservatively, and ensure they can be explained later. Calculation results may be used for internal review, customer explanations, materials for financial institutions, construction plans, and operation plans. If you are later asked "why did this amount of power generation occur?", being able to explain the rationale for the loss settings will increase the credibility of the estimates.

When you are not yet familiar with using PVSyst, the loss settings screen shows many items, making it easy to be unsure where to start. In that case, it is efficient to check the items in order of their impact on energy production. By checking temperature loss, wiring loss, soiling loss, mismatch loss, shading and angle-of-incidence losses, downtime rate, and degradation in that order, you can reduce oversights. This article explains these topics divided into seven practical perspectives you should check to prevent differences in energy production.

Check 1 Align assumptions about temperature loss with on-site conditions

Solar modules have the characteristic that their output decreases as cell temperature rises. Therefore, in PVSyst loss settings, the treatment of temperature losses is important. Although one might imagine that regions with high irradiance produce more energy, stronger irradiance also tends to raise module temperature, causing output reductions due to temperature. Especially on rooftop installations in summer or in poorly ventilated installation environments, module temperatures tend to be high, so underestimating temperature losses can lead to overestimating annual energy production.

In PVSyst you set how much the modules are cooled according to the mounting configuration and thermal conditions. Modules can have different temperatures at the same ambient temperature depending on whether they are installed on the ground where air can easily pass behind them or mounted close to a roof. The height of the racking, the distance to the roofing material, the wind flow, and surrounding obstacles also affect temperature conditions. In practice, it is important not to judge the installation method from drawings alone but to set assumptions based on the site’s ventilation conditions.

When checking temperature losses, first see whether the temperature conditions in the meteorological data differ significantly from actual site conditions. Even when using standard weather data, temperature and wind patterns vary in mountainous areas, coastal areas, urban areas, and snow-prone regions. On rooftops in urban areas, temperatures tend to be higher due to surrounding buildings and paved surfaces. Conversely, for ground-mounted installations with good airflow, module temperature increases may be relatively suppressed. Rather than relying solely on the settings in PVSyst, confirming them together with a description of the installation environment increases the credibility of the estimates.

Temperature loss should be checked not only on its own but also in relation to DC capacity and power conditioner capacity. In designs where module capacity is larger than power conditioner capacity, the input-side power increases during periods of strong solar irradiance. However, because module output decreases during periods of high temperature, judging peak output solely by irradiance can be misleading. Appropriately setting temperature loss changes the frequency of peak clipping events and the apparent annual energy yield.

Practitioners should avoid artificially minimizing temperature losses. Selecting cooling conditions that do not match site conditions in order to make estimated power generation appear higher will make explanations difficult in later stages. In calculations used for customer briefings and business feasibility assessments, it is important to set parameters that are consistent with site photos, layout drawings, racking conditions, installation height, and the like. Since PVSyst reports allow you to check the breakdown of losses, if the temperature loss is unnaturally small or large, you should review the settings.

Temperature loss settings are one of the items in estimated power generation that are prone to variation depending on experience. Even if you proceed with settings close to the initial values, always verify that those initial values are appropriate for the site. Rooftop, ground-mounted, wall-mounted, low-tilt, closely spaced layouts, etc.—temperature losses change when installation conditions change. When using PVSyst, it is important not to stop at entering the settings: check the proportion of temperature losses in the loss diagram and confirm there are no contradictions with the site conditions.

Check 2: Reassess wiring losses from distance and voltage drop

Wiring losses are one of the fundamental losses that occur in photovoltaic (PV) power systems. The power produced by the modules is sent through DC-side cables to junction boxes and power conditioners, and then sent via AC-side wiring to the receiving equipment and loads. In this process, part of the power is lost as heat due to the resistance of the cables. In PVSyst, ohmic losses on the DC and AC sides can be treated separately, so leaving these as rough estimates can cause discrepancies in predicted energy production.

When checking wiring losses, it is important first to understand the cable distances. Even if they look close on the layout diagram, the actual wiring route may require detours and end up longer than expected. In ground-mounted installations in particular, the distance from the string to the power conditioner, the distance from the power conditioner to the point of interconnection, and the routes of buried conduits and racks affect power generation. Even for rooftop installations, the routing from the roof surface to the equipment location and the wiring paths inside the building can change the distances.

When using PVSyst, it is important to consider the DC side and the AC side separately. On the DC side, losses vary with string voltage and current, cable cross-sectional area, and the number of parallel strings. On the AC side, losses depend on the power conditioner output, voltage, distance to the point of interconnection, and the size of the main feeder. For installations that include transformers, also confirm how transformer losses are handled separately from wiring losses. In the initial study phase, when not everything can be finalized in detail, it may be acceptable to provisionally use standard values, but as the design progresses these should be reviewed to match the actual wiring plan.

A common mistake with wiring losses is looking only at installed capacity and not sufficiently accounting for wiring distance. In large ground-mounted installations, because the panel area is extensive, wiring distances can vary greatly depending on equipment layout. Wiring losses change depending on where the power conditioner is placed, how combiner boxes are arranged or grouped, and the route taken to the receiving power equipment. Even a layout that appears to yield high generation in preliminary studies can see increased losses if wiring distances grow during detailed design.

Also, wiring losses affect not only energy yield but also construction costs and maintainability. Using thicker cables makes it easier to suppress losses, but it impacts ease of installation, material costs, and conduit sizes. Conversely, assuming thinner cables or longer wiring increases losses and requires checking voltage drop and heat generation. Because PVSyst’s loss settings do not substitute for the electrical design itself, it is important to ultimately align them with the as-built drawings, voltage drop calculations, protection coordination, and safety standards.

Whether the wiring loss values are appropriate can be checked on the loss diagram. If wiring losses are unnaturally small within the annual losses, review whether the inputs for cable distances and conductor cross-sectional areas are realistic. Conversely, if they are excessively large, there may be room to improve equipment placement or wiring routes. From the perspective of preventing generation discrepancies, it is important to reconcile the final drawings with the PVSyst input conditions and confirm that wiring lengths, cable specifications, and equipment placement have not changed significantly.

Check 3 Set soiling losses by region and maintenance method

When dirt adheres to the surface of a photovoltaic module, the amount of solar radiation it receives is reduced, resulting in lower power generation. PVSyst allows you to set soiling losses, but this parameter can vary depending on site conditions and the default value may not reflect the actual situation. Causes of soiling include sand and dust, pollen, yellow sand (Asian dust), bird droppings, fallen leaves, dust from farmland or factories, and deposits containing salt near the coast. Because the way modules become soiled differs by region and surrounding environment, it is important to set this parameter to match the specific site rather than using a uniform value.

When considering soiling losses, first confirm whether the environment allows rain to naturally wash the surfaces. On installations with a certain tilt angle, rain can more easily wash dirt off the surface. Conversely, on low-slope roofs or installations that are near-horizontal, dirt tends to remain. Mud and dust can accumulate at the lower edge of modules, leading to localized reductions in power generation. When setting soiling losses in PVSyst, you need to check not only the region but also the tilt angle and the cleaning schedule.

In practice, soiling losses are sometimes set too low. If you want to make the expected power generation look higher, you may be tempted to assume smaller soiling losses, but caution is necessary if there are unpaved roads, development sites, farmland, or material storage yards nearby. During construction and in the early stages of operation there is often a lot of dust, and module surfaces can become easily soiled. Additionally, in mountainous areas fallen leaves, tree sap, and bird damage can occur. Along the coast, soiling that contains salt tends to adhere and may not be washed off by rain alone.

Soiling losses become realistic when considered together with a cleaning plan. The value you should set changes depending on whether you assume scheduled cleaning, cleaning only when dirt is visibly noticeable during inspections, or leaving it to natural rainfall. While a higher cleaning frequency makes it easier to reduce soiling losses, if cleaning is not actually carried out there will be a discrepancy with the estimate. If you assume cleaning as a condition in PVSyst simulations, it is important to reflect that in the operations plan as well.

Considering soiling losses on a monthly basis can also be useful. In PVSyst, soiling losses can be treated not only as a constant annual value but as monthly values, so for sites with pronounced seasonality you should consider monthly assumptions. In some regions, soiling increases during dry seasons, pollen-heavy seasons, or periods when yellow sand is likely to arrive. In snowy regions, soiling can remain after snowmelt. Even if the annual average loss rate appears small, if soiling is concentrated during seasons with high power generation, the impact on annual energy production can be significant.

In checking soiling losses, it is important to verbalize the site's surrounding environment. If you briefly record in the estimation documents the nearby road conditions, dust sources, bird damage risk, and cleaning policy, it will be easier to explain the rationale behind the settings later. Regarding the use of PVSyst, it's important not to focus only on numeric inputs but to organize the reasons for adopting those numbers. Soiling losses tend to be overlooked, but they have high practical verification value in preventing discrepancies with actual power generation.

Check 4 Do not underestimate mismatch loss

Mismatch loss is the loss that occurs when modules or strings within the same circuit do not perfectly match in their characteristics. Photovoltaic modules of the same model can have slightly different outputs due to manufacturing variations, temperature conditions, irradiance conditions, and differences in degradation. In series-connected circuits, the lower-performing portions affect the overall current, so the generation will not be perfectly ideal. In PVSyst, such mismatches can be configured as a loss.

One key point to watch for with mismatch losses is that even when the design looks clean, variability occurs in the field. Even if modules have the same nominal output, their actual outputs have allowable tolerances. Also, wind exposure and temperature can differ depending on the installation position, causing operating conditions to change even within the same string. Furthermore, partial soiling, minor shading, and differences in the state of connections during installation can also increase mismatch.

When setting mismatch losses in PVSyst, it is important to check the module specifications, string configuration, and the balance of parallel circuits. Verify that the number of modules in series per string is consistent, that strings with different azimuths or tilts are not mixed on the same MPPT input, and that circuits with significantly different shading patterns are not grouped under the same control unit. If azimuths or tilts are mixed for design reasons, mismatch and operating-point deviations are more likely to occur, so these must be carefully reflected in the energy yield estimates.

Also, it is important not to confuse mismatch loss with shading loss or soiling loss. While shading can cause mismatch, the loss from the shade itself and the loss caused by output differences within the circuit are conceptually different. The way you treat soiling also changes depending on whether you consider it as uniform soiling or as variability caused by partial soiling. If you lump everything together as mismatch loss, it becomes difficult to identify which factor is reducing energy production. To prevent differences in power output, it is important to separate and organize the causes by loss category.

Mismatch losses tend to be underestimated when drawings are simplified during the initial study phase. Even if shading and orientation differences are not considered in a preliminary layout, the detailed design may reveal the effects of lightning protection equipment, fences, buildings, adjacent equipment, and terrain. As a result, solar irradiance conditions within the same circuit can become non-uniform. While it may be acceptable to proceed with standard loss values at the early stage, after detailed design you should review the circuit configuration and shading conditions to confirm that the mismatch loss estimate is realistic.

Mismatch losses may appear small numerically. However, as installed capacity increases, even a slight loss rate becomes a non-negligible difference when converted to annual energy production. Also, when comparing multiple options, if the mismatch loss settings are unfair between options, the comparison results will be distorted. For example, even with the same capacity, an option with aligned orientations and an option with multiple orientations mixed have different mismatch risks. In comparative evaluations, it is important to align the loss settings according to the design conditions of each option.

Check 5 Verify angle-of-incidence loss and near-field shadow effects separately

Solar modules generate electricity more efficiently when sunlight strikes them at angles closer to perpendicular. When sunlight enters at an oblique angle, a larger proportion is reflected at the surface and the amount of light received decreases. This angle-related loss is called incident-angle loss. PVSyst can account for losses due to the angle of incidence of light on the module surface. To prevent differences in energy production, it is important to check this incident-angle loss separately from shading losses caused by buildings, trees, and other obstructions.

Incidence angle losses are related to the azimuth and tilt settings. Designs that are close to south-facing with a moderate tilt tend to receive solar radiation relatively efficiently throughout the year. By contrast, east–west-facing, low-pitch, wall-mounted, or multi-surface installations increase the periods when sunlight arrives at an oblique angle, changing how incidence angle losses appear. After entering azimuth and tilt into PVSyst, checking the loss diagram to see how much incidence angle loss is occurring lets you understand how the installation surface conditions affect energy production.

Near-field shading occurs when surrounding obstacles cast shadows on modules. Buildings, trees, utility poles, rows of mounting racks, fences, equipment, slopes or embankments, and adjacent rows can all be sources of shading. For ground-mounted installations, insufficient spacing between rows can allow the shadow of the front row to fall on the rear rows. For rooftop installations, rooftop equipment, upstands or parapets, guardrails, and adjacent buildings can be problematic. Because shadows change with the time of day and season, it is important to assess how much they affect periods of high energy production rather than relying on a simple annual average.

Confusing incidence angle loss with near-field shading can lead to choosing the wrong corrective measures. If incidence angle loss is large, reviewing the tilt angle, azimuth, and choice of mounting surface is an effective countermeasure. On the other hand, if near-field shading is significant, changing row spacing, equipment layout, separation from obstructions, or module arrangement is effective. Correctly separating the causes of loss clarifies the direction for design improvements. When using PVSyst, after configuring the shading, it is important to check that losses due to shading do not overlap with incidence angle loss or other losses.

Also, when handling shading, you need to consider not only the visual shadow but also its electrical effects. If part of a module is shaded, that effect can spread to the entire string. Considering losses solely based on the proportion of shaded area can lead to underestimation. In PVSyst, the near shading and module layout settings can sometimes be used to approximate the electrical impact of shading. However, since not all site conditions can be fully reproduced, it is practical to prioritize significant obstacles and retain the rationale for your input assumptions.

When assessing near-field shading, it's important not to focus only on the period around the winter solstice but to evaluate its impact on annual energy production. In winter the solar elevation is low and shadows are long, but solar irradiance and energy output vary by season. Even if the effect appears large during periods with long shadows, its impact on annual production can be limited. Conversely, if shading occurs during the spring-to-autumn period when production is higher, it will have a larger effect on annual energy yield. In PVSyst outputs, checking monthly energy production and the breakdown of losses to see which seasons show differences helps with practical decision-making.

Check 6: Anticipate losses from equipment outages and degradation

In generation estimates, assuming that equipment continues to operate normally at all times tends to create discrepancies with actual results. Solar power generation systems can experience inspections, maintenance, equipment outages, communication failures, grid-side shutdowns, protection trips, output curtailment, component replacements, and the like during long-term operation. In PVSyst’s loss settings, thinking about such availability and downtime is also important. Time when equipment is stopped cannot generate power even if there is solar irradiance, and therefore directly affects annual energy production.

When considering losses from equipment downtime, it is easier to organize if planned and unplanned outages are treated separately. Planned outages include regular inspections, electrical equipment inspections, cleaning operations, and refurbishment work. Unplanned outages include equipment failures, operation of protective devices, communication failures, natural disasters, and grid-side troubles. While not everything can be predicted accurately, for long-term feasibility assessments it is realistic to anticipate a certain level of outage risk.

When setting system unavailability in PVSyst, it is important not to casually set losses to zero. In particular, if you prioritize only presenting maximized energy production, it becomes difficult to explain discrepancies with actual performance during the operation phase. In practice, the risk of downtime varies depending on factors such as system size, the presence or absence of remote monitoring, maintenance arrangements, response time to the site, and the availability of spare parts. A well-organized maintenance system makes it easier to keep downtime short, but when the site is remote and response times are long, the losses from downtime need to be assessed carefully.

The percentage of downtime does not necessarily match the rate of energy loss. Stopping at night has little impact on generation, while stopping during periods of strong solar irradiance has a much larger impact. Because the effect also varies with season and weather, when handling unavailability in PVSyst be careful not to present the specified time proportion directly as the annual energy loss rate. In calculation documents, it is important to treat downtime settings as statistical assumptions and not present them as overly precise predictions.

Long-term degradation is also an important item to check. Solar modules are equipment intended for long-term use, and their output gradually decreases over time. In PVSyst, the way degradation is treated differs depending on whether you look only at first-year generation or evaluate long-term generation. Even if degradation has a limited impact in a first-year estimate, it has a cumulative effect over the entire project period. When generation guarantees or financial planning are involved, it is important to account for long-term decline trends, not just single-year generation.

Also, settings for initial degradation require careful attention. For crystalline modules, output can change in the initial period after commissioning due to light-induced degradation. The detailed treatment varies depending on the module type and specifications, but when configuring this in PVSyst you must align it with the datasheets and the characteristics of the equipment you plan to use. If initial degradation, annual degradation, module quality, and mismatch are accounted for redundantly, the estimate can become excessively conservative. Conversely, if they are not considered at all, the reliability of long-term assessments will be reduced.

Losses from equipment downtime and degradation tend to be overlooked because they work to gradually reduce power generation. However, they are important for long-term operation. If a system is evaluated based only on first-year generation, maintenance arrangements and downtime risks can be missed. When using PVSyst, you should not only calculate annual generation but also consciously produce realistic generation figures that reflect operating conditions. In calculation documents, clearly stating how downtime rates and degradation rates were treated makes it easier to verify the assumptions later.

Check 7 Verify consistency with the loss diagram before report output

When you run an energy yield simulation in PVSyst, it is important to check the loss diagram at the end. The loss diagram shows where and how much loss occurs in the flow from solar irradiance reaching the module surface, being converted to DC power, and finally being output as AC power. Rather than looking only at the final energy yield value, examining the breakdown of losses makes it easier to spot configuration errors or unrealistic assumptions.

When checking a loss chart, first focus on the major loss items. Look at how much temperature loss, shading loss, power conditioner-related losses, wiring losses, soiling losses, and so on are occurring. If any items deviate significantly from typical ranges, review the input values and settings. For example, results such as very small temperature loss despite a rooftop installation, almost no wiring loss despite long cable runs, or no shading loss at a site with heavy shading warrant attention.

Next, verify the consistency among loss items. Check whether soiling loss, shading loss, and mismatch loss overlap, or conversely whether any of them are missing. If every factor that reduces energy production is set separately and large, the estimate can become overly conservative. On the other hand, omitting important losses will lead to an overestimated energy production. In loss settings, both overestimation and underestimation are problematic. Depending on the purpose, it is important to be clear whether the estimate is a standard estimate, a conservative estimate, or an estimate for comparative evaluation.

Before outputting the report, we also verify the connection between input conditions and output results. We check that the installed capacity, number of modules, tilt angle, azimuth, equipment capacity, meteorological data, wiring losses, soiling losses, and downtime rate match the drawings and plans. Even if the PVSyst screen looks correct, some settings from another project may remain if they were copied. When reusing files from past projects, be sure to review not only the site name but also the loss settings.

When comparing multiple proposals, fairness in loss settings is important. For example, when comparing layout Option A and layout Option B, if only one has reduced soiling loss or a lower downtime rate, it becomes unclear whether the difference in power generation is due to design differences or to differences in the settings. In comparative evaluations, separate and manage assumptions that should be common from those that should vary by option. Tilt angle, azimuth, and shading conditions will vary by option, while regional meteorological data and basic maintenance conditions should be kept common; it is important to clearly define the rules for comparison.

PVSyst reports may be shared with stakeholders who are not specialists. Therefore, being able to explain how to read the loss diagram will make internal and external meetings run more smoothly. Rather than presenting only the final energy yield, being able to explain which losses are large and which are minor can lead to design improvements and inform investment decisions. In particular, if shading losses or wiring losses are large, they may be improved by changing the layout or reviewing equipment placement. The loss diagram can be used not merely to confirm results but as a hint for design improvement.

Summary: How to Master Loss Settings in PVSyst

PVSyst's loss settings are a critical step that determine the accuracy of energy yield estimates. Even with the same solar irradiance and installed capacity, annual energy production can vary depending on how temperature, wiring, soiling, mismatch, incidence angle, shading, downtime rate, and degradation are treated. Proceeding with loss settings at their default values can fail to reflect site conditions and may result in large discrepancies from actual energy production. Conversely, carefully confirming site conditions and inputting evidence-based values will produce estimates that are easier to explain to stakeholders.

In practice, what matters is not entering every loss in detail but not overlooking the items that affect energy production. For temperature losses, check the installation type and ventilation conditions; for wiring losses, check the distance and cable plan. For soiling losses, reflect the region and cleaning policy; for mismatch losses, look at the circuit configuration and any mixed orientations. Separate and organize the causes of incident-angle losses and shading losses, and have the downtime rate and degradation over time reflect the realities of long-term operation. Finally, check the loss diagram and review whether the input conditions and output results are consistent, which makes it easier to prevent discrepancies in energy production.

When mastering the use of PVSyst, loss settings are unavoidable. It is not enough to simply enter numbers on the screen; you must understand the site, cross-check with drawings, and consider operating conditions. Generation estimates serve as a common document linking design, construction, maintenance, and business feasibility assessments. Therefore, documenting the rationale for loss settings and being able to explain them leads to improved practical quality.

When performing a solar power generation yield estimate, it is efficient to first turn the loss settings into a checklist so you can review each project from the same perspectives. Rough values are acceptable in initial studies, but it is important to update them as you move into detailed design to match drawings, equipment specifications, wiring plans, and maintenance policies. To make PVSyst simulation results usable as on-site decision-making material, improving the accuracy and explainability of the loss settings is indispensable.

In planning solar power generation, it is necessary to improve accuracy across the entire process—not only in power generation estimates but also in site surveys, layout planning, construction management, and operational verification. If you want to prevent differences in generated energy and reduce discrepancies between design and the field, it is important to, alongside the approach to loss settings, establish methods for acquiring on-site data, rules for updating drawings, and procedures for managing inspection records.