6 Input Mistakes and Countermeasures That Trip Up PVSyst Beginners

When forecasting solar power generation and assessing project feasibility, not only the simulation results but also the conditions entered are important. Practitioners using PVsyst for the first time may think they are filling in the on-screen fields in order, but assumptions about meteorological data, installation conditions, modules, inverters, loss conditions, shading conditions, and so on can be off, producing results that are either overly optimistic or too pessimistic compared with reality.

In this article, aimed at beginners searching for "how to use PVSyst", we break down six common mistakes and their countermeasures that often trip people up during the input stage. Rather than just memorizing operational steps, read with an awareness of which inputs are most likely to affect the results and what to check to make your simulation easier to explain.

Note that PVsyst screen names and input methods may vary depending on the version, project type, license, and the database being used. This article does not cover detailed screen operations for a specific version; instead, it organizes practical checkpoints to help prevent input errors.

Table of Contents

• Errors in selecting regional meteorological data

• Failing to align azimuth and tilt angles with on-site conditions

• Mixing up module and capacity specifications

• Mistakes in configuring power conditioner specifications accurately

• Using loss rates unchanged from their initial/default values

• Oversimplifying shading conditions and surrounding obstacles

• Verification procedures to reduce input errors

• Summary

Mistake: Selecting the Wrong Region for Meteorological Data

One thing PVsyst beginners often stumble on first is selecting meteorological data. In solar power simulations, meteorological conditions such as solar irradiance, ambient temperature, and, when necessary, wind speed affect power generation. Therefore, if the meteorological data chosen at the outset deviates from local conditions, even if you carefully enter the system parameters afterward, it becomes difficult to justify the reliability of the results.

A common mistake is to pick a point that seems close to the site by feel and proceed without fully considering differences such as elevation, coastal vs. inland, mountainous areas, snowy regions, or basins. Even if distances look short on a map, the meteorological conditions are not necessarily similar. Within the same prefecture, temperatures and cloud formation can differ between the coast and the interior. In some areas, solar radiation conditions change simply because a mountain lies in between. In practice, it is important to choose not based on “it’s nearby” but from the perspective of “can it represent the locational characteristics of the power plant.”

Another input mistake is failing to record which weather data was adopted when multiple candidates exist. When reviewing a simulation later, if the source, location, covered period, and whether any corrections or conversions were applied to the weather data are unknown, it becomes difficult to explain why the power output is high or low. For internal checks and explanations to clients, being able to trace the assumptions is more important than the numbers themselves. Beginners often focus too much on progressing through the settings screens and tend to finish the work without leaving a record of why they made a given selection.

As a countermeasure, first document the power plant’s location, elevation, surrounding topography, and climatic characteristics, and check the differences between these and candidate meteorological sites. When selecting meteorological data, assess not just proximity to the address but whether the solar radiation and temperature conditions are representative of the site. Pay particular attention in mountainous areas, regions with heavy snowfall, locations influenced by sea breezes, and urban heat environments, since differences with nearby stations are likely to show up in the results.

When you select meteorological data, record the chosen site name, data type, target period, and reason for selection in the project memo and the input conditions list. If another person reviews the same project in the future, knowing why those conditions were chosen will make recalculations and condition changes easier. When learning how to use PVsyst, it is important not only to proceed correctly through the input screens but also to develop the habit of documenting the rationale for your decisions.

When reviewing meteorological data, do not look only at the annual solar radiation; also check monthly trends. Even if the annual values appear reasonable, patterns in winter or during the rainy season may deviate significantly from local observations. In regions affected by snowfall and low temperatures, it is necessary to confirm that winter power generation is not being overestimated relative to actual conditions. Conversely, verify—together with the temperature data—whether output reductions due to high summer temperatures are being accounted for.

For beginners, meteorological data may look like an item you only choose at the beginning, but in reality it is the foundation of the entire simulation. If this is off, subsequent comparisons and improvement evaluations will also be off. Treat the initial inputs with particular care: verify whether they are close to local conditions, whether you can explain the reasons for your selections, and whether the monthly trends feel reasonable—this is the first step toward a reliable simulation.

Failure to match azimuth and tilt angles to site conditions

A common stumbling block in PVsyst is entering the azimuth and tilt angles of the photovoltaic modules. The azimuth indicates which direction the panels face, and the tilt indicates the angle at which they are installed. Both affect energy production and the time-of-day generation profile, so incorrect entries make the results difficult to explain.

A common mistake among beginners is entering the angle shown on a drawing without fully confirming its meaning. If you don’t check whether the angle on the drawing is referenced to true north, magnetic north, or the site’s baseline, the actual orientation and the entered value will be misaligned. In PVsyst, the conventions for azimuth reference and the handling of positive/negative signs can differ from typical architectural drawings, so you need to check the definition of the input field before entering values.

Using a fixed installation in the Northern Hemisphere as an example, PVsyst treats south as 0° and uses the convention of entering west as positive and east as negative. Because drawings in Japan often express orientation relative to north, entering the angle from the drawing unchanged can result in east and west being reversed or north and south being confused. For the tilt angle, confirm whether the value is the angle from the horizontal plane or a roof pitch notation, and, if necessary, convert it to an angle before entering it.

In rooftop installations, the orientation and tilt of each roof surface can differ. If you summarize everything into a single representative value, the breakdown of power generation can end up not matching the actual situation. This is especially true for building installations that include east-, west-, and south-facing surfaces: unless the surfaces are treated separately, it becomes difficult to explain which surface produces more power and which is more prone to losses.

Even for ground-mounted installations, when multiple rows are arranged in the east–west direction, the overall orientation may appear uniform, but local slopes can vary along the terrain. If the graded surface has a slope or the mounting structure design differs by area, a simple bulk input may not fully capture the on-site conditions. In practice, you need to cross-check design drawings, layout plans, field survey results, and construction plans to decide which value to use as the representative value.

The countermeasure is to standardize the definitions of azimuth and inclination within the project before data entry. For azimuth, confirm what reference direction is used and which direction positive and negative angles indicate. For the inclination angle, clarify whether it is the angle from the horizontal plane or a value converted from a roof pitch notation. Because treating a pitch notation directly as an angle can lead to input errors, always verify the units and meaning.

After entering the data, check not only the results screen but also the displayed settings and the azimuth and tilt angles on the report. Compare them with the drawings to confirm that angles are not reversed, roof surfaces have not been mistaken, and tilt angles are not set to extreme values. If possible, have another person compare the drawings and the input values to help prevent simple mix-ups.

The azimuth and tilt angles are basic inputs when using PVsyst. However, precisely because they are basic, mistakes are often overlooked. It is important not only to fill in the numbers on the screen but also to be able to explain which drawings, which site conditions, and which design policies those numbers are based on.

Mistaking module and capacity conditions

When inputting PV modules you handle the model, rated output, number of modules, number in series, number in parallel, total capacity, and so on. Errors here are directly reflected in the simulation results, so this is a point beginners should be especially careful about. PVsyst lets you set many conditions in detail, but because of that high degree of freedom, if you fail to cross-check with design drawings and equipment specifications you may end up simulating a plant that differs from the actual one.

A common mistake is confusing the nominal output per module with the capacity of the entire installation. You may think you have entered the number of modules but end up calculating based on the number of strings or circuits, causing the total capacity to not match the design value. Conversely, you may focus solely on matching the system capacity and proceed while the series and parallel configurations differ from the actual electrical design.

Another mistake is choosing data for similar conditions without checking the module’s specification values. Solar cell modules can differ in voltage, current, temperature coefficients, dimensions, and cell configuration even within the same power output range. If the input data differ from the actual equipment, it will affect compatibility assessments with the power conditioner and the output changes due to temperature. Beginners often pick modules simply because their names are similar, but you need to verify them by cross-checking with the specification sheet.

As a countermeasure, start by organizing from the drawings and equipment lists the module model, rated output, number of modules, number in series, number in parallel, and the capacity for each installation surface. Then verify that the total capacity in PVsyst matches the design documents. What is important here is to check not only the total capacity but also whether the configuration matches. Even if the total capacity is similar, differences in series or parallel counts can change the voltage range and how losses manifest.

After entering the data, check the capacities in order by installation surface, by circuit, and for the entire facility. Especially in projects that are divided into multiple areas, it is easy to forget to enter some areas or to enter the same area twice. Aligning the area names on the drawings with the division names used in the simulation makes later review easier. If the names are inconsistent, you may not be able to tell which conditions refer to which areas during review.

In module capacity specifications, care must be taken not to confuse units. When watts, kilowatts, module counts, and string counts are mixed, input errors by orders of magnitude can occur. After entering numbers, check whether the total system capacity is not significantly different from the capacity in the design documents and whether it falls within a reasonable range. If an extremely high power generation value appears, it is wise to suspect double-counting of capacity before blaming irradiance conditions.

Also, when the module type or number is changed during the design process, verify that no previous simulation conditions remain. If initial study conditions are duplicated and used for detailed analysis, old module conditions can persist. As the project progresses, it is important to make clear at which point in the design timeline the calculation results correspond.

Inputting the module conditions is a part that requires a lot of work and has many on‑screen items to check for PVsyst beginners. However, if you organize this carefully, it will make subsequent loss analysis and explanations of power generation easier. It is important to sequentially verify the system capacity, circuit configuration, specification values, and the breakdown for each installation surface, and to ensure that the input results are linked to the design documentation.

Mistake: Unable to set power conditioner conditions without excess or deficiency

Entering the power conditioner conditions is another item where beginners often stumble. The DC power generated by the photovoltaic modules is converted to AC power through the power conditioner. Therefore, the input conditions involve conversion efficiency, capacity, number of units, input circuitry, voltage range, and how output limits are handled. If this is oversimplified too much, not only the amount of generated power but also the breakdown of losses will deviate from reality.

A common mistake is inputting values without checking the relationship between module capacity and the power conditioner (inverter) capacity. In photovoltaic systems, the DC-side capacity and the AC-side capacity do not necessarily match. The DC side is sometimes designed larger, and in those cases the inverter can reach its output limit during periods of high solar irradiance. If beginners proceed without understanding this, losses due to output limiting or clipping may be underestimated or overestimated.

Another common mistake is entering the number of power conditioners and the circuit configuration as a single consolidated input. Treating equipment that is actually split into multiple units as one large device makes it difficult to see the conditions of each input circuit and the allocation of capacity. While simplification may be acceptable at the rough-estimate stage, when used for detailed review or presentation materials it is necessary to clarify to what extent the actual equipment configuration is being reflected.

As a countermeasure, check the power conditioner's specification sheet and the single-line wiring diagram, organize the input conditions, and then reflect them in PVsyst. In particular, verify the DC input range, maximum input, rated output, conversion efficiency, number of units, and string assignments. Check whether the capacity on the design documents corresponds to the capacity in the simulation. Even if the input values appear correct, an unnatural string assignment can lead to infeasible electrical combinations.

For beginners, it is recommended to check module conditions and power conditioner conditions not separately but as a combination. When the number of modules in series changes, the voltage conditions change and the relationship with the power conditioner’s input range also changes. Whether the voltage becomes too high at low temperatures or whether it goes outside the operating range at high temperatures relates not only to power generation but also to verification in equipment design.

Care is also required in handling output limits. When DC power exceeding the rated output of the power conditioner is fed in during periods of high solar irradiance, the AC output side can become capped. Whether this is regarded as a loss or accepted in the design depends on the project. If simulation results show losses due to output limiting, confirm whether they are the result of an intended design policy or an input error.

In projects where conditions on the AC side or constraints related to grid interconnection must be reflected, simply selecting a power conditioner is not sufficient. If there are limits on the overall plant output, contractual restrictions, control policies, or similar factors, they need to be treated as assumptions for the power generation simulation. However, because it may not always be possible to reproduce detailed control conditions exactly, it is helpful to clarify up front which items will be included as input conditions and which will be evaluated separately, as this makes explanations easier.

Power conditioner settings may feel a little difficult for beginners, but they are an important input item alongside module conditions. By checking the relationships among capacity, number of units, input circuits, and output limits, and carefully reviewing whether the design documentation corresponds to the simulation conditions, you can improve the reliability of the results.

Mistake of leaving the loss rate at its initial value

In PVsyst simulations, you set various loss conditions that affect energy production. Wiring losses, soiling losses, temperature-related conditions, mismatch, downtime, shading effects, and so on — in actual power plants many factors reduce output below the theoretical energy yield. A common pitfall for beginners is not thoroughly checking these loss conditions and simply using initial values or the figures from past projects as-is.

Loss rates may look like small numbers and are often treated lightly. However, a difference of just a few percent in annual generation can affect project viability and explanatory materials. If loss assumptions are more lenient than reality, estimated generation may appear overstated. Conversely, being overly conservative can lead to a harsher assessment than reality and cause the installation’s value to be underestimated.

A common mistake is to set soiling losses to a single uniform value without considering the region or installation environment. In locations with a lot of airborne dust, areas prone to bird fouling, places susceptible to pollen or salt, or low-tilt installations where rain does not easily wash away dirt, the effects of soiling differ. The way losses are considered also changes depending on cleaning frequency and maintenance policy. Leaving the default initial value may not adequately reflect the conditions specific to that project.

Regarding wiring losses, proceeding with approximate values can lead to discrepancies with the actual design. Losses vary depending on cable length, conductor size, circuit configuration, and current conditions. While estimates may be acceptable in the early design phase, during the detailed design stage they need to be aligned with the single-line wiring diagram and the cable plan. If you prepare a report without reviewing the input values, it may later be found to be inconsistent with the design documentation.

The countermeasure is to make it possible to explain, for each loss item, “why that value is used.” Even if you cannot calculate everything in detail, separately record whether you used initial values, in-house standard values, project-specific values, or conservative assumptions. For example, organize the rationale for each item: soiling loss as a value based on local environmental conditions and cleaning policy, wiring loss as a value based on design documents, and shutdown-related conditions as values based on operational assumptions.

Beginners often become unsure which loss factors are important when there are many. In that case, prioritize checking the factors that have the largest impact on the results. Dirt, temperature, shading, wiring, output limits, and shutdowns are items frequently raised when explaining power generation. Rather than trying to make everything perfect, it is important to set the significant losses based on sound justification.

Also, when reusing conditions from past projects, check the differences between the source project and the current one. If the region, installation type, module tilt, maintenance policy, or surrounding environment differ, the same loss rate may not be appropriate. Values from past projects are useful as references, but they are not necessarily correct as-is. Even when reusing them, you must confirm whether they can be applied to the current project without issue.

Loss rates can sometimes appear in simulations as "adjustment items." However, in practice it is the loss conditions that are most likely to be subject to accountability. Rather than reducing losses to produce favorable results, it is important to set the values as outcomes that reflect local site conditions and operating conditions. By developing the habit, from the moment you start using PVsyst, of checking each loss item one by one, you will create simulations that you can explain later.

Mistakes Caused by Oversimplifying Shading Conditions and Surrounding Obstacles

A commonly overlooked input in solar power generation simulations is the shading conditions. When solar irradiance is blocked by surrounding buildings, trees, utility poles, fences, mountains, rooftop equipment, adjacent rows of panels, etc., power generation decreases. PVsyst can account for shading from distant terrain and shading from nearby obstacles, but beginners may find the inputs difficult to handle and therefore oversimplify them or proceed assuming no shading.

If shading conditions are underestimated, estimated power generation can be higher than actual output. This is particularly true for sites that receive shadows in the morning and evening, regions where the sun’s altitude is lower in winter, building rooftops with many pieces of equipment, and locations surrounded by mountains or trees, where the impact of shading can be significant. Because reductions may occur not only in annual energy yield but also during specific times of day or seasons, how shading is handled is important when evaluating self-consumption and peak shaving.

A common mistake is to look only at on-site photos and conclude, "there are no large obstacles." Depending on the time of day when the site survey is conducted, shadows may not be visible. Even if there is no problem in summer, in winter the sun's altitude becomes lower and shadows can lengthen considerably. In other words, the absence of shadows at the time of the survey is not the same as the absence of shadows throughout the year.

Another common mistake is entering approximate values for the heights and distances of surrounding obstructions without accurately determining them. Even if an obstruction appears to be somewhat distant, it can still have an effect if it is tall. Conversely, an obstruction that seems close may have little impact if it lies outside the sun’s path. Do not judge based on distance alone; you need to verify the relationship between azimuth, height, and solar altitude.

As a countermeasure, before entering shadow conditions, organize the types, positions, heights, and orientations of surrounding obstructions. For building-mounted installations, check rooftop equipment, railings, penthouses, and adjacent buildings. For ground-mounted installations, check trees, slopes, adjacent structures, and shadows between rows of panels. If drawings or survey data are available, use them to organize the input conditions. When using site photographs, record the shooting direction and date and time of capture so they can be easily verified later.

Even if shading conditions cannot be reproduced in detail, record that there may be an impact and make the assumptions behind the simplification clear. In the preliminary stage it may be acceptable to proceed with a simplified model, but in that case it is necessary to share internally that "the effect of shading may not be fully reflected." When entering the detailed review stage, reexamine the shading conditions together with on-site surveys and layout studies.

Shading between adjacent rows of panels is also important. For ground-mounted installations, the distance between rows, tilt angle, and panel height can cause the front row to cast shadows on the rear row. Shadows are especially long during mornings and evenings in winter, affecting annual and monthly energy generation. Reducing the distance between rows increases installed capacity but may also increase shading losses. Checking shading conditions is essential when assessing the balance between energy generation and installed capacity.

In the shading input, also check the breakdown of losses on the results screen. If losses due to shading are extremely large, review whether the height or position of obstacles were entered incorrectly. Conversely, if there are obvious nearby obstacles but shading losses are almost zero, you should suspect missing inputs or insufficient modeling. When the results feel odd, it is effective to check not only the total power generation but also the monthly, by-time-of-day, and by-loss-item breakdowns.

Shading conditions, although time-consuming to input, strongly influence the credibility of the results. Beginners should, rather than aiming for a perfect three-dimensional reproduction, start by identifying the factors likely to have an impact and separating those that can be ignored from those that need to be checked. Instead of judging shadows simply as present or absent, it is important to input them from the perspective of "in which seasons, at what times of day, and to what extent they are likely to have an effect."

Verification procedures to reduce input errors

For PVsyst beginners to reduce input errors, it's important not only to understand each field but also to establish a fixed order for checks. Because simulations involve switching between many screens, if you make corrections in the order they occur to you, you can lose track of what you've already checked. This is especially true for projects that have undergone design changes, where old and new conditions tend to become mixed.

First, what you need to confirm are the project's basic conditions. Clarify the location, meteorological data, equipment names, the purpose of the simulation, and the stage of the study. The level of input accuracy required varies depending on whether it is a preliminary estimate or an analysis approaching detailed design. If you proceed with an unclear purpose, you risk refining only the details while important assumptions remain outdated.

Next, check the system capacity and configuration. Review module type, number of modules, mounting surfaces, number of series connections, number of parallel strings, number of power conditioners, and consistency of capacities. Here, cross-check not only the figures in PVsyst but also the design drawings and equipment lists. Confirm that the total capacity matches the design documents, that no area’s capacity is missing, and that there are no anomalies in the circuit configuration.

Next, check the installation conditions. Review the azimuth, tilt angle, mounting height, row spacing, and the conditions of the roof surface or ground-mounted racking. Verify that the reference azimuth on the drawings matches the azimuth entered, and that you have not confused the unit or meaning of the tilt angle. If there are multiple surfaces, listing the conditions for each surface makes it easier to notice any missing inputs.

Next, check the loss conditions. Review, in order, the conditions that affect power generation—dirt, wiring, temperature, mismatch, shutdowns, output limits, shading, etc. It is not necessary to examine every loss to the same depth, but leave justification for items that have a large impact on the results. If initial values are used, confirm why those initial values are acceptable. If company standard values are used, verify whether they can be applied to this project.

Finally, verify the validity of the results. Review the annual generation, monthly generation, performance ratio, and loss breakdown to ensure no extreme values appear. Input errors can show up as anomalies in the results. For example, if winter generation is unnaturally high, if shading is expected but shading losses are almost zero, if generation is excessive relative to the installed capacity, or if output curtailment is abnormally large, return to the input conditions and check.

In practice, it is also effective to separate the person who enters data from the person who verifies it. If the same person performs both entry and verification, they may overlook mistakes due to assumptions. Even having another staff member simply compare the drawings, equipment lists, input conditions, and result reports makes simple input errors easier to catch. Especially for someone using PVsyst for the first time, it is reassuring not to complete everything alone and to establish a system in which an experienced person checks the work once.

When reviewing, file names and version control are also important. If you overwrite a file each time conditions change, you won't be able to tell which results were based on which conditions. Use names that indicate the review date, design conditions, and main changes so that later comparisons are easier. It's important to keep things in a state where, when power output changes, you can trace whether the weather data, the capacity, or the loss conditions were changed.

When learning how to use PVsyst, a shortcut is to have a checking framework, not just to memorize the order of the screens. If you review in the order of basic conditions, system configuration, installation conditions, loss conditions, and result validity, you can more systematically detect input errors. As you become more experienced, you'll be able to infer suspicious input fields just by looking at the results, but for beginners it is safer to proceed carefully, using a checklist.

Summary

Input mistakes that PVsyst beginners stumble over are not merely simple operational errors. Choices of meteorological data, interpretation of azimuth and tilt angles, organization of module and capacity conditions, combinations with inverters, justification for loss rates, and treatment of shading conditions — how design documents and on-site conditions are interpreted plays a major role in the results.

In a simulation, you will get results simply by filling in the on-screen fields. However, what is required in practice is not just producing results, but being able to explain which assumptions those results are based on. Whether power output is high or low, you need to be able to trace which of the meteorological conditions, equipment conditions, loss conditions, or shading conditions are affecting it.

What beginners should be aware of first is not to treat input values as isolated numbers. Weather data should be considered in connection with site conditions. Azimuth and tilt angles should be checked against drawings. Modules and power conditioners should be linked to equipment lists and electrical design. Loss rates should be related to operating conditions and maintenance policies. Shading conditions should be connected to the local surroundings. By verifying these connections, input errors can be greatly reduced.

Also, the results from PVsyst should not be considered separately from on-site surveys and post-construction management. If the conditions assumed during the planning stage differ from the actual site conditions, the estimated power generation will change. By connecting the simulation at the design stage, verification during construction, and power-generation management after operations begin, you can achieve a more realistic evaluation.

To prevent input errors, it's important not to rely solely on the operator's attention but to establish verification procedures, recording methods, and a review system. In particular, meteorological data, system capacity, azimuth, tilt angle, loss conditions, and shading conditions should be fixed as items to check every time. Because power generation simulations are built on the accumulation of small inputs, it's the more mundane checks that support the reliability of the results.

The first step to mastering PVsyst is not to memorize all of its complex functions, but to avoid common input errors and build simulations on assumptions you can explain. Personnel involved in the planning, design, and operation of power plants should not judge based solely on the numbers on the screen; by proceeding with inputs while cross-checking against site conditions, they can expand the ways the results are used.

Furthermore, in managing a solar power plant, it is important to continuously compare the conditions assumed in simulations with the actual on-site situation. Even if the design judged that there would be little shading, power generation can change due to changes in the surrounding environment, soiling, equipment degradation, and operational downtime. By establishing a system to verify causes of reduced generation on-site and to re-evaluate as necessary, simulations become not merely planning documents but foundational materials for operational improvement.

To apply PVsyst power generation forecasts in practice, it is effective to consider the whole process, including acquisition of on-site data, verification of equipment condition, visualization of power generation, and linkage with inspection records. If you can connect the assumptions used in the simulation with on-site measured data, it will help not only to detect input errors but also to improve operations and identify the causes of reduced power generation.