What is PVSyst? Five common misconceptions about power generation forecasting

PVSyst is a specialized simulation software used to predict the energy production of photovoltaic systems and to organize design conditions and loss factors. In practical solar PV work, various conditions affect energy production, including system capacity, installation location, solar irradiance, orientation, tilt, shading, ambient temperature, wiring, and equipment configuration. PVSyst is useful for inputting these conditions and checking annual and monthly energy production, loss breakdowns, and performance indicators.

On the other hand, PVSyst is sometimes misunderstood as "software that automatically produces the correct energy yield if you just input data." In reality, PVSyst's results are heavily influenced by the input conditions. If on-site information is inaccurate, even a well-formatted report can lead to predictions that deviate from actual energy production. This article, aimed at practitioners searching for "What is PVSyst", organizes five common misconceptions about energy yield forecasting and explains how they should be understood in practice.

Table of Contents

• PVSyst is not software that automatically guarantees energy production.

• Misconception 1: If you input data into PVSyst, it will always output accurate energy production.

• Misconception 2: You can judge the quality of a design by looking only at the annual energy production.

• Misconception 3: Smaller losses always result in better outcomes.

• Misconception 4: Shading and terrain won't significantly affect results even if modeled roughly.

• Misconception 5: You can make business decisions based solely on PVSyst results.

• How to correctly use PVSyst power generation forecasts in practice.

• High-quality site information is indispensable for improving PVSyst's accuracy.

• Leverage PVSyst to produce explainable power generation forecasts.

PVSyst is not software that automatically guarantees energy production

PVSyst is analysis software that simulates the energy production of photovoltaic systems and allows users to check the generation and losses for each design condition. In power generation projects and equipment design, it is necessary at the planning stage to forecast future energy production. Even with the same installed capacity, the actual energy output varies depending on local solar irradiance, panel orientation, tilt angle, surrounding shading, temperature, wiring conditions, equipment configuration, and installation method. PVSyst is used to organize these conditions and to build the basis for energy production forecasts.

However, PVSyst does not guarantee energy production. It is software that simply displays calculation results based on the input conditions. If the input conditions closely match the actual site conditions, the reliability of the energy production forecast is likely to improve. Conversely, if the installation location, orientation, tilt, shading, loss conditions, and so on are inaccurate, the reported energy production will also deviate from reality. In other words, the accuracy of PVSyst depends heavily not only on the software itself but also on the quality of the information entered.

What beginners should first understand is that PVSyst's results are not a "correct answer" but predictions based on assumptions. The actual performance of a photovoltaic system varies due to year-to-year weather differences, equipment aging, soiling, maintenance, installation accuracy, and changes in the surrounding environment. PVSyst cannot fully predict these for the future. It is used at the design stage to set reasonable conditions and to check how much energy generation can be expected under those conditions.

If you use PVSyst without this understanding, you tend to judge based solely on the annual energy production shown in the report. However, in practice you need to verify not only the energy production figures but also the conditions from which those figures were derived. You should confirm whether the meteorological data are appropriate, whether the shading settings match the on-site conditions, whether the loss assumptions are realistic, and whether the equipment configuration is feasible; only after these checks can you treat the simulation results as a basis for judgment.

PVSyst is not a magical tool that automatically guarantees energy production; it is a tool to organize the mechanisms that determine energy production and to verify the validity of design conditions. Simply keeping this premise in mind will help you avoid many common misunderstandings in energy production forecasting.

Misconception 1: If you enter data into PVSyst, it will always produce accurate energy generation estimates

The most common misconception about PVSyst is the belief that entering the required inputs will necessarily produce an accurate energy production estimate. Indeed, PVSyst is a professional software that can calculate solar power generation in detail. However, the calculation results depend on the input conditions. If the assumptions about site irradiance, orientation, tilt, panel layout, shading, equipment configuration, temperature conditions, wiring losses, and so on are off, then no matter how precise the calculations are, the results will not reflect reality.

In power generation forecasting, it is necessary to consider separately the precision of the calculation formulas and the accuracy of the input conditions. Even if the software performs advanced calculations, if the on-site conditions entered are too rough, the results will remain rough estimates. For example, if shading from surrounding buildings or trees is not adequately reflected, the simulation may show little shading even though there are times when actual power generation decreases. Also, if the installation orientation or tilt differs from reality, the estimated solar irradiance incident on the panel surface will change.

Care must also be taken in handling meteorological data. In power generation forecasts, solar irradiance and temperature conditions have a large impact. However, if the meteorological data used does not match the actual conditions at the installation site, the results will be off. Even when using data from nearby locations, actual conditions can differ due to local characteristics such as topography, sea breezes, snowfall, fog, and mountain shadows. PVSyst performs calculations based on meteorological data, but a person must judge how well those data represent the site.

Also, the power generation forecast does not perfectly predict future weather. The annual power generation simulation is a projection based on typical weather conditions and representative data. If the actual operational year has more sunny days, it may generate more than the forecast; if it has many rainy or cloudy days, it may fall below the forecast. Therefore, it is risky to interpret PVSyst’s results as “this amount of power will definitely be produced.”

The correct understanding is that PVSyst is software that provides a reasonable projection of energy yield based on input conditions, and that the reliability of the results is determined by those input conditions and by the verification work. To improve the accuracy of energy yield forecasts, it is necessary to carefully gather on-site information, correctly organize the design conditions, and check the results for any anomalies. In practice, it is important not only to learn how to operate PVSyst but also to be able to explain the input conditions.

Misconception 2: You can judge a design solely by its annual energy production

One of the most prominent figures in PVSyst results is the annual energy production. In power generation projects, annual energy production is undoubtedly an important metric because it influences revenue planning and business decisions. However, it is a misconception to judge the quality of a design solely by the annual energy production. Annual energy production is an aggregate value, and unless you examine its breakdown and generation patterns, you cannot identify design-related issues.

For example, even if the annual energy production is the same, the monthly generation patterns can differ greatly. One design may produce a lot in summer and drop sharply in winter. Another design may have a slightly lower annual total but more stable seasonal variation. Depending on the goals of the power generation project and how electricity is used, it can be important not only to look at the annual total but also to know when and how much power can be generated. Because PVSyst lets you check monthly generation, it is important to look at the seasonal variation behind the annual total.

The performance ratio is also an important item to check. Annual generation is strongly affected by system capacity and solar irradiation, but by looking at the performance ratio it becomes easier to understand how effectively the system is generating power relative to the solar energy received. Even if annual generation appears high, that may be simply due to large system capacity and the efficiency may be low. Conversely, even if annual generation alone does not stand out, the system may be generating efficiently relative to its design conditions.

You must also check the breakdown of losses. If annual energy production is lower than expected, you need to isolate whether the cause is solar irradiance conditions, shading, temperature, wiring, or equipment configuration. Annual energy production alone does not tell you where there is room for improvement. By examining PVSyst's loss diagram and loss breakdown, you can identify, step by step, the factors that are reducing energy production.

Also, a proposal with a higher annual energy yield is not necessarily the optimal one. Packing panels densely to increase generation can make maintenance access difficult, complicate construction, or hinder future operations and maintenance. In power generation projects, it is necessary to comprehensively evaluate energy yield, constructability, maintainability, safety, land conditions, and equipment configuration. PVSyst's annual energy yield is an important decision-making input, but it alone cannot be used to evaluate the entire design.

Beginners should first check the annual energy production, and then develop the habit of checking, in order, the monthly production, performance ratio, loss breakdown, and input conditions. Annual energy production is the entry point, not the conclusion. The value of using PVSyst in practice lies not only in the total figure but in being able to read and understand the reasons behind that amount of production.

Misconception 3: Thinking that a smaller loss necessarily means better results

In PVSyst reports and loss diagrams, various losses are shown—temperature losses, shading losses, wiring losses, conversion losses, and losses due to soiling, among others. Beginners tend to think that the smaller these losses are, the better the results. Of course, when comparing under the same conditions, smaller losses generally lead to higher energy production. However, just because losses are displayed as small does not mean the simulation is correct.

Losses occur as a result of site conditions and design conditions. For example, in regions with high ambient temperatures, temperature-related losses occur to some extent. In locations with nearby obstructions, shading losses can occur. In installations with long wiring distances, wiring losses occur. If these losses are set unrealistically low, the estimated power generation may appear higher, but the forecast could become detached from reality.

In power generation forecasting, the important thing is not to make losses appear small, but to ensure that losses are set realistically. If a site with shading shows almost no shading losses, the input for shade conditions may be insufficient. If the wiring distance is long yet wiring losses are extremely small, you need to check the wiring condition settings. Likewise, if the installation tends to get hot but temperature losses are too small, the assumptions should be reviewed.

On the other hand, it is a misconception to immediately conclude that the design is poor just because the losses are large. Some losses are difficult to avoid due to site conditions. In mountainous areas or locations surrounded by structures, a certain amount of shading loss may be unavoidable. In high-temperature regions, output reductions due to temperature are more likely to occur. What is important is not just looking at the size of the losses, but determining why those losses are occurring, whether they can be improved, and whether they should be accepted within the business plan.

The loss diagram in PVSyst is not a tool for making the energy yield look higher, but a tool for explaining the causes of reduced energy production. If there are items with large losses, they become candidates for design improvement. If shading losses are large, you can consider reviewing the layout or the installation area. If wiring losses are large, you can examine wiring routes and equipment placement. If temperature losses are large, it is meaningful to check the mounting method and ventilation conditions. In practice, it is important not to hide losses but to understand them and use that understanding to inform design decisions.

The first step to using PVSyst correctly is to shift from the simplistic view that smaller losses are always better to the view that losses should be checked to verify whether they are reasonable outcomes of the site and design conditions.

Misconception 4: Shadows and terrain can be included only roughly without significantly affecting the results

A common misconception often overlooked in power generation forecasting is the idea that shadows and terrain can be entered roughly without significantly affecting the results. In solar power generation, the effect of shading is extremely important. Surrounding buildings, trees, slopes, equipment structures, terrain undulations, and adjacent rows of panels cast shadows depending on the time of day and season. Especially in the morning and evening and during winter, the sun's altitude is low and shadows tend to lengthen, which can greatly affect power generation.

The effect of shading is not determined solely by the area covered by shadows. The impact on power generation varies depending on what time of day the shading occurs, which circuit is affected, and how the shading pattern changes with the seasons. Even a short-duration shadow can cause a non-negligible loss if it occurs during a time that is important for generation. Also, even partial shading can affect a wide range of output depending on the equipment configuration.

Topography is also an important factor. If the land is flat, layout planning is relatively easy, but actual candidate sites often have slopes and elevation differences. When the terrain is undulating, it affects panel mounting angles, inter-row shading, drainage, constructability, and maintenance access routes. Even slight elevation differences that are barely noticeable can be important for PV layout and shading assessment. To evaluate terrain and shading in PVSyst, it is essential to understand the site's shape as accurately as possible.

When simulations are run with coarse shadow settings, estimated power generation can appear higher than it actually is. In particular, if on-site trees or structures are not taken into account, shadow losses tend to be underestimated. If the impact of shadows is overlooked during the project planning stage, actual power generation after construction or commissioning may fall below expectations, and analysing the causes and implementing countermeasures can take time. Because shadows and terrain are often elements that cannot be easily changed later, it is important to check them carefully from the early stages.

Also, attention must be paid to shading between panels. When trying to install as many panels as possible on limited land, inter-row shading can increase. Increasing installed capacity can lead to higher energy output, but if shading losses increase, the output may not grow as much as expected. By comparing layout conditions in PVSyst, you can assess the balance between installed capacity and shading losses.

The input of shading and terrain is one of the areas in PVSyst where the quality of on-site information is especially critical. Rather than judging based only on drawings, it is important to verify orientation, tilt, elevation differences, and the locations of obstacles on site, and to reflect that information in the design conditions. Avoid the notion that shading and terrain can be treated roughly; they must be regarded as key factors that affect the reliability of power generation forecasts.

Misconception 5: You can complete business decisions based solely on PVSyst results

PVSyst is software that is very useful for forecasting power generation, but it is a misconception to think that decisions on a power generation project can be made solely based on PVSyst results. In a power generation project, you need to comprehensively assess not only the expected power output but also land conditions, ease of construction, maintainability, grid connection conditions, legal procedures, the surrounding environment, project schedule, financing plan, operations structure, and various other factors. Among these, PVSyst is one of the important references for examining power output and losses.

For example, even if PVSyst yields results that predict high power generation, that design is not necessarily easy to construct. If the terrain is complex and earthworks or installation are difficult, if it is hard to secure maintenance access, if wiring routes become long, or if there are constraints from the surrounding environment, it may be necessary to review the plan from perspectives other than power generation. A proposal that predicts high power generation is not necessarily optimal for the project as a whole.

Also, the results from PVSyst are design-stage forecasts and do not fully guarantee actual performance after commissioning. Actual power generation will vary depending on weather, equipment downtime, cleaning status, equipment condition, changes in the surrounding environment, and the quality of maintenance and operations. Rather than being reassured by having produced a planning-stage report alone, it is important to compare actual values after commissioning and, when necessary, perform root cause analysis and implement improvements.

In business decision-making, it is also important to share the simulation assumptions with stakeholders. If the annual power generation figure circulates on its own, recognition gaps such as “this differs from what we assumed” are likely to arise later. You need to be able to explain which meteorological conditions were used, how shading was modeled, what level of losses was assumed, and what the system configuration is. PVSyst reports are useful as materials for building consensus, but it is essential to interpret and explain their contents.

When using PVSyst results, it is also useful to compare multiple design options and conditions. Looking at only a single result can make it difficult to judge whether that condition is optimal. By comparing cases that vary azimuth, tilt, layout, system capacity, and loss conditions, you can identify which factors the energy output is sensitive to. This broadens the range of business decisions and makes it easier to organize risks and potential areas for improvement.

PVSyst is a powerful tool that supports decision-making in power generation projects, but it does not replace the decision-making itself. By linking power output forecasting, on-site surveys, design review, construction planning, and operations management, you can apply PVSyst’s results in practice.

How to Properly Use PVSyst's Power Generation Forecasts in Professional Practice

To use PVSyst correctly in practice, it is important to treat energy yield forecasting not as something completed with a single calculation but as a process of repeating input, verification, comparison, and correction. In solar power generation design, the accuracy of information changes at each stage—from initial feasibility studies to detailed design, pre-construction checks, and post-operation performance comparisons. PVSyst also becomes a more useful basis for decision-making when it is used while updating input conditions to match those stages.

In the initial assessment, we project electricity generation based on the candidate site's solar irradiation conditions and an approximate facility capacity. At this stage, detailed on-site information may be insufficient. Therefore, the results are treated as rough estimates and used as material for judging the viability of the project. Next, as on-site surveys and design studies advance, factors such as orientation, tilt, shading, layout, wiring, and equipment configuration are reflected more concretely. This enables a more realistic electricity generation forecast than the initial assessment.

When comparing design proposals, it is important to change one condition at a time and observe the results. If you change multiple conditions simultaneously, it becomes difficult to determine what caused the differences in energy production. By separating and checking the factors of change—such as altering the azimuth, changing the tilt, modifying the layout, or reflecting shading conditions—you make design decisions easier. PVSyst is well suited to comparing multiple conditions, enabling evaluation based on numerical data rather than intuition.

When checking results, it is important to look at annual generation, monthly generation, the performance ratio, and the breakdown of losses together. Annual generation is important, but by itself it is insufficient. Check seasonal variations from the monthly generation, assess the overall efficiency of the system from the performance ratio, and identify the causes of reduced generation from the breakdown of losses. By examining these comprehensively, you can determine whether the results are realistic.

Also, it is important to document the basis for simulation results. If you record which meteorological data were used, how site conditions were accounted for, and how shading and losses were set, it will be easier to explain the results later. In power generation projects, there are many stakeholders and a long time elapses from planning to operation, so recording the assumptions is extremely important. Organizing the rationale for input conditions, not just the PVSyst report, improves practical reliability.

The correct way to use PVSyst is not to produce numbers but to ensure you can explain them. When presenting the results of a power generation forecast to stakeholders, being able to explain why those numbers are what they are, where losses occur, and which conditions will change the results will improve the quality of design and business decisions.

High-quality on-site information is indispensable for improving PVSyst accuracy

To improve the accuracy of PVSyst's power generation forecasts, the quality of on-site information is essential. Solar power systems are heavily influenced by the conditions of their installation site. Orientation (azimuth), tilt, changes in site elevation, surrounding obstructions, terrain, drainage, maintenance access routes, and the usable installation area are items that are difficult to assess accurately without an on-site inspection. If these details are entered into PVSyst while still ambiguous, the basis for the power generation forecast also becomes unclear.

Particularly important is information related to shadows. Surrounding buildings and trees, the undulation of the terrain, and the position and height of equipment and structures can directly affect power generation. When evaluating shading, you need to consider not only the positions of obstructions but also their heights and their relationship to the movement of the sun. Accurately identifying obstructions on site and reflecting them in PVSyst input conditions will make the assessment of shading losses closer to reality.

Orientation and tilt are also items that require on-site verification. Even if they look correct on drawings, the actual site or roof may have subtle deviations. If the installation angle or orientation differs from what was assumed, the amount of solar radiation reaching the panel surface will change. Especially when using the roofs of existing facilities or complex terrain, it is important to accurately confirm the on-site shape. PVSyst calculates based on the entered orientation and tilt, so if these are inaccurate it will affect the overall results.

On-site information related to wiring and equipment layout also affects power generation forecasts. Even after the equipment layout is decided, if the actual wiring routes are long, electrical losses may increase. It is important to understand the locations of conversion equipment and power-receiving equipment, wiring routes, and construction constraints, and to reflect them in the loss conditions as necessary. PVSyst can also account for electrical losses, so linking it with the site’s design conditions produces forecasts that are closer to real-world practice.

Site information cannot be completed by the initial survey alone. As the project progresses, design and construction conditions may change. If there are changes such as site development, layout adjustments, modifications to equipment configuration, or changes in the surrounding environment, the input conditions in PVSyst should also be reviewed. A power generation forecast is not something you produce once and finish; reviewing it in line with updates to site information improves its accuracy.

To make PVSyst results reliable, it is important to consider site survey, positioning, design, and simulation as an integrated process. The more accurate the on-site information, the clearer the rationale for the input conditions and the stronger the report’s explanatory power. The shortcut to improving PVSyst accuracy is not merely learning how to operate the software, but establishing a practical workflow that correctly captures site conditions and reflects them in the simulation.

Connecting to explainable power generation forecasts using PVSyst

PVSyst is analysis software used to predict the energy production of photovoltaic power systems and to organize design conditions and loss factors. However, unless common misunderstandings about energy production forecasting are avoided, PVSyst’s results cannot be correctly applied in practice. Views such as “if you input data you will always get an accurate energy output,” “it is sufficient to look only at the annual energy production,” “losses are better when smaller,” “shading and terrain can be treated roughly,” or “business decisions can be made based solely on PVSyst results” all require caution.

The results from PVSyst are predictions based on the input conditions. You need to check not only the energy generation figures but also the underlying assumptions, such as the installation site, meteorological conditions, azimuth, tilt, shading, losses, and system configuration. In particular, reading the loss breakdown and the monthly generation makes it easier to explain why the generation values were determined as they were. The value of PVSyst is not merely in producing generation numbers, but in organizing the basis for those numbers and making them explainable to stakeholders.

When using PVSyst in practice, it's important not to treat power output forecasts as a single fixed answer, but as a process for validating design conditions. In preliminary studies, use it to obtain rough estimates; in detailed design, reflect site conditions; before and after construction, check deviations from the plan; and after the start of operation, compare forecasts with actual performance. By using PVSyst at each stage in this way, you can improve decision-making accuracy across the entire power generation project.

Accurate on-site information is essential for that. If you can precisely identify obstacles that cause shading, terrain elevation differences, orientation, tilt, installation area, wiring routes, and so on, the input conditions for PVSyst will more closely reflect real-world conditions. Conversely, if on-site information remains vague, no matter how thorough the reports are, there will be limits to the reliability of power generation forecasts. The accuracy of simulations should be considered together with the accuracy of on-site inspections.

In the design and operation of solar power generation, it is important to bridge desk-based generation forecasts with on-site realities. If location information and positioning data acquired on site can be reflected in the design conditions, it becomes easier to judge shading, layout, and installation area, and the input parameters for PVSyst gain credibility. By utilizing an iPhone-mounted high-precision GNSS positioning device such as LRTK, on-site position checks and surveying work can be streamlined, making it easier to organize the field information needed for generation forecasts. Combining PVSyst simulations with high-precision location information obtained on site can reduce misunderstandings in generation forecasts and lead to design decisions that can be clearly explained.