What is PVSyst? 5 Essential Things to Know for Energy Yield Calculations

Table of Contents

• What does PVSyst do?

• Basic knowledge 1: Meteorological data is the starting point for energy yield calculations

• Basic knowledge 2: Understand the conversion to irradiance on tilted surfaces

• Basic knowledge 3: Do not lump losses together; examine them item by item

• Basic knowledge 4: Shadows are treated differently for far-field and near-field shading

• Basic knowledge 5: Read results not only by annual energy output but also by specific yield and PR

• How to proceed when using PVSyst in practice

• Summary

What does PVSyst do?

It is easiest to understand PVSyst as simulation software for carrying out the study, capacity sizing, and performance analysis of photovoltaic power generation systems in an integrated workflow. Official information positions it as a product for modeling photovoltaic power generation systems and performing design optimization and time-based simulations while utilizing meteorological data and an equipment database. Furthermore, technical documents describe it as a comprehensive design and analysis environment that can handle not only grid-connected systems but also stand-alone power supplies and pump applications, so it is appropriate to regard it as practical, professional software for interpreting generation by building up conditions rather than as a mere rough-calculation tool.

What matters for practitioners is that PVSyst is not a box that only outputs an annual energy figure. PVSyst offers both a preliminary design approach for quickly checking monthly estimates with minimal inputs, and a detailed design approach in which you create projects with site conditions and time-series meteorological data to run detailed hourly simulations. Official documentation shows that within this detailed design framework you can create multiple variants per project to compare differing conditions, and that you can inspect many variables on a daily, monthly, and hourly basis as a result. In other words, PVSyst is software not only for producing total energy output but for validating which assumptions affected the results.

Whether you have this understanding or not greatly changes how you use PVSyst. Many people who search for "What is PVSyst" know the name of the software but tend to start using it with an unclear sense of what and how much can be entrusted to the program. In reality, its true value appears when you consider the chain—from choosing meteorological data, converting irradiance to the installation plane, breaking down losses including temperature and wiring, the geometry of shading and its electrical impacts, to interpreting the resulting performance indicators. Treating PVSyst not merely as an estimation aid but as a foundation for verifying the consistency of design assumptions makes it far more practical in professional use.

Basic Knowledge 1: Meteorological Data Are the Starting Point for Power Output Calculations

When performing energy yield calculations in PVSyst, the first thing to grasp is the meteorological data. The official documentation also states that meteorological data are the starting point for project evaluation and at the same time the largest source of uncertainty. In fact, hourly simulations use information such as global horizontal irradiance, diffuse irradiance, temperature, and wind speed, which determine the hour-by-hour generation behavior. Even with the same system configuration, if the meteorological data you input differ in site, period, quality, or how they are generated, the resulting annual energy yield and the monthly highs and lows will change. Therefore, the first step to understanding PVSyst is to adopt a perspective of questioning the meteorological data before the equipment.

One thing that is easy to overlook in this context is that the meaning of the calculation results changes depending on which type of meteorological data was used. Official documents organize that results run with multi-year averages or data representing a typical year are easier to treat as forecasts of average power generation, whereas results using data from a specific year are strongly influenced by the weather unique to that year. Furthermore, PVSyst validation-related documents explain that the uncertainty of the meteorological data itself is typically around 5–10%, and year-to-year variability can be roughly ±5%. In other words, rather than looking at the numbers alone and concluding that "the equipment is bad" or "the design is inadequate," you should first confirm which meteorological conditions those numbers represent.

In practice, at this point you want to check whether observations from distant locations have been reused, whether horizon conditions at the mountain edge have been ignored, and whether the data are multi-year averages or single-year. PVSyst provides approaches for comparing meteorological data and for generating and managing time-series data, but it does not automatically guarantee the representativeness of the input values. Precisely for that reason, in the early stages of power generation calculations, clarifying "which meteorological conditions the calculation is based on" before "which equipment" will determine how well you can explain the subsequent steps.

Basic Knowledge 2 Understanding the Conversion to Solar Irradiance on Inclined Surfaces

Next point to understand is that PVSyst does not simply convert meteorological data on the horizontal plane directly into power generation. In the official documentation, the process of deriving the irradiation incident on the actual installation surface from horizontal-plane irradiation is described as plane transposition, and the global irradiance, direct irradiance, sky-diffuse, and ground-reflected components incident on the installation surface are calculated separately. In other words, PVSyst’s energy production calculation proceeds not only by considering “how many kWh of irradiation there are,” but by properly calculating “from which directions, in what proportions, and onto which installation surface that irradiation arrives.”

Once you understand this concept, it becomes clear why inputs for azimuth and tilt angle are important. Whether the array is fixed facing south, split east–west, or tracked changes the breakdown of solar radiation incident on the installation surface, and even at a location with the same annual irradiation, the plane-of-array irradiance will not be the same if installation conditions differ. PVSyst’s simulation variables are provided in stages, from values converted from horizontal-plane irradiation to plane-of-array values, and then values that have been further processed through far shading and near shading, incidence-angle correction, and soiling correction. Therefore, when you change installation conditions it is easier to trace why results moved, and the tool is less likely to be just a black box.

Also, the surface transposition model itself needs to be considered according to the quality of the input data. The official documentation provides a more robust, easier-to-handle approach and a more detailed approach that assumes the diffuse component information is of sufficiently high quality. What really matters in practice is not memorizing the names, but being aware of how trustworthy the diffuse irradiance information is and how correctly the installation surface conditions have been entered. When using PVSyst, surface transposition should be recognized not as a mere internal process but as an important step that determines the assumptions for energy production calculations.

Basic Knowledge 3: View losses by item rather than aggregating them

A common mistake when using PVSyst for the first time is to lump all losses into one large number. However, the official documentation shows that PVSyst is designed to treat many losses individually, such as optical losses due to angle of incidence, soiling, performance differences at low irradiance, temperature rise, module quality differences, mismatch, DC wiring, AC wiring, external transformer losses, and auxiliary consumption. Moreover, even if default values are entered for the initial calculation, it is recommended to review each loss according to the project thereafter. In other words, PVSyst’s philosophy is not to round losses together, but to decompose them by source and support each with a rationale.

This idea is directly tied to practical accountability. For example, whether the site is dusty, whether the environment is easily washed by rain, whether the DC wiring is long or short, or whether the equipment’s installation temperature tends to be high will change how losses are allocated. If you combine losses into a single coefficient, you won’t know what to fix when the results are poor; if you allocate them by item, it becomes easier to isolate causes and to explain them internally. PVSyst’s emphasis on the loss diagram is based on the premise that this separation is important.

What is particularly easy to overlook is temperature and the behavior of the conversion unit. Official documentation explains that array temperature is evaluated from the balance of solar radiation absorbed by the module, heat loss to the ambient air, and the power extracted. In other words, higher irradiance does not simply lead to greater generation, because losses due to temperature rise also increase. Also, the conversion stage’s efficiency is not constant; the model treats it as changing with internal consumption and output level. If you select equipment by looking only at peak conditions, you are likely to miss behavior under partial load or when output is limited, so in PVSyst you need to be aware not only of rated values but also of how losses appear according to operating state.

Basic Knowledge 4: Shadows Are Handled Differently for Distant Shadows and Nearby Shadows

Handling shading is an essential point for deepening one’s understanding of PVSyst. In the official documentation, far-field shading and near-field shading are clearly distinguished. Far-field shading is shading that, like the horizon or distant mountains, uniformly affects the entire power-generating surface at a given time. In contrast, near-field shading is shading that locally affects part of the power-generating surface, such as shadowing from surrounding structures or inter-row shading, and it must be treated in three dimensions to determine which areas are obscured and to what extent. PVSyst itself also notes that near-field shading is one of the more difficult aspects within the software and requires detailed 3D descriptions.

What is important here is that losses due to shading are not simply 'the amount by which irradiance is reduced.' Official documents state that near-field shading acts separately on the direct, diffuse, and reflected components at each time, and that the result also changes depending on where on the power-generating surface the shadow falls. Therefore, if surrounding obstacles are entered in an ad hoc manner or inter-row shading is estimated only by area ratio, it becomes difficult to accurately represent the morning/evening dips and wintertime attenuation. In practice, it is important to organize horizon conditions and proximate obstacles separately and to decide how much detail to model based on the project's accuracy requirements.

Even more troublesome are electrical shading losses. Official documentation explains that actual shading losses include not only the linear irradiance loss but also additional losses caused by current limitation in series circuits due to partially shaded cells or modules. In other words, a small shaded area does not necessarily mean a small loss. If you don’t understand this, you cannot explain why an obstacle that looks small on the drawings can produce unexpectedly large differences in simulation. When handling shading in PVSyst, you need to consider not the apparent shaded area but also the electrical connectivity.

Basic Knowledge 5: Read results not only by annual power generation but also by specific yield and PR

When reviewing the results of an energy production calculation, judging solely by the total annual output can easily lead to misreading the quality of the design. In PVSyst’s result screen, you can check the specific yield, which indicates how much electricity is generated per 1 kW of installed capacity per year; this is expressed on an annual basis as kWh/kWp/year. Specific yield is useful for getting a sense of generation relative to project size and solar irradiation conditions, and serves as a basis for investment decisions and comparisons with similar projects. However, this value alone does not tell you whether the favorable result was due to good site conditions or good design.

What you should look at there is PR. Official documents describe PR as the ratio of the energy actually obtained to an ideal reference amount defined from the solar irradiation incident on the installation surface and the rated output. And, unlike specific yield, PR is less directly dependent on weather conditions or the orientation of the installation surface itself, so it is considered a metric that makes it easier to compare system quality across projects at different locations or orientations. In practice, if you use specific yield to evaluate project-level performance and PR to evaluate design quality, discussions in meetings are less likely to go off track.

Also worth checking is the loss diagram. PVSyst’s loss diagram is a display that lets you quickly see how much energy was shaved off at each stage, and it can be used for monthly evaluations as well as annual ones. However, because each loss rate is shown as a proportion of the energy at the previous stage, they cannot be simply added together. If you don’t understand this, it can lead to the misconception of “adding up loss rates to get the total loss.” Practitioners using PVSyst should, when reviewing results, devote effort to correctly interpreting what the numbers mean rather than trying to increase the numbers.

Also, when discussing future projections, the concepts of P50 and P90 become important. In PVSyst’s official documentation, P50 and P90 are described as a framework in which, on top of the initial simulation results, the user assumes and evaluates year-to-year variability in meteorological conditions and additional uncertainties. In other words, PVSyst does not automatically produce guaranteed values; interpreting the representativeness of the meteorological data and the range of variability is a prerequisite. If you plan to use energy yield calculations for internal approvals or contract negotiations, it is essential not to confuse single-year results with probabilistic assessments.

How to proceed when using PVSyst in practice

From a practical workflow perspective, it makes sense to first get a rough estimate with a small number of assumptions and then move on to detailed simulations. Official guidance also frames preliminary design as a phase for quickly performing a simple monthly evaluation, and states that more accurate results are obtained from hourly simulations that include the actual equipment and detailed loss conditions. Therefore, rather than diving into the details immediately, it is better to first grasp the installable capacity and general orientation, and then increase accuracy in the order of location, weather, azimuth, tilt, equipment configuration, losses, and shading, which reduces rework.

What works well here is to run an initial calculation early, then use variant comparisons to check the differences in conditions. PVSyst’s official documentation shows that you can use multiple variants within a project to perform optimizations and condition comparisons. In practice, by lining up and comparing options such as different tilt angles, different inter-row spacing conditions, or options with losses set more conservatively, you can quickly grasp what is having an effect. Rather than choosing the option with the largest numbers, looking at how much the results move in response to changes in assumptions is the quickest way to judge the stability of a design.

Additionally, if you need to hand results off to other internal teams or use them for your own aggregations, you can export hourly, daily, and monthly outputs to external files for analysis. The official documentation explains that variables related to the simulation can be output on an hourly, daily, and monthly basis. This makes practical operations easier, such as tracking causes of monthly performance degradation, organizing seasonal impacts, and transcribing data into external report formats. Mastering PVSyst is not about viewing numbers on the screen, but about organizing them into a form that can explain the figures and connect to related business processes.

PVSyst is also effective when used to analyze existing installations. The official documentation presents the idea of comparing on-site measured data with simulation values on an hourly or daily basis to help detect small faults and to review the actual system conditions. In other words, PVSyst can be used not only for design at the time of new installation but also as a tool to verify whether the system is performing as expected after the start of operation. The fact that it makes it easy to create a common language between design and maintenance departments is also a major strength as practical software.

Summary

PVSyst is a practical software tool that links meteorological data, installation conditions, shading, temperature, wiring, and equipment behavior to build up power generation on an hourly basis. The five basic points to grasp are that meteorological data are the starting point of the calculations, that horizontal-plane irradiance is correctly converted to the irradiance on the installation surface, that losses should be handled decomposed rather than as a single lump sum, that shading must be approached differently for far-field shading and near-field shading, and that results should be read not only as annual energy yield but also as specific yield and PR. If you understand these five points, you will not just passively view PVSyst outputs but will be able to trace which assumptions pushed the results up and which pushed them down.

To further improve the accuracy of power output calculations, it is essential to properly link desk-based assumptions to on-site conditions. In particular, when moving into the stage of confirming obstacle locations, post-development height relationships, and the coordinates of equipment placements, the accuracy of the positional information handled on site becomes important. When you want to carry out such on-site verification efficiently, it is natural to make use of LRTK, an iPhone-mounted GNSS high-precision positioning device. Because it makes it easier to reconcile the assumptions used in simulations with on-site reality on a coordinate basis, it becomes easier to reduce discrepancies in understanding across the design, construction, and maintenance stages.