What is PVSyst? An Overview of the Basics of Design, Generation Forecasting, and Loss Calculations

Table of Contents

• What is PVSyst

• Design Basics 1 Organize conditions at the project level

• Design Basics 2 Determine meteorological data and the mounting surface

• Generation Forecast Basics 1 Calculate array output from incident irradiance

• Generation Forecast Basics 2 Derive the system output

• Loss Calculation Basics 1 Temperature, low irradiance, wiring, mismatch

• Loss Calculation Basics 2 View shading as optical losses and electrical losses

• How to read results Examine the loss diagram and performance indicators

• Common misconceptions beginners may have

• Summary

What is PVSyst?

PVSyst is dedicated software for studying photovoltaic power generation systems, sizing capacity, evaluating performance, and analyzing data. In the official documentation it is presented as an environment capable of handling grid-connected, stand‑alone, pumping, and DC‑system applications, and is described as a professional tool equipped with a meteorological database, a component database, various utilities, and measured‑data comparison functions. In other words, PVSyst is not simply a tool to calculate annual energy production once; understanding it as a platform that organizes a project’s assumptions while linking design and energy‑yield prediction makes it easier to grasp the overall picture.

When practitioners search for "PVSyst", what they want to know is less the meaning of the name and more what can be done in design, how generation forecasts are constructed, and how far loss calculations can be examined. From that perspective, PVSyst's distinguishing feature is that it can handle location, weather, installation surface, system configuration, losses, shading, and result comparison in a single workflow. The official description likewise states that, within the framework of a project, it contains geographic conditions and time-series meteorological data and allows comparison of different simulation runs. A major reason PVSyst is used in practice is that it makes it easy to explain not only the generation figures but also the assumptions from which those figures were derived.

Additionally, PVSyst offers both a phase for quickly producing estimates from only a few general conditions and a phase for detailed design using time-step simulations. This aligns well with the practical workflow of reviewing a project's broad direction in its early stages and, as the project advances, tightening up assessments of losses and shading. Because it encourages starting from a rough proposal and gradually improving accuracy rather than relying on finished numbers from the outset, it is software that is easy to adopt for both beginners and experienced practitioners.

Design Basics 1: Organize Conditions at the Project Level

The first thing to grasp in PVSyst design is the idea of organizing work by project rather than running calculations as isolated, one-off runs. The official documentation describes a project as a central framework that holds geographic conditions and time-series meteorological data, within which multiple simulation runs can be compared. This is not merely a matter of storage format. It’s a way to view different orientations, tilts, loss conditions, shading conditions, and so on side by side within the same case. Understanding this framework first as a basic design principle makes the PVSyst interface and the meaning of its tasks much easier to follow.

In practice, it is rare that a single correct answer is determined from the start. For example, points you will inevitably want to decide by comparison include whether to use a steep south-facing tilt or a shallow east–west tilt, whether to widen or tighten row spacing, and how strictly to treat shading. Because PVSyst makes it easy to organize such differing conditions within a single project, it is also easy to save intermediate design iterations. The official tutorial likewise recommends saving the initial standard scenario and then creating alternate scenarios by sequentially adding distant shading, near shading, loss conditions, and so on. In other words, designing with PVSyst is not a one-shot process but a design approach that develops proposals through comparison.

If you don’t understand this way of thinking, beginners will have difficulty grasping “why create multiple files for the same project” and “why keep previous proposals.” However, design is essentially the work of choosing a reasonable option from among differing conditions. PVSyst is made to match that reality. As a basic rule of design, the important thing is not to aim for a perfect proposal from the start, but to first create a single baseline proposal and then organize things by adding or changing conditions. Once you can do this, explaining the reasons behind the design and reviewing it later becomes much easier.

Design Basics 2: Determining Meteorological Data and the Installation Surface

The next basic step in design is to properly determine the meteorological data and the installation surface conditions. The official description of PVSyst states that, as the foundation of a project, there are geographic conditions and time-series (hourly) meteorological data, and that the meteorological database allows creating site information, generating time-series data, visualizing, comparing, and importing externally. In other words, PVSyst’s calculations do not start by immediately entering system capacity or component types; they start by first defining “which location” and “which meteorological conditions” to assume. Since solar power generation results depend strongly on meteorological conditions, this order is extremely important.

A common stumbling block for beginners is treating meteorological data as if it were a single fixed value. In reality, the way the reference year is selected and differences in data sources can change how annual and monthly values appear. Even official comparisons of data sources note that there are large differences between meteorological datasets, and it is not easy to definitively determine which is optimal. In other words, PVSyst is not software that automatically provides “absolutely correct meteorological data”; it is a tool to be used with an awareness of differences in meteorological assumptions. To improve the quality of a design, it is essential to understand which meteorological conditions are being used for the calculations and, if necessary, compare results under multiple conditions.

Furthermore, PVSyst does not use the horizontal-plane irradiance as-is; it converts it to the irradiance incident on the installation plane. In the official simulation workflow, it is explained that meteorological data are read hourly, after which the global, beam, diffuse, and reflected components on the installation plane are calculated. This is because, even at the same site, different orientation and tilt change the actual amount and timing of incoming light. What you should understand as a basic design principle is that meteorological data are only regional average conditions, and only when these are combined with the installation-plane conditions is the "light this system receives" determined. If orientation and tilt are set carelessly, the entire subsequent simulation becomes more prone to error.

Power Forecasting Basics 1: Calculating Array Output from Incident Solar Irradiance

The basic thing to understand about generation forecasting is that PVSyst tracks, hour by hour, how the received light is converted into electricity. In the official simulation workflow, it is explained that first the irradiance incident on the installation surface is determined from hourly meteorological data, and then the effective irradiance is calculated by taking into account far shading and near shading, the angle of incidence, and optical attenuation. In other words, PVSyst does not output the power generation from the outset; it first determines how much usable light reached that surface. When trying to understand generation forecasts, it is important not to skip this initial step.

After that, PVSyst calculates the DC-side energy through the electrical behavior of the module. In the official description of the single-diode model, the module’s behavior is described by a model that includes not only rated values but also current, voltage, series resistance, shunt resistance, temperature, and so on. Furthermore, the efficiency drop at low irradiance is explicitly explained as “irradiance loss,” which is a loss that naturally arises from the single-diode model. In other words, PVSyst’s power generation forecast is not an ideal calculation assuming only a sunny noon, but reflects realistic behavior including low-irradiance periods and temperature variations.

What often confuses beginners here is the simple assumption that “more incident solar irradiance directly means more power generation.” In reality, efficiency falls at low irradiance and output drops at high temperatures. Because PVSyst models those changes, generation forecasts are influenced not only by the strength of the weather but also by module characteristics. To put the basics of generation forecasting in one sentence: “convert the light coming from the sky into the light that reaches the surface, and sum up, hour by hour, how much of that light becomes electricity in the modules.” With this understanding, the subsequent loss calculations become easier to relate to.

Fundamentals of Power Generation Forecasting, Part 2: Deriving System Output

Seeing the array output on the DC side is not the end. In PVSyst’s power production forecast, that output is further translated into the energy available to the whole system. In the official list of simulation variables, losses relative to the array-side output—such as losses from power conversion equipment efficiency curves, losses due to power thresholds, losses from the input voltage range, and nighttime consumption—are organized as separate variables. In other words, the power obtained on the DC side does not directly become the usable energy on the AC side. In the latter part of the generation forecast, the conditions of the conversion equipment are applied to bring the estimate closer to the actual output.

What beginners often misunderstand at this stage is to think, "If the modules can produce this much, roughly the same amount should come out on the output side." In reality, converters and other equipment also have preferred operating ranges, and their efficiency is not constant. In PVSyst, you can separate not only operational efficiency but also constraints caused by low or high voltage, overloads, and nighttime consumption, which makes it easier to identify where losses are occurring. In this way, it is easier to understand the latter part of generation forecasting if you consider it less as "the equipment generating electricity" and more as "how well the generated electricity can be turned into a usable form."

Also, PVSyst allows you to view hourly behavior here as well as annual values. This makes it easier to detect quirks that aren’t visible from the annual totals alone. For example, you might read the results as showing that conversion-side constraints are strongly apparent only during certain time periods, or that nighttime losses are relatively large in a particular season. The strength of PVSyst is that it lets you track “which process experiences what, and at what times” instead of ending a generation forecast with a simple annual total. This perspective is indispensable for turning calculation results into design improvements.

Basics of Loss Calculation 1: Temperature, Low-Light, Wiring, and Mismatch

What you should grasp first as the basis of loss calculation is that losses are not something subtracted all at once at the end, but are incorporated into the simulation along the way. In PVSyst’s general description, it states that in the second stage of detailed design you can specify and analyze fine effects such as thermal behavior, wiring, module quality differences, mismatch, and incidence angle losses. Furthermore, in the list of simulation variables, losses due to low irradiance, temperature losses, mismatch losses, wiring losses, and module quality losses are defined as independent variables. In other words, losses are not a single coefficient but an accumulation of many small elements.

Among these, low-irradiance loss is something beginners tend to overlook. In the official explanation of irradiance loss, PVSyst’s low-irradiance loss is described as arising from the intrinsic behavior of the module’s single-diode model, and occurring because efficiency drops as irradiance decreases. In solar PV design, it’s easy to assume that “even weak light produces power proportionally,” but in reality it’s not that simple. The concept of low-irradiance loss is crucial for accurately estimating generation in the morning and evening and under cloudy conditions. PVSyst is practical as a power-generation forecasting tool because it includes these easily overlooked losses in its model from the outset.

The same applies to temperature loss, wiring loss, and mismatch loss. As temperature rises, output falls, and any wiring resistance will reduce the power. Differences in module quality and variability between strings also become non-negligible losses in the field. In PVSyst these can be viewed separately, so you don’t have to stop at the conclusion that “total losses are large” — you can break down what is actually contributing. The basic principle of loss calculation is to understand that it is not “one large loss,” but “many small losses stacking up to produce the final result.” With that perspective, loss diagrams and the performance ratio become much easier to interpret.

Basics of Loss Calculations 2: Viewing Shadows as Optical and Electrical Losses

Another important aspect in loss calculations is how you view shading. Beginners in solar PV design tend to think of shading simply as the proportion of shaded area, but PVSyst's documentation explains that shading losses include not only a linear reduction in irradiance but also additional losses due to electrical mismatch. In other words, shading is not just a matter of receiving less light; it also affects the behavior of the system as a circuit. Even a small amount of shading can, depending on how components are connected, cause much larger losses than expected.

In the official simulation variables, electrical shading mismatch losses are defined as ShdElec, and for calculating near shading, methods based on module strings or on module layout can be used. This means that shading issues can be evaluated not only in terms of spatial conditions but also electrical conditions. The reason PVSyst’s shading assessment is strong is that it treats shading not merely as a "lack of brightness" but as a "factor that disrupts the system's power generation behavior." Because of this way of thinking, shading assessments are less likely to be underestimated in practice, reducing the chance of failure.

The official tutorial also shows that distant shading is easy to include in the initial stage for getting a sense of the overall orientation, while near shading is added later during a more detailed design phase. In other words, you don't need to evaluate shading at the maximum level of detail from the start; you can adjust the depth of evaluation according to the importance and complexity of the project. The two attitudes beginners should adopt initially are "don't underestimate shading" and "when necessary, evaluate it more thoroughly." Even just that will help you move beyond the simple area-ratio mindset and make it easier to embrace PVSyst's design philosophy.

How to Read the Results: Viewing the Loss Plot and Performance Metrics

Even if you understand the design, energy yield prediction, and loss calculations up to this point, it won’t be useful in practice unless you know how to read the results. The first thing to look at in PVSyst’s results is not just the annual energy yield. On the official loss diagram page, the loss diagram is described as being particularly useful for quickly assessing the quality of a system design and identifying the main sources of loss. In other words, when reviewing results you need to look not only at the magnitude of the final value but also at where and by how much reductions occurred to reach that value. The loss diagram is the entry point for that.

A common mistake beginners make is to stop after looking at the annual energy production. However, PVSyst has numerous monthly, daily, and hourly variables, so you can track which seasons show differences and which times of day have large losses. For example, if the drop is pronounced only in summer, you should suspect temperature or ventilation conditions; if it’s weak only in the mornings and evenings, you may need to reassess the orientation or shading. The basic approach to interpreting results is to look for where the differences arise, not the total amount. PVSyst has ample information for that purpose.

Performance ratio is also an important metric. Officially, the performance ratio is described as the ratio of the energy actually available for useful use to the ideal amount calculated from the irradiance incident on the installation surface and the nominal output, and it is considered to broadly include optical losses, array losses, and system losses. A proposal with a high annual energy production may simply be due to favorable site conditions. Conversely, a proposal with a high performance ratio may indicate that the system is well-coordinated as a whole. Beginners do not need to master it perfectly at first, but developing the habit of treating "annual energy production" and "performance ratio" separately will make interpreting the results much more practically oriented.

Common Misunderstandings Among Beginners

Based on what has been covered so far, summarizing the points that beginners are particularly likely to misunderstand: First, PVSyst is not software that gives you a final, completed value right away. Preliminary design is for initial judgment and serves a different role than detailed design. Second, the values in the database are a convenient starting point, not a substitute for final verification. Third, losses are not something you subtract in bulk afterward; they are incorporated during the calculations. Fourth, shading is not merely an area ratio but also a circuit-level issue. And fifth, you should not read results based only on annual energy production; you need to look at the loss diagrams and the performance ratio as well. Simply knowing these things early on will make your use of PVSyst much more reliable.

Another important point is that while PVSyst is strong for design and assessment, it is not software that replaces the site survey itself or construction management itself. Detailed evaluation of adjacent shading requires accurate 3D definitions and precise knowledge of module positions. In other words, the accuracy of desktop simulations is supported by the accuracy of site information. If this is not understood, it becomes easy to encounter situations where "I calculated it in detail with the software, but it doesn't match on site." For beginners to use PVSyst effectively, it is essential to adopt the mindset of considering the software together with on-site information gathering, rather than looking only inside the software.

Summary

PVSyst is a professional software that handles photovoltaic system design, energy-yield prediction, and loss calculations in a single workflow—from site and climate to mounting surfaces, system configuration, losses, shading, and results analysis. To outline the basics: you first set project-level conditions, then select the meteorological data and the mounting surface; from there you determine the incident irradiance, calculate the behavior of the modules and the system, sequentially apply losses such as temperature, low irradiance, wiring, and shading, and finally read the results using loss diagrams and the performance ratio. Once this flow is understood, it becomes clear that PVSyst is not merely a generation-calculation tool but software for organizing the rationale behind a design.

In practical work, the value of PVSyst is not simply that it produces numbers. Its value lies in organizing assumptions, comparing differences in conditions, identifying weaknesses, and, when necessary, comparing with actual performance to inform future work. For that reason, beginners do not need to understand everything perfectly at first; it is better to gradually deepen their understanding while keeping in mind the stumbling points summarized in this article. If you view PVSyst not as "difficult software" but as "software that connects design and verification," both learning and use become much easier.

The more carefully you perform desk-based generation forecasts and shading assessments, the more important the accuracy of on-site positioning and equipment layout becomes. Even if you refine design conditions in PVSyst, if site staking and obstacle identification on the ground are unclear, discrepancies between design assumptions and construction reality tend to grow. That is why, during the design phase, organizing desk-based conditions in PVSyst and, during the field phase, combining them with an iPhone-mounted, high-precision GNSS positioning device such as LRTK makes it easier to link design, construction, and operations & maintenance more consistently. The idea of using PVSyst to establish the design rationale and LRTK to align on-site positional accuracy is well suited to improving the overall reproducibility of solar PV work.