What is PVSyst? Understand the basic terms you should know before getting started

Table of Contents

• What is PVSyst?

• Term 1 Project

• Term 2 Variant

• Term 3 Meteorological Data

• Term 4 Orientation and Tilt of the Installation Surface

• Term 5 Array / Subarray / String

• Term 6 Incident Irradiance and Effective Irradiance

• Term 7 Loss Diagram

• Term 8 Performance Ratio

• Term 9 Linear Shading Loss and Electrical Shading Loss

• Term 10 Comparison with Measured Data

• Key Points to Understand Before Getting Started

• Summary

What is PVSyst?

PVSyst is specialized software for feasibility studies, system sizing, performance evaluation, and data analysis of photovoltaic power systems. It is designed to handle not only grid-connected systems but also standalone systems, pump applications, and DC-grid applications, and it includes a meteorological database and a component database, various solar energy–related tools, and even functions for comparing measured data. In other words, rather than being a tool that simply calculates annual energy production once, it is easier to grasp the overall picture if you understand it as practical software for handling the entire workflow from design to verification while organizing a project's assumptions.

What beginners most often stumble over in PVSyst is not the number of screens or features, but that the meanings of terms don't connect. If it's unclear which words relate to "location", which relate to "equipment configuration", and which relate to "how to read the results", you may be able to operate the software but it will be difficult to deepen your understanding of the design. Conversely, once you can organize the meanings and roles of the basic terms, the software suddenly becomes much easier to understand. That's because PVSyst is not only software that produces numbers, but also software that helps organize the reasons behind those numbers.

Here's why it’s important to summarize and understand basic terminology before getting started. You don’t need to master every feature from the outset, but simply knowing the roles of terms such as project, variant, meteorological data, mounting surface, array, loss diagram, performance ratio, shading losses, and comparison with measurements will make using PVSyst considerably more stable. In this article, we organize and summarize the terms that practitioners should grasp first, for introductory purposes.

Term 1 Project

The term you should understand first in PVSyst is "Project." According to the official documentation, a project is a central framework that contains the geographic location and time-based meteorological data, and it is within this framework that design and simulation are conducted. Furthermore, when creating a project, the workflow shows choosing the file name and project name, defining the site, selecting the meteorological file, and configuring the project settings. In other words, a project is not merely a saved file but a container that collects the information forming the foundation of that project.

This way of thinking is important because in solar PV design you often compare multiple options within a single project. Since the location and meteorological conditions are common and different options with varying orientations and loss conditions are typically layered on top, you need to first separate the common foundation as a "project." When you first start using PVSyst, if you think of each calculation as a separate item rather than organizing by project, the point of comparison becomes hard to see. It’s easier to understand if you remember that a project is a term for organizing the common parts: "this site, this location, these basic conditions."

Put in practical terms, a project is like the spine of a set of design documents. You slide multiple design proposals and evaluation options into it; it is not something to be considered separately for each proposal from the outset. The reason PVSyst makes it easy to organize cases is because of this unit called a “project.” If you understand this term before getting started, you will naturally understand why the location and meteorological conditions are defined first.

Term 2 Variant

The next term to understand is "variant." In the official documentation, a variant is referred to as "system definitions" or "calculation versions," and is described as defining the PV system components necessary to meet the conditions required by the user. If a project is the common foundation, a variant is an individual design proposal that sits on top of that foundation. Extensions are managed separately from the project, making it easy to compare multiple proposals in parallel within the same project.

A common source of confusion for beginners here is that the difference between a project and a variant is ambiguous. Simply put, a project is "the project itself," and a variant is "an alternative within that project." For example, if you want to compare a south-facing option and an east–west option at the same site, the location and weather conditions are the same, but orientation and configuration differ. In this case, saving them as separate variants within the same project clarifies the comparison. PVSyst is designed with this kind of comparison in mind, so understanding variants makes its use much more straightforward.

The official tutorial also recommends first creating the initial system configuration with minimal conditions, and then sequentially making separate variants that add elements such as far shading, near shading, and individual losses. In other words, PVSyst is better suited to a workflow of developing alternate designs by incrementally adding conditions to a baseline case, rather than trying to arrive at a completed proposal in a single attempt. Understanding the term "variant" makes it easier to see why multiple patterns are created within the same project.

Term 3 Meteorological Data

In PVSyst, "meteorological data" are the hourly environmental conditions that form the basis for generation forecasts. In the official description of meteorological data sources, PVsyst is said to have a meteorological database and to be able to import external data sources and custom files. The software is also described as being for study, sizing and data analysis of complete PV systems, indicating that meteorological conditions are emphasized as the starting point. In other words, in PVSyst meteorological data are not mere auxiliary information but the starting point for the entire design.

A common stumbling block for beginners is assuming meteorological data is a single fixed value. In reality, which data source you choose, how you treat the target year, and how you approach averaging can all change how both annual energy production and seasonal variability appear. Even official comparisons of data sources reveal large differences among the meteorological datasets available, and it is not easy to determine precisely which is optimal for a given project. PVSyst is notable for its approach of not hiding those differences but using them comparatively.

Understanding this statement correctly will clarify why site setting and weather file selection come at the very start of the design process. You may be tempted to decide equipment capacity and component types first, but photovoltaic power generation fundamentally depends on “what kind of light and how much of it” is available at a location. When reading energy yields in PVSyst, it is important to be aware of which meteorological data those numbers are based on. Not underestimating the term “meteorological data” is the first step to reducing misinterpretations after deployment.

Term 4 Orientation and Inclination of the Installation Surface

"Orientation and tilt of the installation plane" is a fundamental and very important term in PVSyst. In official project design, it is stated that a plane orientation should be defined for each variant, and it is explained that not only fixed mounting structures but also tracker surfaces and row installations can be handled. This is because, even at the same location, the amount and timing of solar radiation actually received change depending on which direction it faces and at what angle it is placed. In other words, orientation and tilt are not "just a single input item" but terms that determine the backbone of the power generation forecast.

This term is difficult for beginners because it tends to be perceived as mixed with meteorological data. Even if the regional irradiance is high, the amount of light a surface receives changes if the orientation or tilt of the mounting surface differs. PVSyst calculates the global, beam, diffuse, and reflected components incident on the mounting surface from horizontal-plane meteorological data, so the orientation and tilt settings directly affect the results. It’s easier to understand if you think of the term not as mere geographic information but as describing “the attitude in which that installation receives light.”

In practice, the quality of a comparison varies depending on whether this term is understood. The reason for creating alternative proposals with slightly different orientation and tilt is that the conditions of the installation surface affect not only energy production but also the loss structure and the way shadows fall. PVSyst makes it easy to compare alternatives, so differences in installation surface conditions are easy to read as numbers, but as a prerequisite you need to grasp the meaning of this term. The orientation and tilt of the installation surface are fundamental terms for interpreting PVSyst results.

Terminology 5 Array · Subarray · String

When understanding design in PVSyst, the terms "array", "sub-array", and "string" are essential. In the official description of sub-arrays, the "system" in a grid-connected project is the entire PV array that includes modules, strings, conversion equipment, and the grid connection, and it is stated that this is composed of multiple sub-arrays. A sub-array is organized as a unit that shares the same module model, the same string configuration, and the same converter conditions, among other things. In other words, it’s easier to grasp if you think of the array as the whole, the sub-array as a grouping within it, and the string as a single row connected in series.

What often confuses beginners is that these three terms look similar. However, in PVSyst this distinction is very important. For example, to treat surfaces with different orientations or systems connected under different input conditions separately, the concept of a sub-array is necessary. On the other hand, a string is a unit of modules connected in series, so it is important when considering electrical conditions or the effects of shading. In other words, thinking of an array as the overall design, a sub-array as a homogeneous grouping, and a string as an electrical unit brings you closer to the practical way of reading things.

PVSyst is favored because it can handle these configuration units without ambiguity. It can organize not only how many kilowatts to install, but also which surface each subarray is assigned to and the string configuration that makes up each subarray. If you understand these terms before getting started, it becomes much easier to see what you are doing on the system definition and module layout screens. This is a term you should remember as a basic word for assessing not just the power-generation figures but also the feasibility of a configuration.

Term 6 Incident Solar Radiation and Effective Solar Radiation

"Incident irradiance" and "effective irradiance" are indispensable terms for understanding the power generation forecasting workflow. In the standard simulation process, hourly meteorological data are first used to determine the global, beam, diffuse, and reflected components with respect to the installation plane; then far-field shading, near-field shading, incidence-angle losses, soiling, and other factors are applied in sequence to translate those components into effective irradiance. In other words, incident irradiance is "the light entering the surface," and effective irradiance is "the light actually available for power generation after accounting for losses."

If you don’t understand this difference, beginners are likely to feel that “there is a lot of irradiance but it doesn’t generate as much power as expected.” However, in reality, even after light reaches the surface, not all of it can be used for power generation. Reflection can increase depending on the angle, and the surface can be affected by dirt or nearby shading. PVSyst treats those processes as individual variables, so you can check incident irradiance and effective irradiance separately. That distinction instantly deepens your interpretation of the results.

For practitioners, understanding this terminology is also the key to reading the first part of the loss diagram. When a region's solar irradiation conditions are good but the results don't improve, it may not be that the meteorological data are poor, but that energy is being lost on the way to effective irradiation. In other words, once you can distinguish incident irradiation from effective irradiation, it's easier to trace where the energy was lost. If you're reading power generation forecasts in PVSyst, these two terms are basic vocabulary you should grasp first.

Term 7 Loss Diagram

The "loss diagram" is one of the most important terms for reading PVSyst results. On the official loss diagram page, the loss diagram is described as a chart for quickly assessing the quality of a PV system design and identifying the main sources of loss. Moreover, because this diagram is always included in the annual report and can be checked month by month, it makes it easy to grasp how losses manifest, including seasonal variations. In other words, it is easy to understand the loss diagram as "a chart that shows, in a single view, how energy decreases down to the final result."

When beginners look only at the annual energy production, they tend to stop at "high" or "low". However, in practical work it is difficult to formulate improvement measures unless you know where and by how much losses occurred. By looking at the loss diagram, you can see how the process starts from solar irradiance conditions and proceeds through optical losses, array losses, and system losses to the final available energy. This indicates that PVSyst is not merely software for calculating energy production, but also a tool for identifying weaknesses in a design.

In design practice, using the loss diagram can greatly change the quality of conversations. When energy production is lower than expected, instead of relying on gut feelings like "the shading looks bad" or "the temperature seems high," you can share, as a diagram, which stage of losses is having an effect. If a beginner is adopting PVSyst, the result term they should learn first might be the loss diagram rather than annual energy production. That is how much this diagram symbolizes the purpose of using PVSyst.

Term 8 Performance Ratio

"The performance ratio" is an evaluation term you absolutely should know when reading PVSyst. According to the official explanation, the performance ratio is an indicator obtained by dividing the energy actually usefully produced by the ideal amount calculated from the solar irradiance incident on the installation surface and the nominal output. Furthermore, this indicator is said to broadly include optical losses such as shading, angle-of-incidence losses, and soiling; array-side conversion losses; temperature effects; quality differences; mismatch; wiring losses; and system losses. In other words, the performance ratio is a term used to summarize "how cleanly this system operated."

Beginners tend to compare systems only by annual energy production, but annual energy production is strongly affected by site conditions and orientation. On the other hand, the performance ratio is less directly dependent on those factors, so it is officially described as being easy to use for comparing system quality even for installations in different locations or with different orientations. In other words, this term helps separate “producing a lot of electricity” from “being a good design.” Especially when comparing alternative proposals or evaluating operational performance, the interpretation changes depending on whether the performance ratio can be observed.

Also, the Performance Ratio (PR) is useful not only for design but also for post‑operation comparisons. Officially, PR is understood to broadly include optical losses, array losses, and system losses, so it can be read as a helpful indicator when connecting predictions and actual measurements. For beginners it may feel a bit abstract, but if you think of it as a term to separate “the magnitude of annual energy production” and “how well the system functions as a whole,” it will be quite useful. If you want to understand PVSyst, it’s a term you should learn early on alongside the loss diagram.

Term 9 Linear shielding loss and electrical shielding loss

"Linear shading loss" and "electrical shading loss" are essential terms for understanding shading in PVSyst. In the official description of linear shading, linear shading loss is said to correspond to the irradiance deficit caused by the apparent obstruction. On the other hand, the official explanation of electrical shading loss refers to the additional loss obtained by subtracting linear shading loss from the actual shading loss; it is said to arise because parts of cells or modules become shaded, which perturbs the I/V characteristics and causes the whole to be pulled down to the most disadvantaged cell or string.

It is important to understand these two separately because thinking of shading as a mere area ratio can be misleading. Even if a shadow appears to cover only a small area visually, the electrical connections can produce losses that are greater than expected. PVSyst can treat the "visible shading" and the "invisible additional losses" separately, making it easier to make design decisions even on projects with severe shading. The reason practitioners are less likely to underestimate shading and fail is that these terms are clearly distinguished within the software.

For beginners it’s a somewhat difficult term, but it’s worth remembering at least that "shading does not end with area ratios." In PVSyst, you can, when necessary, use module layout and string-splitting models to examine electrical impacts in depth. Once you understand what this term means, it becomes easier to see why this software is valued more highly than simple shading-calculation tools. If you are involved in projects with shading conditions, this is an important term to master early.

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Term 10: Comparison of Measured Data

Finally, the term I want to understand is "measured data comparison." According to the official explanation of measured data analysis, the purpose of this feature is to closely compare, on an hourly or daily basis, the data measured on site with simulated values. Furthermore, it is said to not only help verify the validity of the software, but also to assist in operational analysis of systems actually in operation and in detecting minor faults. In other words, measured data comparison is a term for linking prediction and reality.

This phrase is important because it indicates that PVSyst is not software only for pre-design. By comparing the assumptions made during design with the actual results after operation, it becomes easier to judge whether the loss settings were appropriate, whether shading assessment was insufficient, or whether there are problems on the equipment side. The official `measured data` file verification page also explains that tables and graphs by month, day, and hour, as well as synchronization checks, are available. In other words, it is easy not only to compare results but also to verify the quality of the data used for those comparisons.

As a beginner, you may not use this feature immediately. However, it is important to know before implementation that PVSyst is not only a prediction tool but also a validation tool. This is because the ability to look back at the numbers you designed and apply them to subsequent projects is what gives the software value as a long-term tool. Simply knowing the term "comparison with measured data" makes it easier to see PVSyst as something other than a one-off calculation tool.

How to Read This Before You Start

So far we have organized the basic terminology, but one point to keep in mind before diving in is that PVSyst is not software for learning terms in isolation; it is software you understand through the connections between terms. Within a project you define the meteorological data and the location, place the mounting surface, decide on subarrays and strings, determine the effective irradiance from the incident solar radiation, read the results from loss diagrams and the performance ratio, and, if necessary, connect to comparisons with measured data. Once this flow becomes visible, the meaning of each screen naturally becomes easier to understand.

Beginners typically get stuck at the start because they try to memorize terminology by rote. In practice, however, what matters more than memorization is the relationships between concepts. For example, understanding the relationship between meteorological data and the installation surface makes the meaning of incident solar irradiance clear. Understanding the relationship between incident irradiance and losses lets you read loss diagrams. Understanding how loss diagrams relate to the performance ratio makes it easier to interpret comparisons of alternative designs and comparisons with actual measurements. The key to learning PVSyst quickly is not to memorize terms like a vocabulary list, but to grasp where they appear in the flow of the design process.

Summary

When organized for beginners about what PVSyst can do, nine functions are particularly important: preliminary rough estimates in the early planning stage, organization of site and meteorological data, definition of installation surface conditions, verification of system configuration and capacity sizing, hourly power generation simulation, review of array loss breakdown, evaluation of linear shading losses and electrical losses, comparison of alternative designs and economic evaluation, and comparison with measured data and detection of anomalies. Seeing these makes it clear that PVSyst is not merely a power generation calculation tool but an integrated software for organizing design conditions and the reasons behind results.

What’s important for practitioners is not to treat this software as a black box. The more you verify which site, which meteorological data, which orientation, which configuration, which losses you assume, and where the results decreased, the more valuable PVSyst becomes. If you use it not simply to produce numbers but to give those numbers a rationale, the quality of your design and your explanations will improve dramatically. Beginners don’t need to master everything perfectly at first, but just knowing the overall capabilities will greatly reduce confusion after adoption.

The more carefully you check weather, the mounting surface, losses, shading, and comparisons during desk-based reviews, the more important the accuracy of on-site location data and equipment layout becomes. Even if you refine design conditions in PVSyst, if site positioning and obstacle identification are vague, the gap between design assumptions and actual construction tends to widen. That is why, at the design stage, organizing desktop conditions in PVSyst and, at the site stage, combining them with an iPhone-mounted GNSS high-precision positioning device like LRTK makes it easier to link design, construction, and maintenance more consistently. The more items you can check in PVSyst, the better its compatibility with on-site means of supporting positional accuracy, such as LRTK.