9 Values to Check First in PVSyst｜Basics of How to Read Them

In the work of designing photovoltaic systems and forecasting power generation, the quality of your judgments depends on how you read the numbers shown on PVSyst’s result screens. Even if you carefully set the input conditions, unless you correctly understand the meaning of the numbers that come out, you won’t be able to improve the quality of the design, the validity of comparative evaluations, or the persuasiveness of explanations to stakeholders. Especially for practitioners searching for information on “how to read PVSyst,” it is often unclear where on the results screen to look and what to check first.

In actual operations, it is insufficient to end with an interpretation that looks only at annual energy production. While annual energy production is easy to understand, several factors accumulate on the way to that number: solar irradiance conditions, incident irradiance conditions, shading, temperature, array-side losses, and losses related to wiring and conversion. In other words, the figures from PVSyst may appear to be independent, but in reality they form a sequential, interrelated flow. If you extract only some of the numbers without understanding this flow, you are likely to make incorrect judgments in practice.

In this article, I narrow down the figures you should check first when reading PVSyst to nine and organize and explain the order in which to view them to make judgment easier. Rather than merely describing the items, I present a clear, practice-oriented summary of why each figure should be looked at first and how to connect it with other figures. The aim is to make the results screen less confusing and to provide a basic, easy-to-use reading method that can be used directly for comparison and for explanations both inside and outside your organization.

Table of Contents

• As a basic rule, do not look at PVSyst values in isolation

• Number 1｜Annual energy production

• Number 2｜Specific yield

• Number 3｜Irradiation on the horizontal plane

• Number 4｜Irradiation on the inclined plane

• Number 5｜PR

• Number 6｜Losses due to shading

• Number 7｜Losses due to temperature

• Number 8｜Array losses

• Number 9｜System losses including wiring and conversion

• What happens if you read the numbers in the wrong order

• The accuracy of site conditions affects the interpretation of the numbers

• Summary

As a rule, don't view PVSyst numbers in isolation

The PVSyst results screen displays many numerical values, but the important point is not to evaluate any number in isolation. What is often seen in practice is judging a project solely by its annual energy yield or determining performance only from the PR. However, the figures in PVSyst are fundamentally meaningful within the flow in which solar irradiance is incident, the conditions of light reception are applied, losses occur, and those lead to the final output. Therefore, drawing a conclusion from a single number overlooks the context behind the results.

For example, even for projects with high annual energy production, the evaluation changes depending on whether that is simply because the site has favorable solar irradiation, or because the incident irradiance conditions and the way losses are minimized are superior. Conversely, if a project's annual energy production does not grow as much as expected, and the site itself has a harsh solar resource, that should be considered separately from the quality of the design. In other words, PVSyst results should be interpreted by reading the assumptions behind the numbers rather than focusing on the absolute values.

Also, what is important for practitioners is to be able to use numerical values as tools for comparison. If you follow the numbers in the same order, it becomes easier to organize the differences between projects. Start by looking at the baseline solar irradiation conditions, then the incident light conditions, and finally the losses and the final output; keeping this sequence lets you compare projects as differences in structure rather than as a mere list of numbers.

The nine values covered in this article serve as the axes when you first read PVSyst results. Each is important, but what matters is not memorizing them—it's understanding the sequence and how their meanings connect. Once you grasp this foundation, it becomes clear where to start when looking at the results screen, and your ability to read the numbers stabilizes dramatically.

Numeric Value 1｜Annual Power Generation

The figure that is easiest to check first is the annual energy production. Because this expresses the overall generation scale of a project as a single number, it is the most convenient indicator both for internal review and for explaining to clients. When opening PVSyst results, this is also the number that many practitioners look at first. The expected annual amount of electricity becomes the starting point for assessing business viability and for equipment planning.

However, annual power generation is an output — not the end point of interpretation. If you judge a project's merits solely by this figure, you are likely to overlook the underlying solar irradiance conditions and the loss structure along the way. For example, a project may show high annual generation simply because it is located in an area with good solar irradiance, while another project may be suppressing losses under harsh irradiance conditions. Annual power generation is convenient for comparison, but using it without considering the background can be misleading.

When assessing annual generation in practice, the basic approach is to first use it as a figure to grasp the scale. You roughly check what amount of electricity this project is designed to produce and whether it is too large or too small compared with similar projects handled in the past. After that, it is important to follow up by examining the subsequent figures to understand why that value was reached.

Also, while annual electricity generation is easy to explain to stakeholders, care must be taken in how it is presented. If you show only that number, the other party is likely to assume it represents the entire design. In reality, however, that number is the result of the cumulative effect of solar irradiation and losses. Therefore, even if annual electricity generation is the first figure to check, the basic principle of interpretation is not to treat it in isolation but always to read it in conjunction with the subsequent figures.

Value 2｜Specific power generation

Along with annual energy production, the specific yield is something you should check early on. Specific yield helps you understand how much generation is being achieved per unit of installed capacity, so it is particularly important when comparing projects of different scales. With annual generation alone, larger projects tend to show larger numbers simply because they have greater capacity, making it difficult to grasp relative performance or efficiency. Specific yield serves as a useful reference for that.

In practice, when there are multiple candidate sites or several layout proposals, it becomes easier to assess by looking at the specific yield to confirm relative differences in generation efficiency. Even if annual generation is similar, different installed capacities change the meaning, and conversely, annual generation may differ while the specific yield is close. In other words, by looking at specific yield you can more easily see performance differences that are obscured by differences in scale.

However, specific yield is not a panacea. A simple comparison between projects with different solar irradiation conditions will reflect not only differences in design quality but also differences in site conditions. Therefore, rather than judging specific yield solely on whether it is high or low, you need to consider the regional and installation conditions under which that value was obtained. As with annual energy production, using specific yield on its own can be misleading; combined with other metrics, however, it becomes a very powerful indicator.

As a basic approach to reading, it’s easy to grasp the scale of a project from its annual energy production and to get a sense of generation efficiency from its specific yield. Even just placing these two side by side makes it clear whether a project simply produces a large amount of energy or is efficient on a per‑capacity basis. In practical PVSyst analysis, it’s important to make a habit of checking these two together first.

Data 3｜Solar radiation on a horizontal surface

Next, what we want to check is the solar irradiance on the horizontal plane. This is a figure that represents the meteorological conditions that form the basis of power generation, and it is indispensable for understanding the inherent potential of the project.

When reading PVSyst results, it’s easy to focus on generation and PR, but before that you should confirm how much solar resource exists at the site; otherwise it becomes difficult to judge the validity of the results.

Horizontal-plane solar irradiance is, so to speak, the inherent condition for power generation that a location possesses. Sites with a high value for this figure have a naturally advantageous basis, whereas sites with a low value face limits on how much the final power output can increase even with good design. In other words, by checking the horizontal-plane solar irradiance, it becomes easier to separate what is due to site conditions and what stems from design measures.

In practice, these figures are important to avoid misjudging the scope of design responsibility. For example, when power generation is lower than expected, it is necessary to distinguish whether the cause lies in the site's solar irradiance conditions or in the receiving conditions or losses. If the amount of solar radiation on the horizontal plane is low in that region to begin with, it may be difficult for the design side alone to significantly raise the outcome. Conversely, if the site's baseline solar radiation is sufficient but the results do not improve, there may be room to revise the design or the condition settings.

Also, this value is important when comparing multiple projects. When there are differences in annual energy production or specific yield, it serves as a benchmark for determining whether those differences are due to site conditions. As a basic principle for reading PVSyst, it is important to first grasp the natural conditions and then assess the quality of the design. The solar irradiance on the horizontal plane is the figure that serves as the starting point in that sequence.

Numerical value 4 | Solar radiation received on an inclined surface

Once you have checked the solar irradiance on a horizontal plane, the next thing to look at is the irradiance received on an inclined surface. This is a value that indicates how effectively the solar radiation provided by the site's natural conditions is being received under the actual installation conditions. In other words, it is a value that shows how much of the site's potential is being converted, through settings such as orientation and tilt, into solar irradiance that is usable by the equipment.

By examining this, you can see the impact of design conditions rather than just site conditions. Even within the same region, the amount of solar radiation that equipment receives changes with orientation and tilt. Therefore, the efficiency of light reception that could not be determined from horizontal-plane solar radiation alone becomes visible through these values. If horizontal-plane solar radiation is sufficient but the solar radiation received on the tilted surface is not increasing, there may be room for improvement in the light-receiving conditions or installation conditions.

In practice, it is important to use these values to verify the connection between solar radiation conditions and installation conditions. A common pitfall when reading PVSyst is to conflate natural conditions with design conditions, but by comparing the irradiance on the horizontal plane with the irradiance received on the tilted plane, it becomes easier to distinguish them. This is because you can see how much natural baseline there is and to what extent the design is able to capture it.

Also, these values are useful as a preliminary step before considering the effects of shading and surrounding conditions. If the conditions on the receiving surface are clarified, it becomes easier to determine at which stage the subsequent losses are taking effect. When using PVSyst in practice, the basic approach is not to simply look at solar irradiation, but to read it in the sequence from the horizontal plane to the tilted plane. Checking up to this point makes the meaning of the subsequent PR and losses much easier to interpret.

Numeric value 5 | PR

A commonly noted figure in PVSyst results is PR. Because PR summarizes the performance of the entire system, it is frequently used both in practical work and in explanatory materials. As a number that is easy to read and useful for comparing projects, many people place importance on it. However, when interpreting PVSyst results, PR is convenient but at the same time a typical metric that should not be judged in isolation.

PR summarizes how well a system is performing under given solar irradiance conditions. It therefore makes it easier to grasp a slightly more in-depth sense of performance than annual energy production, but if you ignore the loss structure that leads to that figure, there is a risk of judging solely by how the numbers look. For example, even if PR is high, PR alone cannot reveal whether that is due to inherently favorable temperature or installation conditions, or to design choices.

Also, for projects that keep losses well suppressed under harsh site conditions, the PR may not end up as high as it looks. Conversely, at sites with favorable conditions, the apparent PR can look good even if the design hasn’t been thoroughly refined. In other words, PR is a useful metric, but it needs to be interpreted together with the underlying assumptions. When interpreting PVSyst results, it’s important not to treat PR as a magic number.

For practitioners, PR is an effective summary metric for grasping the overall picture. First check it to get an overview, then proceed to examine shading losses, temperature losses, array losses, system losses, and so on. If PR is high, identify which conditions are supporting it. If PR is low, investigate which stage is dragging it down. Following this order makes PR a very practical number to use.

Numerical Value 6 | Losses Due to Shading

Losses due to shading are a figure that should always be checked at the initial stage. In solar power system design, the effects of shading are easy to overlook, yet they tend to have a significant impact on actual power generation. Especially if you only look at annual values, the extent of shading's impact can become obscured. However, because shading is a loss that occurs upstream, it affects all subsequent outputs.

The purpose of looking at these numbers is not simply to determine whether there is shading or not. In practice, even if the impact of shading appears small on an annual average, it may be concentrated in specific seasons or times of day. In such cases, the annual loss rate alone can be hard to appreciate, yet it can have a significant effect on operations and reporting. If shading losses are identified at an early stage, it becomes easier to maintain a consistent perspective when later checking monthly results or the validity of the layout.

Shading losses are strongly related not only to the design but also to site conditions. Small differences in surrounding buildings, trees, topography, and equipment placement can be reflected in the results. Therefore, shading loss figures can serve as a clue as to how well desk-based layout studies align with on-site conditions. When interpreting PVSyst, it is important to look at the shading losses and adopt the perspective of returning to the layout plan and site understanding.

In projects where shading losses are large, improving only downstream efficiency may not lead to significant improvement. Because these losses occur upstream, they should be checked as a priority. In practical PVSyst interpretation, it is easier to understand shading losses not as merely an item, but as a numerical indicator used to verify the consistency between the layout and site conditions.

Numerical value 7｜Losses due to temperature

Temperature-related losses are another important metric you should check early in PVSyst. In solar power generation, it’s easy to assume that the stronger the irradiance, the more electricity will be generated, but in reality module temperature rises cause output to decrease. For that reason, even under conditions where generation should increase, temperature losses can prevent it from rising as much as expected. If you overlook this metric when reviewing PVSyst results, you can easily misinterpret why the results are not improving.

In practical work, it is important to correctly understand what temperature loss means. This value is not merely an ambient temperature issue; it also tends to reflect installation methods, ventilation conditions, and the influence of the surrounding environment. For example, even within the same region, the way temperature loss manifests will differ between installation conditions that readily ensure ventilation and those where heat tends to become trapped. In other words, temperature loss is a metric connected not only to site conditions but also to design measures and installation circumstances.

Also, temperature losses become more important in projects with favorable solar irradiance and incident-light conditions. Even if strong solar irradiance is present, large temperature losses can offset that advantage. Conversely, if temperature losses are kept low, the same irradiance conditions are more likely to lead to stable results. Therefore, this metric is very useful for taking one step beyond simply looking at annual energy generation or PR and considering why those values occurred.

When operational staff look at temperature losses, it's important not to simply judge them as high or low but to consider whether they are reasonable in light of site conditions and the mounting configuration. If they are larger than expected, it may mean that, even with good incident irradiance conditions, energy is being lost further downstream. As a basic rule for reading PVSyst, temperature loss should be regarded as a figure that helps explain why the amount of solar irradiance does not directly translate into power generation.

Numerical 8 | Array Loss

Array loss is one of the PVSyst outputs that carries significant practical meaning. It includes not only losses that are easy to consider individually, such as shading and temperature, but also various losses that occur on the module side. Therefore, by looking at this value, it becomes easier to identify DC-side issues that are not readily apparent on the surface. Design quirks that cannot be seen from annual energy production or PR alone can sometimes be reflected in this number.

The reason array losses are important in practice is that there is not a single cause. Not only shading and temperature, but also variability in received irradiance, electrical mismatches, and condition-dependent efficiency reductions — multiple factors affect the DC side. When interpreting PVSyst, it is important not to view this figure as merely a single item, but to read it as a comprehensive signal that may indicate some undue stress or abnormality on the DC side.

Also, array loss is an area where the care taken in the design tends to show. Even if the annual energy production is reasonably high, a large array loss may indicate there is room to tighten up some part of the design. Conversely, if array loss is properly kept low even under harsh conditions, it becomes easier to judge that the results are reliable and the design is stable. In other words, array loss is a metric for reading the quality of the DC side.

For practitioners, it makes sense to look at this value after reviewing the individual losses. After confirming obvious factors like shading and temperature, they take a comprehensive view to see whether there is still any undue stress on the DC side. If you want to deepen your reading of PVSyst, array loss is a value you cannot overlook. By looking at it, you can understand the results not as mere numbers but as the state of the design.

Numerical Value 9 | System losses including wiring and conversions

The final item to check is the system losses, including wiring and conversion. This number shows how much of the power produced on the DC side is lost in the downstream stages before becoming the final AC output. Many people pay attention to upstream factors such as irradiance, shading, and temperature, but tend to overlook downstream losses. However, because the AC-side output is closer to the practical final evaluation, checking this part is indispensable.

By examining system losses, you can understand how well the power generation secured in the upstream stages is being reflected in the final output. If the upstream figures look good but the final results do not improve, there may be issues in the downstream stages, including wiring and conversion. Conversely, even if the upstream conditions are not particularly favorable, a project can still turn out well if downstream losses are properly controlled.

Also, system losses are an area that can be relatively easily reviewed in the design phase. Site conditions and irradiance conditions are difficult to change, but the downstream configuration may have room for reconsideration. Therefore, when this value is large, it becomes easy to consider it as an area for improvement from the design perspective. As a way to read PVSyst, after confirming the upstream natural conditions and layout conditions, having a flow that looks at system losses last makes it easier to understand the entire project in a three-dimensional way.

The reason for placing this figure at the end is clear. PVSyst’s results start with solar irradiation, proceed through incoming irradiance conditions, losses, and the DC-side status, and ultimately lead to the AC-side results. System losses are a figure positioned toward the end of that sequence. By confirming it after reviewing the preceding items in order, it can be viewed not merely as a loss rate but as the culmination of the overall results.

What can happen if you read numbers in the wrong order

All the figures in PVSyst are important, but if you read them in the wrong order you can easily make inconsistent judgments in practice. For example, if you look at PR first and decide a project's merits solely on that, you may overlook solar irradiance conditions and shading impacts. Conversely, if you evaluate based only on annual generation, you won't be able to tell whether the result is due to site conditions or to design optimizations. The meaning of the numbers only becomes clear when they are read in context.

Also, if you look at losses in the wrong order, you can easily misprioritize improvements. If a project has significant shading effects in the upstream stages, focusing only on conversion efficiency in the downstream stages will not lead to major improvements. Conversely, when the upstream stages are well organized, tightening downstream system losses is effective. In other words, which numbers you examine directly determines your improvement strategy.

In practice, there are times when you must explain results briefly at meetings or in reports. If the sequence is not organized, the explanation can jump around and it will be difficult for the audience to follow. First present the overall picture using annual power generation, then explain the solar irradiance conditions and the incident light conditions, then organize PR and losses, and finally review the downstream system losses. If this flow is in place, both the explanation and understanding become much easier.

The basics of reading PVSyst are not about memorizing every number. It’s about understanding which numbers to look at, why to view them in that order, and internalizing that sequence. When you can read in a structured order, the way the results screens appear becomes more consistent, and your ability to compare, evaluate, and explain is improved.

The accuracy of site conditions changes how numerical values appear

No matter how carefully you read the numbers from PVSyst, if your understanding of the underlying site conditions is vague, there are limits to the accuracy of your interpretation. In particular, shading losses, solar irradiance on tilted surfaces, and temperature losses are closely tied to an understanding of the site’s relative positions and surrounding conditions. In other words, for practical interpretation it is very important not to rely solely on the results screen but to be conscious of which site conditions produced those numbers.

For example, even if a layout appears acceptable on drawings, on site the way shadows form can change due to the positional relationships with nearby obstructions and slight elevation differences. Those differences affect shadow losses, irradiance conditions, and even how monthly variability appears. In other words, PVSyst’s figures are desk-based values, but they also reflect on-site positional relationships. Therefore, the higher the accuracy of the on-site survey, the easier it becomes to interpret the meaning of the numbers.

In practice, precisely determining positional relationships has great value for preparing the preconditions for simulations. If confirmation of planned installation positions, clearances from obstacles, understanding of orientation, and sharing of positions among stakeholders become accurate, the validity of the conditions entered into PVSyst improves, and you can be more confident in interpreting the results. Especially in projects where shadows and layout have a large impact, this difference in precision directly translates into differences in design decisions.

In that sense, in professional practice where you want to grasp on-site positional relationships with high accuracy, you naturally turn to LRTK, the iPhone-mounted GNSS high-precision positioning device. Making it easier to perform high-precision location checks and orientation determinations on site also makes it easier to set the input conditions in PVSyst, and it makes the numerical values that appear in the results easier to interpret practically. Deepening your understanding of how to read PVSyst is not just about looking at the numbers on the screen, but also about improving alignment with the field.

Summary

When using PVSyst, the first values you should check are nine items: annual energy production, specific yield, solar irradiance on the horizontal plane, irradiance received on the tilted plane, PR, losses due to shading, losses due to temperature, array losses, and system losses including wiring and conversion. Keeping track of these makes it much easier to read the overall results reliably. What matters is not evaluating each of them individually, but looking at them in sequence along the flow from irradiance to final output.

The basic principle for reading PVSyst is not to look only at the flashy numbers. It is to distinguish which figures are the foundation, which are intermediate results, and which are the final outcomes, and to understand the connections between them. Once you understand this flow, it becomes easier to compare projects and to organize explanations both inside and outside the company. Not only will you be less likely to be confused when looking at the result screens, but you will also find it easier to identify opportunities for design improvement.

To make the interpretation of these figures more reliable, it is essential to grasp the on-site spatial relationships with high precision. If you want to accurately establish the assumptions related to shadows, layout, and orientation, employing LRTK, an iPhone-mounted GNSS high-precision positioning device, is also an effective approach. By combining the ability to correctly read PVSyst’s numbers with the ability to accurately capture site conditions, you can more easily arrive at power generation forecasts and design decisions that are more convincing.