5 Ways to Read the PVSyst Loss Diagram｜Understanding the Loss Diagram

In professional practice—when designing solar power systems and forecasting energy production—relying solely on numerical data can easily lead to overlooking where generation is being lost.

This is why a loss diagram that allows you to grasp the flow of losses on a single sheet is important. A loss diagram visually organizes how much of the incident solar irradiance decreases at each stage and how that ultimately translates into output and generated energy.

However, although loss diagrams are visually simple, misreading them can lead to incorrect judgments. For example, even a loss that appears numerically small can carry different practical significance depending on whether it is acting in the upstream stage or the downstream stage. Also, if the effects of differences in design conditions, equipment performance, and site conditions are treated on the same level, it is easy to lose sight of the priorities for improvement.

This article, aimed at practitioners searching for "how to read PVSyst", organizes how to interpret the loss diagram from five perspectives. Rather than a mere walkthrough of the screen, it explains how to read the diagram in a way that links to design, internal explanations, comparative evaluation, and on-site verification. The goal is to help readers understand the structure of the loss diagram and to judge which losses can be addressed by design and which should be accepted as assumptions.

Table of Contents

• What does the loss diagram show?

• How to read 1｜First, follow the flow vertically from solar radiation to electrical power

• How to read 2｜Consider losses by separating meteorological conditions and design conditions

• How to read 3｜Focus on the sequence rather than the magnitude of loss rates

• How to read 4｜Distinguish between array losses and electrical losses when assessing

• How to read 5｜Look at intermediate stage differences as well as the final output

• Common misconceptions when reading loss diagrams

• How to use loss diagrams in practice

• The accuracy of verifying site conditions affects the readability of the loss diagram

• Summary

What does the loss plot show?

A loss diagram shows, step by step, where the energy entering a photovoltaic system is reduced. The starting point is solar irradiance, and from there it flows to the radiation reaching the receiving surface, the irradiance effectively used at the module plane, the output on the DC side, and the output on the AC side. Along the way, energy is reduced by various factors such as angular mismatch, reflection, temperature rise, wiring, and conversion efficiency. A loss diagram visualizes those reductions in sequence.

The usefulness of this diagram is that it shows not just the total loss, but where and in what order losses occur. When generation does not grow as expected, it lets you roughly determine whether the cause is irradiance conditions, layout, equipment selection, or electrical design. In practice, it is helpful for finding major bottlenecks in the early stages of design, explaining differences between projects, and aligning understanding among stakeholders.

On the other hand, loss diagrams are not a panacea. The values shown in the figure depend on the input conditions and the modeling approach. Therefore, when interpreting the diagram, you need to be aware not only of the numbers themselves but also under which assumptions they were calculated. To use a loss diagram correctly, it is important to regard the diagram not as a result but as an aggregation of design hypotheses.

How to Read 1｜First, trace the flow vertically from solar radiation to electric power

When people see a loss diagram for the first time, many tend to focus on the items with the largest loss rates. However, the first thing to do is to follow the flow of energy in order, from top to bottom or from input to output. By grasping, as if in a time series, what is being reduced at each stage, the overall structure of the diagram becomes clear.

In practice, what's important is to read the loss diagram not as a collection of individual items but as a series of conversion processes. For example, there is solar irradiance on the horizontal plane, which becomes effective irradiance on the tilted surface, then is subject to optical losses and partial shading and is converted into DC output. After that, it passes through temperature effects, low-irradiance conditions, mismatch, wiring losses, and conversion losses to become the final AC output.

The key point when following this flow is to understand that the meaning of the numbers changes at each stage. The first half mainly concerns how solar radiation is received and geometric conditions, while the second half concerns electrical conversion efficiency and system configuration. If you skip the first half and only look at the second half, you won't understand why the DC output turned out the way it did. Conversely, if you don't look at the second half, you can't judge how much favorable solar conditions are reflected in the final power generation.

The important thing here is not to try to understand the loss diagram all at once. First, determine which stage changes into which stage. Next, identify the factors that are being lost at that stage. Then consider which of those factors can be altered by design and which are more dependent on site conditions. Reading it in this order makes the loss diagram suddenly much easier to use as a practical tool.

This way of reading is also effective when comparing projects. When you compare two projects, looking only at the final power generation can make the difference seem large, but by tracing the flow in the loss diagram you can see whether it’s due to differences in solar irradiance conditions in the first half or differences in system losses in the latter half. This is very important when considering improvement measures. If the difference is mainly due to irradiance conditions, trying to solve it by changing equipment will not lead to a fundamental improvement. First, follow the flow and identify where the differences arise — that is the first step.

How to Read 2 | Consider Losses by Separating Meteorological Conditions and Design Conditions

Another important point in understanding loss diagrams is not to lump losses together. In practice, it is easier to organize if you broadly divide losses into those closer to meteorological conditions and those closer to design conditions. This also helps clarify scopes of responsibility and is directly tied to assessing the potential for improvement.

Factors that are close to meteorological conditions include the solar irradiance itself, the effect of ambient temperature on module temperature, and differences in solar incidence conditions due to season and region. These cannot be completely controlled. Of course, the amount of radiation received changes with installation angle and orientation, but the basic climatic conditions themselves cannot be altered by design. Therefore, differences that arise in this domain must be treated as assumptions specific to each project.

On the other hand, losses that are related to design conditions include orientation and tilt settings, array layout, how shading is experienced, DC wiring, AC wiring, equipment conversion efficiency, string configuration, and so on. These can be altered through design choices and equipment selection. In other words, when looking at a loss diagram, if you can distinguish which parts are due to natural conditions and which are targets for design improvement, the value of the diagram rises dramatically.

For example, if temperature-related losses are relatively large in a given case, that may indicate a strong influence from high-temperature environments or ventilation conditions. In such cases, rather than simply questioning the equipment’s performance, it is necessary to interpret the results including installation methods and the tendency for heat to build up. Conversely, if wiring or conversion losses are larger than expected, there may be room to review the choice of equipment and the system configuration.

This viewpoint is also effective for internal reports and client explanations. If you explain all losses with the same weight, the other party may simply interpret it as “this project has a lot of losses.” However, if you separate losses that are hard to avoid due to natural conditions from losses that can be reduced through design and explain them, the evaluation of the project will be more appropriate. Especially when comparing multiple options, without this decomposition the discussion tends to become abstract and design decisions can waver.

When viewing a loss chart, it's important not to take the displayed item names at face value but to get into the habit of considering which type of characteristic each item has. By doing so, the chart can be used not merely to check results but as a tool for determining the priority of corrective actions.

How to Read 3｜Look at the order (before-and-after relationship) rather than the magnitude of loss rates

When you look at a loss diagram, your eye inevitably goes to the items with large values. Of course, the magnitude of the numbers is important. But what you should pay even more attention to is the sequence — at which stage the losses occur. Even the same few percent can feel different in terms of their impact on the final output depending on whether the loss takes effect in an earlier stage or a later stage.

Losses in earlier stages reduce the baseline amount for all subsequent stages. In other words, if a large reduction occurs early on, it cannot ultimately be recovered even if later stages are more efficient. By contrast, losses in later stages act on an already narrowed amount of energy. Therefore, even when the numerical values are the same, the practical implications differ.

If you don't understand this difference, you'll get the priorities for improvement wrong. For example, if the front-end light-receiving conditions have deteriorated, concentrating only on the downstream conversion efficiency will yield only limited overall improvement. Conversely, in cases where the front-end is properly secured, even slight improvements in the downstream stage can affect the total power generation. The loss chart is also a diagram for reading the effects of this order.

Also, by looking at the upstream and downstream context, it becomes easier to notice interdependent losses. For example, as a result of reducing shading through layout improvements, the light-receiving conditions may improve and the downstream DC output may increase. Alternatively, revising the configuration can change temperature conditions or wiring conditions, affecting multiple loss items. Rather than viewing loss items as independent boxes, viewing them as a chain that connects upstream and downstream makes it easier to visualize the effects of design changes.

When practitioners handle loss diagrams, questions like "which part is the worst?" tend to come up in meetings and reports. At that point, rather than simply naming the items with the largest numerical values, being able to explain things like "this loss is occurring in an earlier stage, so its impact is large" or "this is in a later stage, so there is room for improvement but its effect on the whole is limited" raises the quality of the discussion. The core of a practical interpretation is the perspective that looks not only at the absolute value of the loss rate but also at where it is taking effect.

How to Read 4｜Differentiate and Assess Array Losses and Electrical Losses

One purpose of using a loss diagram is to isolate where a problem lies. In practice, a very convenient way to divide this is to distinguish between array-side losses and electrical-side losses. The array side mainly covers the areas related to how the modules receive solar irradiance and convert it into electricity. The electrical side covers the areas related to how the generated power is collected, converted, and connected to the output.

On the array side, the main factors are azimuth, tilt, shading, reflection, temperature, low irradiance, and mismatch. A characteristic of this area is that it is strongly affected by site conditions and layout planning. In other words, not only the design drawings but also on-site obstructions, installation methods, ventilation, and racking conditions affect how it is interpreted. If the loss chart shows a large decrease on the array side, you should first return to the layout and irradiance conditions.

On the other hand, on the electrical side the focus is on wiring losses, conversion efficiency, coordination between equipment, and output limits. This area is also relatively easy to refine through design. Of course there are external factors such as grid conditions, but in many cases it is easier to achieve improvements by revising the configuration than on the array side. If the downstream drop in a loss diagram is unnaturally large, it becomes a prompt to question the validity of the electrical-system approach and the choice of equipment.

This breakdown is useful not only for identifying root causes. It is also meaningful because it reveals where in the process checks should be made. If you are concerned about losses on the array side, you should first carry out layout planning, shading analysis, and organize the installation conditions. If you are concerned about losses on the electrical side, you should prioritize the single-line approach, circuit configuration, and clarifying the conditions of the conversion systems. In other words, a loss diagram also indicates which department or which stage of the process should recheck the issue.

Furthermore, this perspective is also useful when reviewing on-site troubles. When the generated power falls short of expectations, if it remains unclear whether the cause is local shading or installation conditions, or electrical losses, countermeasures can end up being ineffective. By using a loss diagram as a reference and separating the array side from the electrical side, the items that need to be checked become organized, and the efficiency of the investigation improves.

How to Read 5｜Look at the differences in intermediate stages as well as the final output

When dealing with loss diagrams, it's common to stop after looking only at the final AC output or the annual energy production. However, in practice, that alone is not sufficient. What matters more is understanding how much difference has emerged at the intermediate stages. The final value is the outcome, but discrepancies at intermediate stages tend to reveal the underlying causes.

For example, if the effective solar irradiance on the receiving surface is sufficiently secured but the conversion to DC output is falling more than expected, you should pay attention to temperature conditions, module conditions, and system configuration. Conversely, if the DC side is not that poor but the discrepancy widens on the AC side, conversion equipment or output control may be having a strong influence. In this way, by looking at which section shows the largest drop, the likely direction of the problem becomes apparent.

Interpreting the differences at intermediate stages is especially useful when comparing multiple proposals. Even if the final power output of one proposal is similar to that of another, their intermediate loss structures can differ greatly. At first glance they may look the same, but one may enjoy better light‑receiving conditions while suffering large downstream losses, whereas the other may face harsher light‑receiving conditions but operate more efficiently in later stages. Making an adoption decision without recognizing these differences can leave you disadvantaged in terms of future operability and reproducibility.

Furthermore, differences at intermediate stages are also useful when verifying the impact of design changes. For example, by tracking which stages improved when the support-frame layout was changed or which stages shifted when the equipment configuration was altered, it becomes easier to justify the changes. Even if the final value makes the improvement appear small, meaningful changes may have occurred at intermediate stages. Such changes are worth accumulating as design assets for future use.

In practical work, there's a tendency to want to explain whether a result is good or bad with a single number. However, to improve the quality of power generation forecasts, it is essential to develop the habit of examining the intermediate structure rather than just the final outcome. A loss diagram is precisely the material for that purpose. When you can see the differences at intermediate stages, the resolution of the diagram increases and it can be used not merely as a confirmation document but as the basis for design decisions.

Common Misconceptions When Reading Loss Plots

Loss diagrams may appear easy to understand, but they are actually prone to misinterpretation. One common misunderstanding is treating all losses as if they can simply be added together. In reality, losses are applied in stages, so they can be difficult to grasp with a simple summation approach. If you look at the percentages shown in the diagram and assume that adding them all up gives the total loss, you will end up with a result that does not match the actual structure.

The second misconception is the belief that items with larger losses should always be prioritized. In reality, you cannot determine priority without examining whether those losses can be improved, whether the improvement justifies the cost, and whether they are linked to other items. Treating losses that are inherent to natural conditions the same as losses that can be reduced through design makes improvement measures unrealistic.

The third misconception is lining up the figures for each case side by side and judging them as good or bad as they are. If the cases differ, regional conditions, orientation, and equipment conditions will also differ. For that reason, it is dangerous to determine superiority or inferiority based solely on the numbers in a loss diagram. When comparing, you need to align the assumptions as much as possible and then examine at which stage the differences occur.

The fourth misconception is using loss diagrams only as explanatory materials for the final results. In fact, loss diagrams are also highly effective during intermediate stages of design. By checking the loss structure early on, you can prevent major rework later. In particular, placement and shading conditions are difficult to revise in later stages, so it is worthwhile to adopt the practice of reviewing loss diagrams from the initial stages.

The fifth misconception is thinking that the more precise the numbers on a figure are, the more correct they must be. A loss diagram is merely the result of a simulation based on input conditions. If the validity of the inputs is low, no matter how detailed the numbers are, the quality of your judgment will not improve. To read a loss diagram correctly, you need, alongside understanding how to read the figure, the ability to assess the reliability of the input conditions.

How to Use Loss Diagrams in Practice

For operational staff, the important thing is not to stop at looking at the loss diagram, but to determine how to link it to actual operations. The basic starting point is an initial comparison of projects. When there are multiple candidate sites or layout proposals, comparing not only the final power output but also the locations where losses occur makes it easier to see which option has a more stable configuration. Even if the apparent power outputs are similar, options with a large concentration of losses may be more vulnerable to changes in conditions.

Another effective use is internal and customer briefings. Power generation forecasts can be hard to accept if you show only numbers. In such cases, using a loss diagram to sequentially show "this much solar irradiance is received, but under these conditions this part is reduced" increases the persuasiveness of the explanation. Especially in projects where losses of different natures—such as shading, temperature, wiring, and conversion—are mixed, there is great value in explaining them step by step with diagrams.

It can also be used for improvement studies. Examine the loss diagram to identify sections that appear to have room for improvement, then apply measures such as rearranging the layout, revising the configuration, or reviewing equipment. What’s important here is not to change all items at once. If you alter many conditions at once, you won’t know what was effective. By using the loss diagram as a starting point to form hypotheses and evaluating the differences one by one, it becomes easier to accumulate design knowledge.

Furthermore, it is also effective as handover documentation. When stakeholders with different roles—design staff, sales staff, construction staff, maintenance staff, etc.—are involved in a project, loss diagrams tend to serve as a common language. Even if their specialties differ, they can easily share the perspective of "where losses are occurring." Including the loss structure along with the numerical results reduces misunderstandings in subsequent processes compared to handing over numbers alone.

In this way, loss plots are not merely a results display. They are practical documents that can be used widely for design decisions, comparative evaluations, explanations, improvements, and handovers. To unlock their value, it is essential not to simply look at the numbers but to read the structure, distinguish meanings, and adopt a perspective that leads to improvement.

The accuracy of on-site condition verification affects the readability of loss diagrams

No matter how carefully you read a loss diagram, if your grasp of the underlying site conditions is vague, there is a limit to the accuracy of your interpretation. Losses related to shadowing, orientation, tilt, surrounding obstructions, and the relative positions of installations are especially dependent on the precision of on-site information. In other words, the deeper you go into reading loss diagrams, the more apparent the importance of how you define the input conditions becomes.

In practice, even when drawings appear problem-free, subtle elevation differences or shifts in the positions of obstacles on site can make a difference. These discrepancies can lead to misreading the receiving and shading conditions in the initial stages of a loss diagram. As a result, the overall evaluation, including subsequent losses, can become inconsistent. To use loss diagrams correctly, it is necessary to capture the on-site positional relationships with as high a degree of accuracy as possible.

What is important here is having means to grasp on-site positional relationships with high accuracy. If the installation position, the sense of distance to surrounding objects, orientation checks, and the reproducibility of planned positions are improved, the assumptions underlying loss diagrams become easier to organize. In particular, for projects that involve consideration of shadows or layouts, the accuracy of position directly ties to the accuracy of design decisions.

In that sense, in practical work where it is important to grasp on-site positional relationships with high accuracy, interest naturally increases in LRTK for iPhone-mounted GNSS high-precision positioning devices. By making it easier to perform quick, high-precision on-site position checks, organizing design assumptions, considering layouts, and sharing among stakeholders becomes simpler. If you can adopt the perspective of not only reading loss diagrams but also establishing more readable assumptions on-site, design and on-site verification will connect and help improve the accuracy of power generation forecasts.

Summary

When reading PVSyst's loss diagram, rather than simply looking for items with large loss rates, it is important to follow the flow from irradiance to output in order, separate meteorological conditions from design conditions, examine the sequence and cause-and-effect relationships, distinguish the array side from the electrical side, and check the differences at intermediate stages. By keeping these five perspectives in mind, the loss diagram transforms from a mere results screen into a resource usable for design decisions.

What is truly useful in practice is not memorizing the numerical values on loss diagrams. It is discerning which losses are difficult to avoid and which can be improved. Supporting that judgment are the organization of input conditions and an accurate understanding of site conditions. Deepening the interpretation of loss diagrams not only enhances the ability to explain results but also leads to improving the quality of the design itself.

If you want to put "how to read PVSyst" to practical use, don't view the loss diagram as a single result; use it as a map that shows where in the design you should focus. Furthermore, if you want to establish assumptions while accurately grasping the positional relationships on site, using LRTK, an iPhone-mounted GNSS high-precision positioning device, is also a useful option. By connecting the accuracy of interpreting the design screen with the accuracy of on-site understanding, you can move closer to more convincing power generation forecasts and layout considerations.