A gentle explanation of how to read PVSyst's Loss Diagram from seven perspectives

When checking the simulation results of a photovoltaic system, it is not uncommon to judge based only on the final generation figure. However, in practice it is important to be able to explain "why that generation value was obtained." The PVSyst Loss Diagram makes it easier to organize those reasons.

The Loss Diagram is an indicator that shows, as a flow, where and how losses occur from when sunlight enters the array until it is finally delivered to the grid. By understanding not only the magnitude of the numbers but also the locations and order in which losses occur, you can greatly improve design review and the accuracy of explanatory materials.

On the other hand, many practitioners may feel that "there are too many items and I don't understand what they mean," "I don't know which parts to focus on," or "I don't know how to judge the reasonableness of loss rates." Therefore, this article gently explains the key points to keep in mind when reading PVSyst's Loss Diagram, divided into seven perspectives. It is organized so that even first-time reviewers can grasp the overall picture of the results.

Contents

• The Loss Diagram is a diagram for understanding the breakdown of generation as a flow

• First check the reference irradiance condition

• Read incident losses as changes before and after reaching the array

• Read array losses by organizing temperature and low-irradiance effects

• Confirm electrical losses as drops in wiring and conversion stages

• Judge system losses by consistency with design conditions

• Evaluate the final generation together with the intermediate path, not alone

• Tips for reading the Loss Diagram to make it practical

The Loss Diagram is a diagram for understanding the breakdown of generation as a flow

PVSyst's Loss Diagram is a diagram that allows you to follow, from upstream to downstream, how energy in a photovoltaic system is reduced at each stage. The starting point is irradiance; from there it is narrowed to the effective irradiance received on the array surface, then to module output, DC energy, and finally AC energy. Along the way, factors such as shading, reflection, temperature rise, wiring, and conversion efficiency come into play, and losses are subtracted at each stage.

The major role of this diagram is to enable understanding of generation not as a single final value but as the accumulation of multiple elements. For example, even if the annual generation is lower than expected, the Loss Diagram makes it easy to separate whether the irradiance condition settings are too strict, the impact of shading is large, temperature conditions are severe, or wiring and conversion are causing larger-than-expected drops.

In practice, what matters is reading the Loss Diagram not as a mere list of results but as the basis for explaining the design. The direction of countermeasures changes completely depending on whether the difference originates upstream or downstream. It helps determine whether the issue is related to site formation and layout planning, electrical design, or operation planning, so understanding the structure of this diagram is highly valuable.

The Loss Diagram shows not only the numerical values of each loss but also the order in which they occur. Even if losses are the same percentage, whether they occur in an earlier stage or a later stage affects the final outcome differently. Therefore, instead of judging solely by large numbers, it is important to be aware of when in the process those losses are subtracted.

Once you can read the Loss Diagram well, you can more easily explain simulation results such as "this project is dominated by temperature effects rather than shading," "array-side losses are influencing the result more than the irradiance assumptions," or "verify upstream settings before worrying about AC-side conversion." This helps the designer's own judgment and is a powerful tool for internal sharing and owner explanations.

First check the reference irradiance condition

The first thing to confirm when reading the Loss Diagram is the irradiance condition at the starting point. If you focus only on downstream losses while this is ambiguous, you are likely to misjudge the overall evaluation. This is because the final generation value heavily depends on the irradiance assumptions.

In photovoltaic simulations, irradiance conditions for horizontal and tilted planes are set first. In the Loss Diagram, you start from this reference radiative energy and narrow it down to the energy that can be effectively used on the array surface. What you should check here is whether the adopted meteorological or irradiance conditions are reasonable for the target location and project conditions.

In practice, when the final generation seems low, you may be tempted to suspect shading or equipment efficiency. However, if the initial irradiance assumption is low, subsequent numbers will be generally low. Conversely, if the assumed irradiance is too high, the intermediate stages may look neat but the final expectation may be overly optimistic. Therefore, confirming the irradiance condition at the entry point is the first step in interpreting the Loss Diagram.

When checking this, it is important not to judge the numbers solely by absolute values but to consider consistency with the project's location, surrounding environment, tilt angle, and orientation. A nearly south-facing layout and one that is not will receive different light even at the same site. Tilt angle settings also change the relationship between horizontal-plane irradiance and array-plane irradiance. Thus, near the starting point of the Loss Diagram, it is easier to understand if you consciously check how much effective irradiance the array surface is assumed to receive.

Another important point is to treat the irradiance condition as a premise rather than a loss. Because the Loss Diagram lists many numbers, people tend to view all of them as loss items. However, the figures at the very beginning are the foundation of the entire simulation rather than causes of loss. If you develop the habit of confirming whether the foundation is reasonable before proceeding to evaluate subsequent losses, your interpretation will become much more stable.

This is especially important when comparing multiple proposals: align the starting points. That way you can distinguish whether differences between design options are really due to layout or equipment conditions, or due to different irradiance or setting assumptions. The Loss Diagram can be used for comparative studies, and in those cases checking the initial premises is even more critical.

Read incident losses as changes before and after reaching the array

In the early part of the Loss Diagram, focus on the losses related to incident irradiance that occur before sunlight reaches the array or immediately after it arrives. Understanding this section makes it easier to interpret the effects of layout planning and installation conditions.

Representative items include changes in irradiance due to orientation and tilt, effects of distant and nearby obstructions, the impact of incidence angle, and losses due to reflection. Since these occur before conversion to electricity, their nature differs from later-stage equipment efficiencies. By looking at how much effective irradiance is being reduced in the early part of the Loss Diagram, you can see how severe the site and layout conditions are.

Be careful not to interpret shading loss as a single number. Shading changes with time of day and season, and a single annual averaged value cannot capture its full behavior. However, the Loss Diagram organizes these complex shading effects into an annual value, so it is very useful for grasping the overall trend. If losses in this part are large, there may be room to reconsider spacing, positions of structures, terrain conditions, or the influence of surrounding obstacles.

Losses related to incidence angle are also easy to overlook. Sunlight does not always strike module surfaces ideally; at shallow angles reflection increases and the energy captured decreases. Because this is affected by morning/evening and seasonal variations, it must be read in conjunction with orientation and tilt considerations. It is natural for this portion to appear to some extent in the Loss Diagram, but if it is extremely large relative to the project conditions, it signals a need to question the installation assumptions.

Also important is that early-stage losses are often difficult to improve by equipment selection alone. For example, temperature losses and some electrical losses can be improved by reviewing equipment choices, but losses caused by site conditions or layout constraints are more strongly tied to the actual site. Therefore, when the early Loss Diagram numbers are large, you must also explain the limits on design flexibility and construction conditions.

When practitioners look at this part, it helps to ask, "To what degree is this project affected by site and layout?" In other words, early losses often reflect the harshness of the installation environment more than the merits of the equipment. With this perspective, the early section of the Loss Diagram becomes information for interpreting site conditions rather than a mere list of numbers.

Read array losses by organizing temperature and low-irradiance effects

In the middle of the Loss Diagram, focus on array losses that occur when modules generate electricity. This is a very important part for understanding photovoltaic performance. Even if irradiance is sufficient, not all of it is converted to power ideally. Output is gradually reduced by temperature and irradiance conditions.

A representative loss is temperature-related loss. Solar modules exhibit reduced output when they get hot; environments with strong irradiance tend to produce more generation but also larger temperature losses. In the Loss Diagram, temperature effects often appear as a relatively large item and attract attention in practice. You should carefully check this especially where module temperatures tend to rise in the summer or in installation types with poor ventilation.

Losses related to low irradiance are another item to watch. Under low irradiance periods or cloudy conditions, module conversion efficiency may drop below ideal levels. Annual generation is not determined solely by strong-irradiance periods, so accumulated behavior during low-irradiance times can be significant. While these items may be less conspicuous than temperature losses in the Loss Diagram, differences can appear depending on project conditions, so do not neglect them.

The important way to read this section is to understand array losses as physical phenomena rather than isolated bad numbers. For example, it is natural for some temperature loss to occur in a photovoltaic system. The issue is whether it is too large relative to project conditions. To judge that, consider background factors such as rooftop vs. ground installation, installation height and ventilation conditions, and regional temperature conditions.

Also, when array losses are large, it is not appropriate to immediately suspect only equipment performance. In reality, many factors such as installation conditions, thermal environment, and operating conditions interact. The Loss Diagram organizes these results for you, but rather than attributing causes to a single factor, evaluate them within the context of overall design conditions.

Once you master reading the middle section, for projects with underperforming generation you can explain not simply "lack of irradiance" or "shading," but "which array conditions are having an effect." This helps both design improvement and client explanations. The Loss Diagram serves as an entry point for reading underlying physical phenomena rather than leaving generation shortfall as a mere result.

Confirm electrical losses as drops in wiring and conversion stages

In the latter part of the Loss Diagram, confirm the electrical losses from the generated DC power through conversion to AC power and finally to delivered output. This area reflects equipment configuration, circuit design, and operating conditions. While the early and middle parts are inclined toward irradiance and module conditions, the latter part requires thinking about the validity of electrical design.

Representative losses include wiring losses, conversion device losses, and losses related to operating point shift. Wiring losses can occur on both the DC and AC sides. Wherever current flows, some power is lost as heat and cannot be zero. However, if this item appears relatively large in the Loss Diagram, there may be room to review cable lengths, circuit configuration, and conductor sizing.

Conversion-stage losses are also important. Because PV systems require DC-to-AC conversion, some loss occurs in this process. Usually this stays within the expected range, but depending on design and operating conditions the impact may appear somewhat large. When checking this part of the Loss Diagram, don't just look for the presence or absence of loss; consider whether the level is reasonable within the overall system.

Also, when reading electrical losses, do not separate them from upstream conditions. For example, if upstream irradiance or array output changes greatly, the absolute magnitude of downstream losses will also be affected. Evaluating the latter-stage numbers in isolation can lead to misunderstanding the cause. Since the Loss Diagram is a flow diagram, maintain the mindset of considering it continuously from upstream to downstream.

In practice, do not be lulled into complacency by relatively small numbers in this latter section. Even if the impact on overall generation seems small, these items have important implications for explaining design quality. In comparison or proposal cases, when premises are similar, differences in latter-stage losses can be perceived as differences in design competence. Therefore, reading the later part of the Loss Diagram ties into not only checking generation but also the precision of design explanation.

Electrical losses tend to reflect design decisions more than site conditions. Precisely because of this, carefully reading this area makes it easier to infer design quality from simulation results. The latter part of the Loss Diagram may seem modest, but it is an unmissable viewpoint in practice.

Judge system losses by consistency with design conditions

In addition to the relatively straightforward items such as irradiance, array, wiring, and conversion, the Loss Diagram also shows items that reflect system-level losses or constraints. When reading these, it is important not to judge by individual numbers alone but to assess consistency with design conditions.

For example, in some projects the way capacity is arranged or operating conditions can prevent part of the output from being utilized effectively. Also, assumed operating or control conditions can cause a gap between theoretically possible generation and the actual energy extractable. The Loss Diagram visualizes such system-level constraints, helping you understand gaps that cannot be explained by simple equipment losses.

Be careful not to overreact simply because system losses exist. Photovoltaic systems do not operate at theoretical maximums at all times. Rather, they are designed balancing safety, stability, and operating conditions, so some constraints are natural. What matters is whether those losses align with design intent and fall within an explainable range.

For example, if capacity was aggressively increased to boost generation and as a result some output is curtailed during certain hours, it is premature to judge the design a failure by looking only at the corresponding item in the Loss Diagram and saying "loss is large." If the annual generation and equipment utilization strategy are reasonable, that loss should be regarded as part of the design decision.

Conversely, if system losses are large without clear design intent, you may need to revisit condition settings. The Loss Diagram does more than indicate the presence of loss; it prompts you to consider whether the loss is justifiable. This is where the designer's interpretive skill appears.

As a practitioner, aim to be able to explain in one sentence "why this loss occurred." If you can separate and explain whether it is due to orientation or shading, temperature conditions, or operating conditions and capacity design, your understanding of the Loss Diagram will have deepened significantly. System loss items may look abstract, but become easier to read when you link them to design.

Evaluate the final generation together with the intermediate path, not alone

People who look at the Loss Diagram often first notice the final annual generation or the energy delivered to the grid. Of course that number is extremely important. However, the value of the Loss Diagram lies less in the final value itself and more in visualizing the path to it. Therefore, it is unfortunate to evaluate the final generation alone.

For example, two projects may show the same annual generation, yet one may be heavily affected by shading while the other mainly suffers from temperature loss. Even with the same outcome, the direction for improvement is completely different. If shading is the cause you need to consider layout and obstacles; if temperature is the cause you must focus on installation conditions and thermal environment. In short, the final value alone does not lead easily to next actions.

Also, even if a project looks like it has high final generation, it could have an unnatural loss composition along the way. If good irradiance conditions happen to secure the final value but shading or electrical losses are underestimated, caution is needed in practice. Conversely, if the final generation is modest but the Loss Diagram's structure is natural and the premises are consistent, you can evaluate the simulation as solid.

This perspective is important for internal reviews and external explanations. Simply stating "this is how much it generates annually" may not provide enough reassurance. Using the Loss Diagram to explain "how much loss occurs at each stage and as a result this is the generation" greatly increases persuasiveness. The generation figure then gains grounding.

Furthermore, this approach is useful for comparing options. Even when multiple proposals show small differences in final generation, differences in loss structure reveal which option might be more exposed to future risks or which is more resilient to condition changes. The Loss Diagram is not just a result table but an analysis tool for design comparison.

While valuing final generation, do not fix it as the sole goal; read it together with the intermediate loss structure. This habit elevates the quality of design and explanations beyond a superficial glance at PVSyst results.

Tips for reading the Loss Diagram to make it practical

So far we have looked at the Loss Diagram from seven perspectives; finally, here are practical tips to make it easy to use. The key is not to treat all items with the same intensity, but to read them with awareness of order and role.

First, confirm the starting irradiance condition, then follow the flow of early incident losses, mid-array losses, and late electrical losses. Focusing on a single midstream item without context makes the position within the whole unclear. The Loss Diagram is a flowchart, so respect the flow when reading it.

Next, do not judge merely by the magnitude of losses. Some level of loss naturally occurs in photovoltaic systems. What you should check is whether those values match the project conditions. Roof vs. ground, flat vs. undulating terrain, dense vs. spacious layouts—all naturally lead to different loss tendencies. Loss Diagram numbers make sense when read together with the project background.

Also, reading with comparison targets rather than a single-year result is effective. For instance, placing different layout options, different tilt angles, or scenarios with adjusted loss settings side by side shows what each setting affects. The Loss Diagram is useful on its own, but comparison deepens understanding considerably.

When using it for explanatory materials, you do not need to discuss every item in detail. Organize and communicate in three stages: premise, main losses, and final result. For example, say "first the irradiance condition is this much, then there are certain losses from shading and temperature, and finally after electrical conversion we get this generation." That structure makes it easier for non-specialists to understand. The Loss Diagram is both an analysis diagram for experts and a tool for explanation.

Also, remember to iterate with field information. If the Loss Diagram shows large shading losses, review the actual terrain and obstacle layout; if temperature losses are large, recheck installation and ventilation conditions. Do not treat simulation results as final—corroborate with on-site conditions and design assumptions.

PVSyst's Loss Diagram may look complex at first glance, but once you grasp the reading order it becomes highly practical. Even simply following the flow from irradiance start through early, middle, late, and final stages greatly improves readability. Mastering this reading method helps not only in validating generation but also in proposals, design, internal sharing, and owner explanations.

In photovoltaic design and construction, the ability to interpret simulation results directly influences the quality of decisions. If you want to improve collection of site information and positional accuracy while organizing design and construction information, using a positioning tool like LRTK (iPhone-mounted GNSS high-precision positioning device) can be effective. As the accuracy of equipment layout and on-site verification improves, organizing simulation premises becomes easier and interpreting results more consistent. Combining the ability to correctly read PVSyst's Loss Diagram with reliable handling of on-site positional information will help lead to more convincing photovoltaic practice.