6 Steps to Read PVSyst's Loss Diagram | For Beginners

When working on solar power system design or energy yield forecasting in practice, PVSyst’s Loss Diagram is one of the documents you consult most often. However, beginners often do not know where to start and tend to pick out only the items with large loss rates and stop there. In reality, the Loss Diagram is not simply a chart that lists losses; it is a document that shows, step by step, how solar irradiance is received, at which stages it is reduced, and how it ultimately leads to the final output. PVSyst itself also explains that the Loss Diagram is a figure for quickly grasping the quality of a system design and for identifying the main sources of loss.

Also, PVSyst’s Loss Diagram is not only always shown in the annual report but can also be checked on a monthly basis. This is very important for confirming seasonal differences and identifying when specific losses are having an effect—things that are hard to see from annual totals alone. Furthermore, PVSyst explicitly states that each loss rate listed in the Loss Diagram is a proportion of the previous energy amount and cannot be simply added together. If you view the diagram without understanding this, it is easy to misjudge the weighting of the losses.

The way of reading that is truly useful in practice is not simply looking at the diagram from left to right. It is knowing the order in which to look so that causes and countermeasures can be organized more easily. Specifically, first check the reference energy on the far left, then the losses due to irradiance conditions, the module- and array-side losses, the degree of attainment up to EArrMPP, then the System Losses after the inverter, and finally return to the monthly variations; reading in that sequence makes it less likely to become confused. In PVSyst’s official documentation, Array and system losses are also organized in the same order in the figure, so this way of looking is well aligned with that presentation.

In this article, I organize and explain PVSyst's Loss Diagram in six steps so that even beginners can read it and work with it without difficulty. Rather than memorizing the English labels on the screen, this guide is arranged so you can see where to look to find the root cause and at which stage you should consider countermeasures. Finally, it connects to the approach of capturing the site's positional relationships with high precision in order to more accurately refine shading and layout conditions.

Table of Contents

• What to Know Before Reading the Loss Diagram

• Step 1｜Start with the reference energy on the far left

• Step 2｜First examine the losses closest to the irradiance conditions

• Step 3｜Look at module and array-side losses

• Step 4｜Check whether it reaches EArrMPP

• Step 5｜Look at the System Loss after the inverter

• Step 6｜Finally return to the monthly Loss Diagram

• Common Pitfalls for Beginners

• The accuracy of on-site conditions influences the credibility of the Loss Diagram

• Summary

What you should know before reading the Loss Diagram

The Loss Diagram is not a chart for checking only the final energy production. In PVSyst, it is structured to organize both array losses and system losses and to show where and by how much energy is being reduced. In the official documentation as well, IAM, soiling, irradiance, temperature, LID, module quality, mismatch, optimizer losses, DC wiring losses, followed by AC wiring losses, external transformer losses, auxiliary consumption, and system shutdown losses are explained in the order of the Loss Diagram. In other words, the Loss Diagram is not merely a list of results but a diagram that breaks down the power generation process itself.

First, what you should grasp here is that each loss rate in the Loss Diagram is a percentage relative to the energy amount immediately before it. PVSyst clearly states that each loss is a percentage of the previous energy amount, and that summing the detailed losses does not yield the overall rate. Therefore, the way beginners often read it—adding up all the loss rates to obtain the total loss—is incorrect. The Loss Diagram should be used as a chart to see at which stage the losses are occurring, rather than to focus on the absolute size of the rates.

Also, PVSyst result variables include important reference points related to Loss Diagram, such as EArrRef, EArrNom, EArrMPP, and EOutInv. EArrRef is the reference energy for PR calculation, EArrNom is the starting point for array evaluation on the Loss Diagram, and EArrMPP is the MPP energy after array losses. If these are read without distinction, the PR reference and the Loss Diagram reference become mixed, making the overall understanding of the diagram unstable.

For beginners, put simply, a Loss Diagram is a "diagram for finding where losses occurred," not a "diagram for summing up loss rates." That's precisely why it's important to decide the order in which you read it. Rather than just scanning from the left edge to the right edge, reading it in meaningful chunks makes it easier to sort out which losses are due to site conditions and which are due to equipment or configuration issues.

Step 1 | Viewing from the leftmost reference energy

When reading a Loss Diagram, the first thing you should check is the reference energy at the far left. Beginners tend to focus immediately on the loss items, but if you don't understand where the losses begin, the meaning of the entire diagram becomes unclear. In PVSyst's result variables, EArrNom is defined as "the starting point for the array energy assessment in the Loss Diagram," and it is the nominal array energy based on GlobEff and Pmpp at STC.

The important point here is that a Loss Diagram does not necessarily begin by being worked backwards from the "actual final output." First, an ideal starting point based on effective irradiance and the concept of STC is established, and realistic module and system losses are then stacked on top of that. Therefore, if you look only at the intermediate losses without checking the reference at the left end, you cannot tell whether a given loss is large or small relative to what.

Also, it is important here not to confuse EArrRef and EArrNom. EArrRef is the reference for PR calculations, and EArrNom is the starting point for array energy assessment on the Loss Diagram. Beginners often feel these two are similar references, but because their purposes differ, it is easier to stay organized when reading the Loss Diagram if you focus on the EArrNom side.

The reason this order is useful in practice is that, by first looking at the leftmost baseline, you can more easily judge whether a case's loss is "bad because it's large" or simply appears different because the baseline itself is large. If you read the Loss Diagram as a chart where you first look at the baseline energy and then check how straightforwardly each subsequent stage steps down, you'll find it much less likely to get confused.

Step 2｜Look at losses closest to the light-receiving conditions first

After checking the leftmost reference, the next thing to look at is the losses related to the light-receiving conditions. In PVSyst's Array and system losses, IAM, soiling, irradiance, temperature, etc. are listed in the first half of the Loss Diagram, but what beginners should first focus on are the losses related to "how directly the solar irradiance is being received." Specifically, the effects of incidence angle, soiling, shading from proximity (mutual) or electrical shading, and efficiency reductions at low irradiance.

The reason for viewing it in this order is that this is considerably upstream in the power generation process. In projects where losses close to the light-receiving conditions are large, no matter how much you tighten module performance or inverter efficiency afterward, overall improvement tends to be limited. Conversely, in projects where this upstream area is well organized, revisiting downstream system losses is more likely to be effective. In practice, reading this upstream section correctly is extremely important for determining where to start and setting priorities.

For example, ShdElec is defined as an electrical shading loss based on the shading calculation according to module strings and ModuleLayout; it is not merely the visually shaded area but a loss that also includes module interconnections. When this item is large, it may indicate that not only is incident solar radiation being lost at the point of power collection, but losses are being amplified by the way they propagate electrically. If a beginner overlooks this, they are likely to be confused, thinking "there is only a little shade, yet the loss is large."

If you first look at the losses closest to the light-receiving conditions, it becomes easier to grasp the character of the project. This is because it makes it clearer whether the project has large losses stemming from location or layout, or whether there is room to improve things through adjustments on the equipment side. If you’re unsure how to read the Loss Diagram, simply examining this preliminary stage carefully will significantly clarify the overall picture.

Step 3｜Check module and array-side losses

Once you’ve examined losses close to the incident irradiance conditions, next move on to module and array-side losses. These include temperature losses, LID, module quality, mismatch, optimizer losses, DC wiring losses, and so on. In the PVSyst documentation these are also organized under the Array and system losses section in the order shown in the Loss Diagram. What beginners should be aware of here is to look at “how much is being lost on the module side even though the power entering the system isn’t bad.”

For example, TempLoss is defined as the loss due to temperature, MisLoss as the module array mismatch loss, and OhmLoss as the DC wiring loss. Unlike losses that are easily visible from the outside, such as shading or soiling, these vary depending on the installation’s design, configuration, and equipment assumptions. In other words, when these are large, it is easier to interpret that there is room to review module selection, wiring, and array configuration, rather than the solar irradiance conditions.

Also, PVSyst explains that for ohmic wiring losses, in MPP operation the wiring losses are applied before the MPP calculation. This can be a little difficult for beginners to understand, but essentially it means that DC wiring losses are treated not as a simple post‑hoc subtraction, but as losses that affect the array-side operating point. Therefore, when OhmLoss is active in the Loss Diagram, it is more practical to view it as part of the array’s behavior rather than simply dismissing it as “less because the wires are long.”

In practice, when the upstream incoming irradiance conditions are in order but power generation isn’t as high as expected, this way of reading the module/array side becomes useful. If it’s not shading and not irradiance, then when asking what is reducing output, looking in the order of temperature, quality, mismatch, and wiring makes it easier to isolate the cause. The Loss Diagram is easier to read if, rather than treating the loss items as a list, you read it in the order “after external-condition losses, look at internal array losses.”

Step 4｜Check whether you can reach EArrMPP

Once you have read the first half of the Loss Diagram, the next thing to check is whether you have properly reached EArrMPP. In PVSyst's result variables, EArrMPP is defined as "Array MPP energy after all array losses on the Loss Diagram." In other words, this is the point reached once the array-side losses have all been accounted for, and it is a very important milestone within the Loss Diagram.

What’s important for beginners about this point is that it makes it easier to separate “array-side problems” from “system-side problems.” If the drop up to EArrMPP is large, the primary cause lies on the array side, in the upstream portion. Conversely, if things reach EArrMPP relatively straightforwardly but then fall off significantly afterward, you should suspect losses on the system side. Once that dividing line becomes visible, the way you read the entire Loss Diagram is clarified at once.

Also, PVSyst's result variables include an item called EArray, which is the array output that includes deviations in the inverter operating point, and it is explicitly stated that this does not appear in the Loss Diagram. Beginners tend to confuse EArrMPP and EArray, but the one to look at first in the Loss Diagram is EArrMPP. When reading the Loss Diagram, it's clearer to first check "how far the array itself reached," and then, if necessary, follow the interaction with the inverter in more detail.

In practice, being aware of this demarcation greatly clarifies the discussion. For example, it becomes easier to determine whether you should revisit the layout and irradiance conditions, or reconsider inverter selection and the DC/AC ratio. When reading a Loss Diagram, keeping EArrMPP in mind as the dividing line is very effective for sorting out the question that often confuses beginners—"where do system losses begin?"

Step 5 | Check System Loss After the Inverter

After confirming EArrMPP, next we look at the system losses downstream of the inverter. In PVSyst's Array and system losses, after DC wiring losses come AC wiring losses, external transformer losses, auxiliary consumption, and system downtime losses. Also, in Array losses, general considerations it is explained that unused energy due to being outside the inverter input voltage range or due to overload is normally treated as inverter losses, i.e. handled on the system losses side.

The key to reading this here is to view System Loss not as a single lump but as groups of losses with different natures—such as the inverter, AC wiring, transformer, auxiliary equipment, and downtime. For inverter-related items, in addition to the efficiency itself, it is important to check whether energy is being lost due to overloads or operation outside the allowable voltage window. On the AC side, wiring length and routing, transformer no-load and load losses, and how auxiliary equipment is operated or configured all have an effect. In other words, System Loss needs to be interpreted not only as “the performance of the generating equipment” but as the result of “the equipment configuration and the design up to the point of injection.”

Also, it’s important to note that the downstream section of the Loss Diagram applies to the energy that has passed through the upstream section. For cases where the upstream section has suffered large losses versus cases where the upstream section is neatly consolidated, the appearance of downstream losses changes even under the same inverter conditions. For that reason, you should not look at System Loss in isolation; you must always read it in connection with the flow up to EArrMPP. If a beginner skips this and looks only at the downstream section, they are likely to lose sight of what the real bottleneck is.

In practice, when the downstream portion of the Loss Diagram is large, it’s important not to suspect inverter efficiency alone but to revisit the DC/AC ratio, input voltage conditions, AC routing, distance to the injection point, auxiliary equipment settings, and so on. System Loss is both the final finishing loss and a loss that directly reduces the amount of electricity sold or injected. That is precisely why even beginners should keep this as an independent checkpoint.

Step 6｜Finally return to the monthly Loss Diagram

It's a shame to stop at reading the Loss Diagram only by annual values. PVSyst explicitly states that the Loss Diagram can also be viewed by month, which allows you to evaluate the seasonal variations and how each loss manifests. For beginners, after grasping the overall picture with the annual diagram, going back to the monthly diagrams to see "when each loss is in effect" is the finishing touch.

The reason this order is important is that annual values alone do not reveal the timing of the causes. For example, even if you find that shading losses are large, their significance differs depending on whether they are concentrated in winter mornings or affect the period around midday in spring and autumn. The same applies to temperature losses: whether they drop sharply only in summer or remain high throughout the year changes the countermeasures that should be considered. System Loss also has a different meaning depending on whether inverter overloads occur only in midsummer or whether resistive losses on the AC side are significant year‑round.

What often happens in practice is that one is broadly satisfied with the annual Loss Diagram and does not return to the monthly breakdown. However, viewing the data by month can reveal imbalances that were not visible in the annual view. Trends such as much larger shading only in winter, inverter-related losses standing out only in midsummer, or increases in low-irradiance losses during the rainy season are much easier to see on a monthly basis. This makes losses that were abstract when looking only at annual values become apparent as concrete issues for design and operation.

If a beginner wants to master the Loss Diagram, it’s important to get into the habit of always returning to the monthly view after reading the annual view. The annual view shows the overall picture, and the monthly view shows the reasons. Once you can switch back and forth between the two, the Loss Diagram becomes not just a results screen but a very strong resource for design decisions.

Common Pitfalls for Beginners

One of the easiest things for beginners to get wrong with the Loss Diagram is adding the loss percentages together. PVSyst clearly explains that each loss is a percentage relative to the immediately preceding energy amount and cannot be summed. Nevertheless, if you add up all the percentages for each item and talk about the total loss, the meaning of the diagram is greatly distorted. The Loss Diagram is a sequential diagram, not a simple aggregation chart.

Another common mistake is confusing the reference for PR with the reference for the Loss Diagram. EArrRef is for PR calculation, while EArrNom is the starting point for the Loss Diagram. If you are not aware of this difference, it is easy to misunderstand the relationship between the losses shown on the Loss Diagram and PR. For beginners, it is easier to stay organized if you read the Loss Diagram using the Loss Diagram reference and treat PR as a separate summary value.

Also, a common mistake is to jump straight to the downstream System Loss without first looking at the upstream irradiance conditions and array losses. The downstream part of the Loss Diagram applies to the energy that has passed through the upstream stages, so it means different things for cases with large upstream losses versus those with small upstream losses. If you look only at the downstream section and treat it as an equipment-efficiency issue, you can miss the real cause.

Furthermore, it's dangerous to be reassured by the annual chart alone. PVSyst provides a monthly view because checking for seasonal variations is necessary. Losses that appear small on an annual basis can be heavily concentrated in specific periods when viewed monthly. What beginners should avoid above all is the short‑sighted conclusion, "It's fine because it looks okay on an annual basis."

The accuracy of on-site conditions affects the credibility of the Loss Diagram

The Loss Diagram may appear to be a self-contained results chart within the software, but how convincing it is depends heavily on the accuracy of the on-site condition assessment. In particular, shading losses and wiring losses are directly influenced by actual site conditions such as equipment layout, obstacle locations, spacing between rows, and the positional relationships of electrical panels and transformers. PVSyst models losses in detail, but if the underlying assumptions are off, a neat Loss Diagram will have limited persuasive power in practice.

For example, if Near shading is not adequately reflected, the front-end incident-light loss may appear too small. Conversely, if the positional relationship between the AC route and the injection point is assumed to be shorter than it actually is, the downstream System Loss may appear smaller. In other words, the Loss Diagram is not just a "result" but also a reflection of how accurately site conditions were entered.

In that sense, having a high-precision grasp of on-site positional relationships is highly meaningful for tightening the assumptions of the Loss Diagram. If you can more accurately organize clearances from obstacles, equipment azimuths, and layouts up to the panels and injection points, it becomes easier to establish the assumptions for shading losses and wiring losses. If you want to use PVSyst’s Loss Diagram as a document closer to actual practice, a perspective that improves the accuracy of on-site assessment—not just desktop input—is indispensable.

From this perspective, it naturally leads to LRTK, an iPhone-mounted GNSS high-precision positioning device, as a means of accurately grasping on-site positional relationships. By making it easier to capture equipment placement, obstacle locations, and a sense of route distances on site with high precision, it becomes easier to refine the preconditions that go into PVSyst’s Loss Diagram. In practical work where the goal is not just to read the Loss Diagram but to bring it closer to a more convincing representation, this level of accuracy in site assessment is a great help.

Summary

For beginners to read PVSyst's Loss Diagram without getting lost, it's important to first check the reference energy on the far left, then follow this order: losses related to the incident irradiance conditions, module/array-side losses, the degree of attainment up to EArrMPP, System Loss after the inverter, and finally return to the monthly chart. Simply following these six steps makes it easier to avoid being swayed by items with large loss rates and to identify where the real bottleneck is.

What’s important is to read the Loss Diagram not as a list of loss rates but as the flow of the power generation process. As PVSyst itself indicates, the Loss Diagram is a chart for getting an overview of design quality, and each loss is expressed as a proportion relative to the preceding stage. For that reason, reading it in order and organizing causes according to their sequential/causal relationships is a basic practice that, while aimed at beginners, also holds up in professional work.

And to make that interpretation even more reliable, it is essential to grasp the positional relationships on site more accurately. If you want to organize shadows, layouts, and route conditions with high precision, the perspective of utilizing the LRTK, an iPhone-mounted GNSS high-precision positioning device, is also effective. By combining the ability to correctly read the Loss Diagram with the ability to accurately capture the site, it becomes easier to arrive at more convincing power generation forecasts and design decisions.