Six Ways to Understand the Loss Diagram in the PVSyst Manual

Table of Contents

• What to Know Before Reading the Loss Diagram in the PVSyst Manual

• Perspective 1: Trace the flow of energy from top to bottom

• Perspective 2: Do not simply add loss rates

• View 3: Examine the losses from solar radiation to effective solar radiation

• Perspective 4: Examine the reasons for reduced power output due to array-side losses

• Perspective 5: Look at the final output based on losses on the PCS and AC sides

• Interpretation 6: Evaluate together with PR and monthly results

• Reading mistakes to avoid when using loss plots in practice

• Approach to turning simulation results into improvements

• Summary

Basics to Understand Before Reading the Loss Diagram in the PVSyst Manual

Many people who want to understand the loss diagram in the PVSyst manual struggle with where to look in the simulation results to judge the validity of the predicted energy production, which losses are large enough to require a design review, and how to explain the figures in a report. The loss diagram is not simply a screen to glance at “what percentage of losses there are”; it is a flowchart for checking at which stages the incoming solar energy to a photovoltaic system is reduced and how much ultimately remains as usable electrical energy.

In PVSyst's official documentation, the loss diagram is described as a tool for quickly assessing the quality of a photovoltaic system design and identifying the main sources of loss. The simulation report also displays an annual loss diagram, and the monthly detailed results are said to allow verification of the seasonal variations of losses.

The most important thing when reading a loss diagram is to treat the loss diagram not as the "answer" but as the "sequence of checks." Even if the final power generation is low, you cannot determine whether the cause is solar irradiance, shading, temperature, module settings, or PCS capacity or efficiency unless you follow the loss diagram from the top down. Conversely, judging good or bad solely by the final power generation can cause you to overlook design problems.

For example, even if the annual energy production is lower than expected, what needs to be improved can vary completely depending on whether the site's solar irradiation conditions are inherently poor, the orientation or tilt results in unfavorable irradiance conditions, the nearby shading configuration is having a strong impact, or temperature-related losses are significant. The loss diagram functions like a map for breaking down and checking each of these causes one by one.

Also, while loss diagrams are often used in sales materials and internal briefings, if misread they can lead to confusion such as "the total loss rates don't add up," "the relationship between PR and energy production is unclear," and "it's not clear which losses can be improved." When reading a loss diagram in the PVSyst manual, it's important to pay attention not only to the sizes of the numbers but also to where the losses occur, their sequence and interrelationships, and whether each item can be altered by design changes.

Perspective 1: Follow the flow of energy from top to bottom

The basic principle of a loss diagram is to follow the flow of energy from top to bottom. At the very top, values related to the solar irradiance reaching the site and the irradiance incident on the tilted surface are shown. From there, accounting for effects such as azimuth and tilt, distant shading, nearby shading, IAM, and soiling, the values are organized into the irradiance actually available at the module plane. That irradiance is then converted to DC power by the modules, which then incurs losses from temperature, low irradiance, mismatch, wiring, degradation, and so on, passes through the PCS, and finally proceeds to AC output.

If you look only at the intermediate loss rates without understanding this flow, you can misjudge the situation. For example, even if shading losses appear large, their impact on annual energy production can be limited. Conversely, even if individual loss rates seem small, multiple losses can accumulate and lead to a large difference in final output. A loss diagram is useful for examining individual losses, but it also serves to show "at which stage the energy is being significantly reduced."

What you should look at first is not the final power output. Start by checking whether the input-side solar irradiance conditions are reasonable. Next, check whether there is a large drop at the tilted-surface irradiance or effective irradiance stages. Then confirm how much the DC side has fallen after module conversion, and finally check losses in the PCS and on the AC side. Simply following this order will greatly improve the readability of the loss diagram.

PVSyst’s loss diagram explains that array losses begin with the evaluation of nominal energy using the effective global irradiance and the array MPP nominal efficiency under STC conditions, and that the PV model’s behavior is then organized according to environmental conditions. In other words, the loss diagram is not merely a summary table but a means of visually following the calculation process from irradiance to PV array behavior and onward to system output.

In practice, when you open a loss diagram, you first check "where this diagram starts and where it ends." The input is solar irradiance, and the output is the amount of electricity sent to the grid or the available electrical energy. Once you have identified this inlet and outlet, examining the intermediate losses makes it easier to explain the reasons for low power generation.

Especially when comparing multiple proposals, it is important not to simply place the final energy output side by side, but to examine where in the flow of the loss diagram the differences arise. Proposals that change azimuth or tilt will alter tilted-surface irradiance and the impact of nearby shading. Proposals that change the modules or PCS will alter how temperature losses, mismatch, PCS losses, and output limitations appear. In this way, loss diagrams are also useful for forming the basis for comparative evaluation.

Perspective 2: Do not simply add loss rates

When reading the loss diagram in the PVSyst manual, the most commonly misunderstood aspect is how loss percentages are handled. Because the loss diagram lists multiple loss items, it’s tempting to simply add them together and ask “what is the total loss percentage?” However, the percentages in the loss diagram are not all relative to the same baseline. Since each subsequent loss is calculated against the energy remaining after the previous stage, simple addition does not match the overall loss percentage.

The official PVSyst documentation explicitly states that each loss is defined as a percentage of the preceding energy quantity, so percentage values cannot be summed, and the overall percentage for multiple combined losses does not equal the sum of the individual detailed values.

This way of thinking is very important when explaining practical work. For example, if there is a 5% loss at one stage and then a 3% loss at the next stage, it does not mean that a simple 8 is lost from the original 100. Instead, the next 3% is applied to the remaining 95 after the first loss. Therefore, adding up the numbers shown on each row of a loss diagram may not exactly match the difference between the initial and final values.

If you don't understand this mechanism, you'll have trouble explaining it when customers or internal reviewers point out, "Isn't the sum of the loss rates wrong?" In a loss diagram, it's important to know which stage's energy quantity each loss is based on. If the energy has been greatly reduced in an earlier stage, the absolute impact will change even if the loss rate in a later stage is the same.

Also, it's risky to devise countermeasures by looking only at items with high loss rates. Even if a particular loss rate is large, it may be difficult to avoid due to design conditions. For example, in regions with high temperatures, temperature losses tend to be large and cannot be eliminated entirely. On the other hand, shading, wiring, PCS capacity, and string configuration, among other factors, may be improved by reviewing the design.

When evaluating loss rates, consider not only whether they are large or small, but also what reference those losses are measured against, whether they can be reduced through design changes, and whether they are increasing or decreasing in relation to other losses. This allows a loss chart to be used not merely as a list of numbers but as a basis for identifying opportunities for improvement.

Especially during the proposal stage, it is easier to convey your point by separating and explaining the major contributing items—for example, “The main causes of reduction in final output are shading, temperature, and PCS conversion”—rather than by presenting an explanation that simply adds up loss rates. The essence of a loss diagram is not to produce a total sum but to understand the structure of the reductions.

Perspective 3: Examining the losses from solar radiation to effective solar radiation

In the first half of the loss diagram, we check how effectively the solar irradiance entering the photovoltaic system reaches the modules. This part involves horizontal-plane irradiance, tilted-plane irradiance, distant shading, near shading, IAM, and soiling. If significant losses occur here, there is a limit to how much power generation can be improved by reviewing downstream equipment settings. This is because the irradiance reaching the modules themselves is reduced.

First, what needs to be confirmed are the light-receiving conditions determined by orientation and tilt. Even with the same installed capacity, configurations closer to south-facing and those oriented east–west produce different annual generation patterns. For roof-mounted installations, building orientation and roof pitch constraints may prevent freely selecting the optimal azimuth and tilt. For ground-mounted installations, it is necessary to adjust the balance between solar irradiance capture and shading according to racking pitch, tilt angle, and land shape.

The next thing to consider is the effect of shading. Shadows can be divided into distant shading caused by mountains and terrain, and near shading caused by buildings, trees, equipment, and rows of mounting structures. Distant shading varies with morning and evening and with the seasons, while near shading is strongly influenced by the layout and the configuration of surrounding obstacles. If the loss diagram shows large shading losses, check whether the 3D model inputs, obstacle heights, installation spacing, and module arrangement are appropriate.

IAM is a parameter that represents reflection losses due to the angle of incidence. The more obliquely sunlight strikes the module, the greater the proportion reflected by the glass surface, reducing the solar irradiance that can be effectively used. It tends to have an effect in the mornings and evenings and during winter, and is also related to the tilt angle and installation azimuth. A large IAM loss is not necessarily a problem, but in extreme layouts or under special conditions, it is necessary to verify the validity of the results.

Soiling losses are also important in the step from irradiance to effective irradiance. The impact varies with site conditions such as dust, pollen, bird droppings, soiling after snowfall, and coastal salt. In PVSyst they are often set as a fixed rate, but in actual operation they change depending on cleaning frequency, rainfall conditions, and installation tilt angle. When checking soiling losses on the loss diagram, consider not simply whether a value has been entered but whether it is consistent with the site’s operation and maintenance policy.

The key point to check in this first part is whether the energy input the system receives is reasonable. If the solar irradiance data are not appropriate, the reliability of the entire loss diagram decreases. If the shading inputs are coarse, the predicted power generation may be higher or lower than actual production. If the soiling loss does not match the site conditions, it will also affect the profitability assessment.

In PVSyst's PR description, the elements included in PR are listed as optical losses such as shading, IAM, and soiling; array losses such as PV conversion, degradation, module quality, mismatch, and wiring; and system losses for grid connection such as inverter efficiency. With this classification in mind, the first part of the loss diagram is easier to understand as primarily the area that examines how effectively the light reached the system.

In practice, when a loss diagram shows a large loss in the early stages, the first things we question are the installation conditions and the input parameters. We reconfirm surrounding building heights, ground elevation, azimuth, tilt, array spacing, and conditions such as snow cover and soiling. By carefully reviewing these factors, it becomes easier to reduce later occurrences of “the simulation looked good but the measured generation was low.”

Approach 4: Examining Reasons for Reduced Power Output Due to Array-Side Losses

In the midsection of the loss diagram, we check how much of the effective irradiance reaching the module surface can be extracted as DC power. This region involves temperature loss, low-irradiance loss, module quality, mismatch, wiring loss, degradation, and so on. This is where the design quality of a photovoltaic system tends to become apparent.

The most noticeable loss is temperature loss. Solar photovoltaic modules experience a drop in power output as cell temperature rises. Although areas with high solar irradiance tend to generate more energy, ambient temperature and module temperature also tend to increase, which can lead to larger temperature losses. For rooftop installations, the degree of temperature rise varies depending on the distance from the roof surface and ventilation conditions. For ground-mounted installations, mounting height and ventilation conditions are relatively easier to manage, so the trend can differ from rooftop installations.

When evaluating temperature losses, don't just look at whether the value is large; also check whether it is consistent with the local climate, the installation method, and the module's temperature coefficient. It's natural for temperature losses to be somewhat high in hot regions, but if they are significantly larger than expected, verify the module type and the configuration of the installation conditions. Conversely, if temperature losses are too small given the site conditions, be cautious that the input assumptions aren't overly optimistic.

Next, what we want to check is mismatch loss. Mismatch is caused by differences in output between modules, the way shadows fall, differences in string configuration, and so on. Especially at sites where partial shading occurs, there can be additional losses due to electrical mismatch, not just simple irradiance loss. If mismatch is large in the loss diagram, there is room to review string design, the extent of shading, and module layout.

Wiring losses cannot be ignored in practice. Wiring losses increase under conditions such as long DC cables, high currents, undersized cables, and long distances to combiner boxes or the PCS. If wiring losses are large in the loss diagram, check the cable route, cable cross-sectional area, number of strings, junction box placement, and PCS placement. Because wiring losses can potentially be reduced through design, decisions are made while balancing power generation improvements against increased costs.

Module quality and degradation factors are also important. Output tolerances assumed in the first year, degradation over time, and variability in product specifications all affect long-term energy production assessments. When looking at a loss diagram, you need to confirm whether it represents a first-year simulation or assumes a long-term average. For documents prepared for financial institutions or investment decision-making, not only first-year energy production but also long-term evaluations that include degradation may be required.

Low-irradiance losses also vary in significance depending on the region and installation conditions. In areas with frequent cloudy weather, layouts with a high proportion of generation in the morning and evening, or east–west oriented designs, module characteristics in the low-irradiance range can have an impact. If low-irradiance losses are large, it is advisable to check the module characteristic settings together with the trends in solar irradiance data.

When examining array-side losses, it's easier to organize them by dividing into three categories: "equipment performance," "appropriateness of layout," and "electrical design." Temperature losses relate to the installation method and module characteristics. Mismatch losses are related to layout and string design. Wiring losses relate to cable design and equipment placement. Reading them separately in this way clarifies the specific targets for improvement.

Perspective 5: Evaluate the final output based on PCS and AC-side losses

In the latter part of the loss diagram, the DC power passes through the PCS and is converted into AC power, and the losses up to the point it is finally supplied to the grid and loads are checked. This involves PCS conversion losses, clipping due to PCS capacity, AC wiring losses, transformer losses, output limitations, standby losses, and so on. Even if sufficient DC power is obtained in the first and middle stages, if it falls significantly in the latter stage, the final generated power will not increase.

PCS conversion losses relate to the efficiency when converting DC to AC. A PCS has the characteristic that its efficiency changes with load factor, so it does not always operate at peak efficiency. If the PCS capacity is small relative to the installed capacity, output can become capped during periods of strong solar irradiance. This can show up on loss diagrams as clipping or output limitation.

In designs that increase the DC/AC ratio, the goal is to raise the operating rate during low irradiance by increasing PV capacity relative to PCS capacity. However, if it is made excessively large, clipping during high irradiance increases, and PCS-related losses appear large on the loss diagram. Therefore, when looking at the loss diagram, you should not immediately judge a large PCS loss as bad in itself, but evaluate it together with annual energy generation, equipment utilization rate, cost, and power sales conditions.

AC-side wiring and transformer losses cannot be ignored for the plant as a whole. Especially in large-scale projects, the distance from the PCS to the substation/transformer equipment and to the point of interconnection becomes longer, and losses vary depending on the voltage class and cable design. If the loss diagram shows large AC-side losses, check the cable size, equipment layout, collection method, and transformer specifications.

With self-consumption systems and battery-coupled projects, the way to interpret the final output becomes even more complex. It is necessary to account not only for the amount of energy sent to the grid, but also the energy used directly by loads, the energy entering the battery, the energy leaving the battery, and charging/discharging losses. In PVSyst’s PR documentation, for grid-connected systems that include self-consumption and battery storage, the approach shown is to include E_Solar as well as E_Grid in the PR evaluation.

This point is important when comparing conventional sell-to-grid projects and self-consumption projects. In sell-to-grid projects, the amount of electricity exported to the grid is a straightforward indicator. In contrast, in self-consumption projects, the amount of electricity consumed within the facility also has value. Therefore, when reading a loss diagram, it is necessary to check the final destination of the energy and be clear about what will be evaluated as the outcome.

When explaining PCS and AC-side losses, it's clearer to separate "losses lost in conversion" from "losses that cannot be used due to capacity limits or constraints." Conversion losses are an issue of equipment efficiency. Clipping and output limits are issues of design policy and interconnection conditions. AC wiring and transformer losses are issues of site layout and electrical design. If you lump these together as the same "losses around the PCS," countermeasures become ambiguous.

Interpretation 6: Assess together with PR and monthly results

Loss diagrams are very useful, but it is dangerous to judge everything about a system solely from a loss diagram. In practice, they are evaluated together with PR, annual energy production, monthly energy production, specific yield, curtailment, cash flow, and so on. In particular, PR is a metric often used to compare system quality.

In PVSyst's official documentation, PR is described as an indicator that compares the energy actually effectively produced to the energy that would be obtained if the system operated continuously at its nominal efficiency under STC conditions. For grid-connected systems, it is shown in the form of E_Grid divided by GlobInc and PnomPV.

When looking at PR, also check which items in the loss diagram are lowering it. Even if PR is low, the meaning differs depending on whether the cause is temperature loss, shading loss, or PCS loss. In high-temperature regions, a large temperature loss causing PR to drop can be natural to some extent. On the other hand, if nearby shading, wiring losses, or mismatch are significantly lowering PR, there may be room for design improvement.

Also, while PR cannot completely ignore the influence of location and orientation, it is a metric that makes it easier to compare system quality than simple annual energy production. In PVSyst's explanation of PR, it is stated that, unlike specific yield, PR does not directly depend on meteorological data or surface orientation, and therefore can be used to compare system quality across different locations and orientations.

However, the objective is not simply to increase PR. For example, with designs such as east–west installations or high DC/AC ratios, the way PR appears and profitability do not necessarily align. Unless you comprehensively consider the generation curve, the selling price of electricity, the self-consumption rate, PCS capacity, land use efficiency, and so on, you cannot determine the practically optimal option. Loss diagrams should be used as auxiliary lines to explain the background of PR.

Combining monthly results is also important. In the annual loss diagram, seasonal characteristics appear averaged. Whether temperature losses are larger in summer, shading losses larger in winter, or the effects of low irradiance during the rainy season are strong can be difficult to discern unless you look at the data month by month. Because PVSyst’s loss diagram can be viewed not only in the annual report but also on a monthly basis, it helps in evaluating seasonality.

For example, in projects where nearby shading is significant during winter mornings and evenings, the impact can appear small if you only look at the annual loss chart. However, checking the monthly results for winter losses may reveal that generation falls sharply during specific periods. Monthly checks are especially important for projects in snowy regions, high-latitude areas, mountainous terrain, and rooftop sites with many obstructions.

By combining loss diagrams, PR, and monthly results, you can make more practical judgments such as "acceptable on an annual basis but watch for reduced generation in winter," "good PR but absolute generation is difficult to increase due to site conditions," and "high generation but large PCS clipping, indicating room to reconsider capacity design."

Misinterpretations to Avoid When Using Loss Diagrams in Practice

One common misreading to avoid when using a loss diagram in practice is treating all items with large losses as inherently bad. Losses include those that are difficult to avoid by design and those that can be improved by revising inputs or the design. Temperature losses and IAM losses tend to arise from environmental and installation conditions, whereas wiring losses, some shading, mismatch, and the selection of PCS capacity can potentially be adjusted through design.

Another misreading is evaluating the loss diagram in isolation. Loss diagrams are useful for decomposing results, but whether the outcome is good or bad ultimately depends on the project’s objectives. For feed-in (export-to-grid) projects, annual power generation and revenue from electricity sales are emphasized. For self-consumption projects, alignment with facility load, whether reverse power flow occurs, and how batteries are used are important. For agrivoltaic or rooftop installations, installation constraints and operating conditions should also be included in the evaluation.

Also, it is dangerous to rely solely on the loss diagram without checking the input conditions. The loss diagram is produced based on the input meteorological data, equipment specifications, layout, shading model, wiring conditions, soiling conditions, and other assumptions. If the assumptions are incorrect, the loss diagram may look clean but does not necessarily reflect results close to reality. In particular, the 3D shading model, terrain conditions, roof pitch, module placement, PCS specifications, and wiring lengths should be checked carefully.

When using a loss diagram in sales materials, it is important to translate and explain the meanings rather than simply listing technical terms. For example, explain IAM as "the effect of increased reflection when light strikes at an angle," mismatch as "the effect of outputs not matching due to differences in module or string conditions," and clipping as "the effect of being unable to utilize some output because of the PCS capacity limit," which makes it easier for non-expert stakeholders to understand.

In internal reviews, it is useful to standardize the order of checks using a loss diagram. If you establish a flow—first solar irradiance data, then orientation and tilt, then shading, then temperature, then mismatch and wiring, and finally the PCS and the AC side—you can reduce omissions in checks by individual reviewers. The loss diagram also serves as a checklist for assessing the validity of simulation results.

Furthermore, when comparing multiple cases, it is important to look at the differences in the loss diagrams. With only a single-case loss diagram, it can be difficult to judge whether the figures are good or bad. By comparing proposals that change the orientation, proposals that change the tilt, proposals that change the PCS capacity, and proposals that change the module type, you can see which losses respond to which design changes. These differences are precisely the practical value of using PVSyst.

Approaches for Translating Simulation Results into Improvements

After understanding the loss diagram, you need a perspective geared toward improvement. Rather than stopping at merely looking at the loss diagram, clarify which items can be changed and which should be accepted as site conditions. By identifying losses with room for improvement, you can refine the design while balancing power generation, cost, constructability, and maintainability.

If shading losses are large, first check the accuracy of the shading input. Verify whether the heights and positions of obstacles are correct, whether the 3D model is over- or under-represented, and whether the array spacing and height match reality. Based on that, consider layout changes, adjusting racking height, increasing clearance from obstacles, or changing module placement. However, if there are constraints on land area or roof shape, reducing shading losses may require reducing system capacity, which can lower total energy generation.

If temperature losses are large, check the installation method and ventilation conditions. Arrangements placed too close to the roof surface tend to have higher temperatures. Even for ground-mounted installations, the rack height and surrounding airflow affect temperatures. The module's temperature coefficient should also be taken into account. However, changing to a high-cost design solely to reduce temperature losses may not be justified by the payback on the investment.

If wiring losses are large, review the cable length, cross-sectional area, voltage, current, and the placement of junction boxes and PCS. Wiring losses are relatively easy to improve through design, but increasing cable size affects material costs and constructability. Therefore, check the balance between the increase in power generation from loss reduction and the increase in construction costs.

When mismatch losses are large, check the string configuration, groupings of shaded modules, and how surfaces with different orientations and tilts are handled. Verify whether modules with different orientations have been combined into the same input and whether areas subject to partial shading are excessively mixed. On roof-mounted installations, designs often span multiple surfaces, so designing to minimize mismatch is important.

If PCS-related losses are large, we look at PCS capacity, the DC-to-AC ratio, output limit conditions, and load patterns. In sell-to-grid systems, even if some clipping occurs, the overall profitability of the facility can increase. In self-consumption systems, the optimal PCS capacity varies depending on matching with daytime load and how the battery is used. Decisions should be based not only on the figures in the loss diagram but also on the business conditions.

When considering countermeasures, it is more practical not to aim to minimize every loss. In photovoltaic system design, maximizing power generation and optimizing costs do not necessarily align. If you widen the racking spacing to reduce shading, the installed capacity may decrease. If you use thicker cables to reduce wiring losses, construction costs will increase. If you increase PCS capacity to reduce PCS clipping, equipment costs may increase.

Therefore, a loss diagram should be used not as a "table for eliminating losses" but as a "table for prioritizing design decisions." By considering in order which losses are largest, which losses can be improved, how much those improvements will cost, and whether improving them will have a business impact, you can achieve realistic design improvements.

Summary

To understand the loss diagram in the PVSyst manual, it is important to read the loss diagram not as a mere list of numbers but as the flow of energy from solar irradiance to final output. First check the input-side solar conditions, then look at optical losses such as shading, IAM, and soiling. After that, review array-side losses such as temperature, low irradiance, mismatch, wiring, and degradation, and finally examine PCS and AC-side losses.

What is particularly important is not to simply add the loss rates. In PVSyst’s loss diagram, each loss is treated as a percentage of the energy remaining at the previous stage, so summing the displayed percentages does not yield the overall loss. Understanding this mechanism makes it easier to avoid misunderstandings when explaining reports or during internal reviews.

Six ways to read a loss diagram are: follow the flow from top to bottom; do not add the loss rates together; examine the progression from solar irradiance to effective irradiance; inspect array-side losses to identify reasons for reduced power generation; inspect PCS/AC-side losses to see the final output; and interpret the diagram together with PR and monthly results. If you keep these six points in mind, it becomes clear which parts of the loss diagram to look at.

In practice, it is important not to stop at simply looking at the loss diagram but to use it to drive design improvements. If shading losses are large, check the layout and 3D model; if temperature losses are large, check the mounting method and ventilation conditions. If wiring losses are large, review cable design and equipment placement; if PCS losses are large, check the DC-to-AC ratio and output limiting conditions.

However, minimizing all losses is not necessarily optimal. You need to comprehensively consider power generation, cost, constructability, maintainability, feed-in tariff conditions, self-consumption rate, and other factors to decide which losses to accept and which to improve. A loss diagram is an important document for explaining that decision with numbers and structure rather than by intuition.

Understanding the loss diagram in the PVSyst manual makes it easier to explain why energy production is low and to compare design proposals. Moreover, you can clearly convey to customers and internal stakeholders why the energy production turns out as it does, which losses are the main contributors, and where there is room for improvement. Correctly reading the loss diagram is a fundamental skill for using PVSyst effectively in practice and an important first step toward improving the design quality of photovoltaic power generation systems.