6 Ways to Interpret PVSyst Detailed Losses | In-Depth Explanation

When reading a PVSyst report, the items that first catch your eye are annual energy production, PR, Specific production, and the Loss diagram. However, if you need to go deeper for design reviews, project feasibility assessments, comparisons with third‑party reports, or to determine why production is lower than expected, understanding how to read the Detailed Losses becomes important.

Detailed Losses is not simply a page where loss rates are listed. It is the page used to examine in detail which conditions are reflected and where energy is being lost in the process by which PVSyst calculates annual energy production. If the Loss diagram is a single chart for viewing the overall flow at a glance, Detailed Losses is positioned more like a detailed statement for delving into its breakdown.

Especially in the design and energy-yield analysis of solar power plants, even with the same installed capacity, results can vary greatly depending on solar irradiance conditions, temperature conditions, how shading is handled, module characteristics, wiring losses, PCS losses, auxiliary equipment losses, output limitations, and so on. If you can correctly read the Detailed Losses, you will be able to explain why energy yield is low by breaking it down item by item rather than relying on intuition.

This article organizes six perspectives to focus on when reading PVSyst's Detailed Losses in a practical, easy-to-use format. It explains them so that even those unfamiliar with PVSyst can understand which numbers to look at, which items to compare, and which losses are likely to become design issues.

Table of Contents

• Detailed Losses is the page for verifying the basis of the energy production

• The first looks at losses from solar irradiance to array input

• The second checks temperature losses and module characteristics

• The third reads mismatch and low-irradiance losses

• The fourth separates wiring losses into DC-side and AC-side

• The fifth considers PCS losses and output limiting separately

• The sixth checks auxiliary equipment losses and external conditions

• Points to note when comparing Detailed Losses

• Perspectives for connecting PVSyst interpretation to on-site verification

• Summary

Detailed Losses is the page for verifying the basis of the power output

PVSyst's Detailed Losses is an important page for verifying the validity of simulation results. Looking only at annual energy production or PR, you cannot tell why the results turned out that way. Whether generation is high or low, you need to check the Detailed Losses to trace the underlying reasons.

In PVSyst, calculations proceed from the horizontal-plane irradiance based on meteorological data to the tilted-plane irradiance, the effective irradiance incident on the module surface, the array output, the PCS output, and the output at the grid connection point. In that process, losses such as shading, reflection, soiling, temperature, module quality, mismatch, wiring, PCS conversion, auxiliaries, and output limitation are reflected in sequence.

In Detailed Losses, these losses are shown with greater granularity. Therefore, when reading it you should not simply look at the magnitude of the loss rates, but pay attention to which stage the losses occur. Whether solar irradiance has decreased, losses occur during the stage where modules convert sunlight into electrical power, or losses occur after the PCS, the causes and countermeasures will differ.

For example, if shading losses are large, check the layout, topography, and surrounding obstacles. If temperature losses are large, check the local ambient temperature, racking type, ventilation conditions on the rear of the module, and the setting of the thermal loss coefficient. If wiring losses are large, check the cable length, cross-sectional area, voltage, current, and collection method. In this way, the Detailed Losses page directly connects to items to be checked in the design.

Also, Detailed Losses is extremely useful when comparing reports from other companies. If you only compare PR or annual energy production, it is difficult to judge which analysis is appropriate, but by comparing each item in Detailed Losses you can break down the causes of the differences. For example, if the energy production differs despite the same installed capacity, it becomes easier to distinguish whether the cause is differences in meteorological data, shading, wiring losses, or PCS losses.

The first examines the losses from solar irradiance to the array input

When reading Detailed Losses, the first thing to check is the losses related to solar irradiance. The power output of a photovoltaic system ultimately depends heavily on the solar irradiance. Therefore, before looking at electrical losses, first confirm how much effective irradiance reaches the module surface.

In PVSyst, the irradiance on tilted surfaces is calculated from the global horizontal irradiance and diffuse irradiance contained in the meteorological data. This calculation involves the installation azimuth, tilt angle, ground surface reflectance, near shading, and far shading. In Detailed Losses, you can check to what extent these factors are reflected as losses or gains.

Particularly important are the items related to Near shading, Far shading, IAM, Soiling, and Albedo. Near shading refers to local shading caused by mounting structures shading each other, surrounding structures, terrain, trees, and similar features. It can have a greater impact during periods of low solar altitude and in winter. Far shading is the influence of shadows from distant mountains or the horizon. This is something that is easy to overlook in mountainous areas and on land development sites.

IAM refers to losses related to incident-angle correction. As sunlight strikes a module more obliquely, reflection at the glass surface and similar effects reduce the irradiance that can be effectively captured. If IAM losses are large, you need to check the tilt angle and azimuth, module characteristics, and the contribution of morning and evening irradiance.

Soiling refers to contamination losses. Sand and dust, pollen, yellow sand, bird droppings, snowfall, and dust from agricultural fields or development sites affect it. In PVSyst, soiling losses can be set monthly, so it is important to check whether a uniform annual value is used or whether it is varied by season. In particular, in snowy regions, it is necessary to clarify whether the influence of snowfall is included in Soiling or is reflected by a different approach.

Albedo refers to the effect of ground surface reflectance. It is usually treated as a small positive factor, but in snowy areas or with bifacial modules it can have a noticeable impact on energy production. When reviewing Detailed Losses, check to what extent albedo is acting as a positive factor and whether the configured value matches local conditions.

The important point here is that solar irradiance-related losses are different in nature from downstream electrical losses. Losses in wiring and the PCS are items that reduce the power that has been generated, whereas solar irradiance-related losses are items that change the energy entering the modules in the first place. Therefore, they can have a very large impact as a cause of differences in power generation.

The second is to check temperature losses and module characteristics

Next, we need to check the items concerning temperature losses and module characteristics. Photovoltaic (PV) modules generate more power when solar irradiance is strong, but their output decreases as cell temperature rises. For this reason, temperature losses tend to be larger in hot regions and under installation conditions with poor ventilation.

PVSyst's Detailed Losses shows losses due to temperature. This value depends on meteorological data such as ambient temperature and wind speed, the mounting configuration, the thermal loss coefficient, the module temperature coefficient, and other factors. A large temperature loss does not necessarily mean a poor design, but it is important to verify that the settings match the local conditions.

For example, cell temperature can differ at the same ambient temperature depending on whether a module is ground-mounted with sufficient airflow behind it or installed flush against a roof. Roof-mounted or poorly ventilated conditions tend to result in larger thermal losses, while ground-mounted racks or well-ventilated installation conditions tend to keep them relatively lower.

Module temperature coefficients are also important. Modules with larger temperature coefficients tend to see greater output decreases at high temperatures. Even when using values registered in PVSyst's module database, you need to confirm that they match the specific module model you plan to use and are consistent with the values in the manufacturer's datasheet.

In Detailed Losses, items related to module quality and degradation—such as Module quality loss, LID, and LeTID—are also subject to review. Module quality loss relates to how much a module’s actual output varies from its nominal value, or how positive tolerances are handled. If a manufacturer’s output tolerance is positive-only, depending on the settings it may act as an increase rather than a loss.

LID is sometimes treated as initial light-induced degradation, and LeTID as degradation associated with high temperature and light exposure. Whether these are reflected varies with the module type and the configuration policy. Care is needed for reports submitted to banks or for third-party evaluations, because the treatment differs depending on whether it concerns first-year energy production or a representative year that takes degradation into account.

Temperature losses and module characteristics can have a considerable impact on power generation. In particular, when PR appears low, large temperature losses can be one of the causes. Conversely, if temperature losses are extremely small, you should check whether the thermal loss coefficient or the installation condition settings are overly optimistic.

Third: Read mismatch and low-light loss

When digging deeper into Detailed Losses, mismatch loss and low-irradiance loss are especially important. These may seem like small numbers at first glance, but because they relate to design quality, module selection, string configuration, and the handling of shading, they cannot be overlooked in practical work.

Mismatch loss is the loss that occurs due to variations in electrical characteristics between modules and differences in conditions between strings. In solar power generation, multiple modules are connected in series to form strings. In a series circuit, the current is constrained by the module with the lower current, so if there are output differences between modules, losses occur.

Causes of mismatch include manufacturing variability, differences in module degradation, variations in soiling, partial shading, differences in orientation and tilt, and differences in string length. In PVSyst, a standard mismatch loss is often set, but for projects with complex layouts or extensive partial shading, it is important to carefully verify whether the chosen value is appropriate.

In particular, when arrays with different orientations or tilts are connected to the same MPPT, mismatch and MPPT losses can become significant. If mismatch-related losses are large in PVSyst's Detailed Losses, you should check the string configuration, MPPT assignment, number of PCS units, and how the input circuits are divided.

Low-irradiance loss is the loss caused by a decrease in module efficiency when solar irradiance is weak. Solar photovoltaic modules affect energy production not only through their output under standard test conditions but also through their performance under low irradiance. In regions where periods of weak irradiance—such as mornings and evenings, cloudy weather, and winter—contribute significantly, differences in low-irradiance characteristics are reflected in annual energy yield.

When evaluating low-irradiance losses, it's important not only to look at whether the value is large or small, but also to verify that the module data being used is correct. The representation of low-irradiance characteristics can vary depending on the manufacturer's datasheet or the contents of the PAN file. If you are using an approximate model from PVSyst's database, you should also confirm that it closely matches the actual module type.

Also, care must be taken not to confuse mismatch losses and shading losses. The reduction in irradiance itself caused by shading is treated as an irradiance-side loss, but the effects of partial shading that limit string current or shift the MPPT operating point can appear as electrical losses. In projects with significant shading, it is important to look not only at the Loss diagram but also at the related items in Detailed Losses.

The fourth point is to examine wiring losses separately for the DC side and the AC side.

When reading the Detailed Losses in PVSyst, wiring losses are an item you should always check. Wiring losses are losses where power is dissipated as heat due to the resistance of the cables. In general, they increase with higher current, longer cable length, and smaller cross-sectional area.

When examining wiring losses, it is important to check the DC side and the AC side separately. The DC side relates to the direct-current wiring from the modules to the junction boxes and from the junction boxes to the PCS. The AC side relates to the alternating-current wiring from the PCS to the substation/transformer equipment and to the point of interconnection. Furthermore, in large-scale power plants, losses occur at multiple stages such as the low-voltage side, transformers, the medium-voltage side, and the extra-high-voltage side.

DC wiring losses depend on the string wiring, the wiring up to the combiner box, and the trunk wiring from the combiner box to the PCS. In designs that distribute PCS and place them closer to the modules, DC trunk wiring is shorter and losses are easier to reduce. Conversely, designs that centralize PCS require longer DC cables, which can increase losses.

AC wiring losses vary depending on the distance from the PCS output to the transformer or the point of interconnection, the voltage class, cable size, and the cable routing. In particular, in large-scale projects, AC-side losses can change significantly depending on the power plant’s collector configuration. If AC wiring losses are large in Detailed Losses, verify that the cable lengths and voltage class settings match the actual design.

What you should be aware of regarding wiring losses is that the input may be a uniform percentage value, or it may be calculated from cable length and cross-sectional area. In PVSyst, you can enter detailed cable parameters to calculate losses, but you can also simply specify a loss rate. When comparing reports, differences in these input methods can lead to differences in results.

For example, even if the DC wiring loss is set to the same 1.5 percent, its meaning changes depending on whether it is based on the actual cable configuration or entered as a general assumed value. During reviews, it is important to check not only the loss rate itself but also whether its basis is consistent with the design drawings, single-line wiring diagrams, cable lists, and layout drawings.

Wiring losses are difficult to change significantly after the power plant is completed. Because cable routes, PCS placement, and voltage levels are determined in the early design stages, if large losses are identified at the PVSyst stage, it is worthwhile to provide feedback to the design team early.

The fifth is to consider PCS losses and output limits separately.

In Detailed Losses, losses related to the PCS are also important. However, when examining losses around the PCS, it is necessary to separate losses due to conversion efficiency from those due to output limiting.

PCS losses are the losses that occur when converting DC power into AC power. A PCS has a conversion efficiency, so not all input power becomes AC output. Even high-efficiency PCS units incur a certain amount of conversion loss. Also, because PCS efficiency varies with output load factor, it does not always operate at its rated efficiency.

In PVSyst, the PCS efficiency curves, rated capacity, MPPT range, input voltage range, and so on are reflected in the calculations. When viewing PCS losses in Detailed Losses, check that the PCS model in use is correct, the rated capacity is correct, and the DC/AC ratio is reasonable.

Output limiting is the loss that occurs when part of the power that could be generated is cut due to the PCS's rated output or grid interconnection conditions. It is commonly called clipping loss. If the PCS capacity is designed smaller than the DC capacity, under conditions such as strong irradiance and low temperature the array may try to produce power that exceeds the PCS rating. That excess is lost as output limiting.

The important point here is that PCS conversion losses and output-limiting losses have different causes. PCS conversion losses occur to some extent whenever the PCS is operating. By contrast, output limiting occurs at high output and is influenced by the DC/AC ratio, weather conditions, temperature conditions, PCS capacity, power factor setting, and the limit at the point of interconnection.

If the output curtailment shown in Detailed Losses is large, always check the DC/AC ratio. Designing to increase DC capacity while constraining PCS capacity raises the PCS utilization rate but increases clipping at peak times. It is not necessarily bad from an economic perspective, but the generation report must be able to clearly explain those losses.

Also, power factor settings can be relevant. Depending on whether the PCS capacity is defined in active power or apparent power, the way output limits are handled when the power factor is changed can differ. It is important to verify that the settings in PVSyst, the actual PCS specifications, and the output limit conditions in the grid-connection contract are consistent.

Care should be taken when output curtailment or grid-side constraints are being considered separately. You need to distinguish whether the interconnection point’s output limit has been entered in PVSyst, whether the restriction is only due to the PCS rating, or whether output control for plant operation is being calculated separately. When reading Detailed Losses, it is important not to summarize everything with the single term “PCS losses,” but to check conversion losses, rating limits, interconnection point limits, and power factor conditions separately.

The sixth is to check auxiliary equipment losses and external conditions

Lastly, what you should confirm are the items related to auxiliary equipment losses and external conditions. In a solar power plant, in addition to modules and the PCS, devices such as monitoring equipment, communication equipment, air conditioning, tracker drive units, PCS auxiliary equipment, and auxiliary power supplies for substation/transformer equipment may consume power. Because these are deducted from the generated output, they need to be checked in Detailed Losses.

Auxiliary losses vary widely depending on the project. In small-scale power plants the impact may be minor, but in large-scale plants and high-voltage or extra-high-voltage projects they can be significant. In particular, when the standby power of PCS and substation equipment, monitoring devices, meteorological instruments, security systems, and communication equipment continuously consumes power, this becomes a measurable annual loss.

In PVSyst, you can set auxiliary losses as Auxiliary loss. Whether this value is constant throughout the year, applies only during daytime, or varies with output will affect energy production. If auxiliary losses are displayed in Detailed Losses, it is important to verify the basis for the setting.

Transformer losses are also important. Transformers have no-load losses, which occur regardless of load, and load losses, which occur according to the load current. In large-scale power plants, the settings for transformer losses can affect PR and annual transmitted energy. If transformer losses are included in Detailed Losses, check whether they are consistent with the values in the transformer datasheet and whether they are being evaluated on the low-voltage side or the medium-voltage side.

As an external condition, the definition of the grid interconnection point is also important. Depending on whether PVSyst's final energy output is at the PCS output terminal, after the transformer, at the point of delivery, or equivalent to the revenue (sales) meter, the measured value you should compare will differ. If you do not check how far losses are included in the Detailed Losses, misunderstandings will arise when comparing with measured values.

For example, if PVSyst's results are based on PCS output but the actual values are taken from the feed-in meter, transformer losses and on-site consumption will appear as differences. Conversely, if PVSyst includes auxiliary losses and transformer losses but the comparison target is the PCS output value, PVSyst may appear lower.

When reading Detailed Losses, always confirm the reference point for the final output. For a business viability assessment of the entire plant, the amount of electricity that can be sold is important, but when verifying equipment performance you may look at PCS output or array output. Depending on the purpose, it is necessary to make clear which stage of energy is being compared.

Points to Note When Comparing Detailed Losses

PVSyst's Detailed Losses is useful not only to read on its own but also for comparing multiple cases. It helps when comparing design proposals, module changes, PCS capacity changes, racking tilt angle changes, wiring route changes, and verifying the effectiveness of shading countermeasures when comparing multiple simulation results.

When comparing, first confirm that the baseline conditions are the same. If weather data, system capacity, module model, PCS model, tilt angle, azimuth, ground surface reflectance, soiling losses, degradation conditions, or output limitation conditions differ, you cannot simply interpret differences in Detailed Losses. It is important to keep all conditions other than the items you want to compare as consistent as possible.

Be especially careful when comparing PR alone. PR is a convenient indicator of a plant’s efficiency, but its interpretation can vary depending on irradiance conditions, temperature conditions, the definition of losses, and the reference points used for evaluation. Using Detailed Losses in conjunction makes it easier to explain the breakdown of PR differences.

For example, if there is a difference in annual power generation between Plan A and Plan B, you can break down whether that difference is due to differences in the amount of solar irradiation captured because of the tilt angle, differences in shading losses, differences in temperature losses, or differences in PCS clipping. Being able to make this decomposition makes it easier to explain the effects of design changes.

Also, when comparing reports from other companies, even if the item names are the same their definitions may not fully match. In one report, soiling losses may include the effects of snow, while another report may treat snow effects separately. Wiring losses may be shown as DC only in some cases, while in others they may include AC and transformers. When creating a comparison table, you need to check not only the names but also what is included.

When comparing Detailed Losses, it is most efficient to check items in order of largest differences. Rather than scrutinizing items that have a small impact on differences in generation, first review items that tend to have a large impact, such as solar irradiance, shading, temperature, PCS limits, wiring, and auxiliary equipment. Then dig into the details as necessary.

Perspectives for Linking PVSyst Interpretation to On-site Verification

The purpose of reading Detailed Losses is not merely to understand the numbers in the report. It is important to translate them into actual design, construction, operation, and on-site verification. If PVSyst shows large shading losses, you should check the site’s topography, obstructions, and racking spacing. If wiring losses are large, there may be scope to review cable routing and PCS placement. If temperature losses are large, it is worth checking the racking structure and ventilation conditions.

In recent years, methods for on-site verification have been changing. Rather than relying solely on paper drawings or photographs, combining drone surveying, point-cloud data, smartphone-based location checks, and AR displays makes it possible to verify differences between designs and the actual site more accurately.

For example, by leveraging GNSS positioning and AR visualization on an iPhone—such as with LRTK—you can more easily verify drawing locations and equipment positions on site while cross-checking the assumptions in PVSyst against actual construction conditions. If you can confirm on site the racking/mounting positions, surrounding obstructions, site grading, cable routes, and the locations of items to be inspected, the field team can more easily validate any items of concern identified in Detailed Losses.

PVSyst is a desk-based analysis tool, but to use its results correctly, linking them to on-site conditions is indispensable. By understanding the breakdown of losses in Detailed Losses and verifying those assumptions on site, you can increase the reliability of energy yield analysis.

Summary

PVSyst's "Detailed Losses" page is an important resource for digging deeper into the basis for energy yield. Because it lets you review the breakdown of losses that cannot be determined from annual energy yield or PR alone, it is useful for design reviews, comparing other companies' reports, project feasibility assessments, and analyzing causes of reduced energy output.

The basic approach to interpretation is to keep the flow from solar irradiance to final output in mind. First, check how much solar irradiance is reaching the module surface. Next, examine temperature losses and module characteristics. Then, in order, check mismatch, low‑irradiance losses, wiring losses, PCS losses, output limitations, auxiliary losses, and transformer losses.

When reading the Detailed Losses, it is important not just to look at the magnitude of the loss rates, but to check at which stage the losses occur, what design conditions they originate from, and whether they match the site conditions. In particular, shading, temperature, wiring, PCS limits, and auxiliary equipment losses tend to affect energy generation and are common practical checkpoints.

When comparing multiple reports, you should not judge them by item names alone; you need to read them with the configuration settings and evaluation reference points aligned. Even with the same PVSyst report, if the meteorological data, loss settings, PCS capacity, output limits, or the handling of auxiliary equipment losses differ, the results will change. By using Detailed Losses, you can break down and explain those differences one by one.

Once you can correctly read PVSyst's Detailed Losses, energy-yield analysis ceases to be just a numbers check and instead becomes a decision-making tool that links design and the field. To organize the reasons for high or low generation, the differences versus other companies' results, and the items that can be improved, the Detailed Losses page is one you should always review.