4 Ways to Interpret IAM Loss in PVSyst | Explaining Incidence Angle Loss

What does the IAM Loss item in PVSyst show?

When reading a PVSyst report, if you only look at the energy yield and PR, you may not be able to correctly explain why those numbers occurred. In particular, when evaluating the energy production of a solar power plant, many losses accumulate, such as solar irradiance, temperature, shading, wiring, PCS, transformers, and so on. Among these, IAM Loss is the item that represents the optical loss that occurs when sunlight strikes the module surface at an oblique angle.

IAM stands for Incident Angle Modifier, and in Japanese it is expressed as 入射角補正 (incident angle correction), 入射角修正係数 (incident angle correction coefficient), 入射角特性 (incident angle characteristic). PVSyst reflects that the amount of solar irradiance actually captured by a module changes depending on whether sunlight strikes the module surface at an angle close to perpendicular or at an oblique angle, such as in the morning, evening, or winter. In other words, even with the same horizontal-plane or tilted-plane irradiance, if reflection at the module surface increases, the light reaching the cells decreases, and the generated power falls accordingly. This reduction is accounted for as IAM Loss.

What’s important when interpreting PVSyst’s IAM Loss is not simply looking at what percentage loss it represents. You need to check which screen or loss chart it is shown on, which component(s) of irradiance it applies to, how it relates to the module’s glass characteristics and installation angle, and whether it is being double-counted with other loss items. IAM Loss refers not to a deficiency of irradiance itself, but to the portion of the light that reached the module surface that could not be effectively used because of the effect of the angle of incidence.

For example, at a south-facing fixed-tilt PV plant, around noon sunlight strikes the module surface relatively efficiently. However, in the morning and evening the solar altitude is low and sunlight hits the module surface at an oblique angle. In these cases the proportion reflected at the glass surface increases and less light reaches the cells. Also, in winter the solar altitude is low, so even during the day the incidence angle has a greater effect. The cumulative result of these seasonal and time-of-day influences over the year is reflected in PVSyst's IAM Loss.

IAM Loss is not a metric that indicates the quality of the irradiance data itself. Separate from whether the annual irradiance in the meteorological data is large or small, it is a parameter that corrects for how much light the module surface can capture. Therefore, a large IAM Loss does not necessarily mean that the meteorological data are poor. Rather, it is more accurate to understand it as reflecting the optical capture efficiency, including mounting tilt, azimuth, module surface properties, the treatment of diffuse irradiance, and the handling of ground reflection.

In PVSyst, IAM Loss mainly appears at the stage from tilted irradiance to effective plane-of-array irradiance. On the Loss Diagram, like Near Shadings and Soiling, it is often seen as an optical loss occurring before the light enters the module. What should be noted here is that IAM Loss is not an electrical loss. Its nature is different from wiring losses, mismatch losses, temperature losses, and PCS losses. It is not a loss of current or voltage, but a reduction in the amount of light available for power generation.

When explaining a PVSyst report in practice, describing IAM Loss as "the effect where light entering at oblique angles is reflected off the glass surface, slightly reducing the irradiance available for power generation" makes it easier to understand. For technical audiences, you can explain it as an optical loss due to the incidence-angle characteristics of the module surface. For owners or financial institutions, it is better to describe it as a natural loss that occurs from oblique irradiation in the mornings, evenings, and winter, and as an item that PVSyst corrects for according to the design conditions.

IAM Loss is not usually the largest loss that largely determines total energy production. However, when comparing differences in PR or reconciling other companies’ reports with your own analysis, it is an item that cannot be ignored. In particular, IAM Loss values change when module type, glass specifications, racking angle, tracking method, bifacial settings, ground reflectance conditions, and so on differ. Therefore, when comparing multiple PVSyst reports, it is important to check not only the IAM Loss figures but also whether the underlying assumptions are the same.

Reading 1: Read IAM Loss as reflection loss caused by the angle of incidence

When interpreting PVSyst's IAM Loss, the first point to grasp is that it represents the loss caused by sunlight striking the module surface at an oblique angle and being reflected. Solar modules capture light most efficiently when sunlight strikes them head-on. Conversely, when sunlight hits the module surface at a shallow angle, reflection at the glass surface increases and the light reaching the cells decreases. PVSyst applies a correction for this difference and aggregates it as an annual loss, which is the IAM Loss.

In solar PV design, it’s common to assume that more irradiance means more power generation. However, PVSyst considers not only the simple irradiance amount but also the angle at which that irradiance strikes the module. For example, even with the same irradiance, the effective irradiance available for power generation changes depending on whether sunlight hits the module surface nearly perpendicularly or comes in from the side at a low solar altitude. This difference is the background behind IAM Loss.

IAM Loss is a concept that is primarily intuitive to understand with respect to direct solar radiation. Direct solar radiation is light that reaches the module directly from the sun’s direction, and the relationship between the sun’s position and the module surface angle is clear. When the sun is close to normal to the module surface, reflection losses are small. When the sun is at a shallow angle relative to the module surface, reflection losses are large. This is why IAM Loss tends to be more pronounced in the mornings and evenings and during winter.

However, PVSyst's IAM Loss does not simply consider direct irradiance alone. Diffuse irradiance and ground-reflected irradiance are also subject to incidence-angle-dependent corrections in the model. Diffuse irradiance is light arriving from the entire sky and does not come from a single direction like direct irradiance. Therefore, the treatment of IAM for diffuse irradiance should be understood as an average correction. Ground-reflected irradiance, being light reflected from the ground that reaches the module, is also influenced by reflection effects at the surface.

In practice, IAM Loss is easier to understand if you read it as "the factor by which the efficiency of capturing solar irradiance changes depending on the solar plant's orientation and tilt angle." At plants with low racking angles, the panels tend to match the high summer sun relatively well, while the incidence conditions for the low winter sun change. Conversely, at plants with high racking angles, they may capture winter irradiance more effectively, but the distribution of incidence angles varies with season and time of day. These differences in design conditions affect the annual average IAM Loss.

IAM Loss is also related to performance of the module surface glass and coatings. Modules with anti-reflective coatings can more effectively suppress reflections at oblique incidence, which may reduce IAM Loss. Conversely, settings that use typical glass characteristics apply a standard incidence-angle correction. The reported loss values will vary depending on which IAM model is selected in PVSyst and what optical properties are included in the module database.

One thing to note here is not to misconstrue IAM Loss as "installation defects" or "module degradation." IAM Loss does not mean that the module is broken, poorly positioned, or improperly wired. It is a simulation parameter that accounts for the unavoidable physical reflection when sunlight enters the glass surface. Of course, the value changes depending on the design angle and orientation, but the existence of IAM Loss itself is natural.

When reading a PVSyst report, check at which stage the IAM Loss is shown. If it appears as an irradiance-related loss in the Loss Diagram, it is a loss that occurs relatively upstream in the energy production calculation. In other words, it means that the effective light received by the module is slightly reduced before electrical conversion. Understanding this placement allows you to explain it without confusing it with later-stage losses such as temperature loss or wiring loss.

Also, IAM Loss can differ when comparing plants. For example, even if the region, annual irradiance, and module capacity are the same, differences in mounting tilt angle or azimuth will change the distribution of incidence angles. As a result, the IAM Loss value may also vary slightly. When comparing PVSyst results, it is important not to judge performance based solely on IAM Loss, but to read it together with the overall plant design conditions.

When explaining to a client, you can say, "IAM Loss is the loss accounted for when sunlight strikes the module at an angle and is reflected off the module surface. It tends to occur at dawn and dusk and in winter, and PVSyst applies a correction to make the annual energy production estimate more realistic." This allows you to convey the meaning of the loss without overusing technical terms.

Reading 2: In the Loss Diagram, read it as losses on the solar radiation side

PVSyst's IAM Loss is easier to understand when viewed in the Loss Diagram. The Loss Diagram is a chart that shows how much the solar irradiance entering the plant is reduced at each stage until it becomes the final power at the grid connection point. In this diagram, IAM Loss is shown as a loss at the stage where irradiance enters the module, not as a loss after electrical conversion.

This categorization is extremely important. PVSyst's loss categories include irradiance-side losses, module-side losses, DC-side losses, PCS-side losses, AC-side losses, and transformer and grid-side losses. IAM Loss is, among these, an irradiance-side loss—that is, a loss related to the effective use of the light incident on the array surface. Therefore, it is not something that is lost after it has been generated as electricity, like wiring losses or PCS losses.

On the Loss Diagram, IAM Loss is reflected in the flow of Global horizontal irradiation, Global incident in collector plane, and Effective irradiation on collectors. The exact display names and order may vary depending on settings and report format, but conceptually it is handled at the stage where the irradiation incident on the tilted surface is converted into effective irradiation that accounts for reflection, soiling, shading, and so on. By subtracting IAM Loss here, the irradiation amount used for subsequent module energy production calculations is determined.

Therefore, rather than reading the percent for IAM Loss as a simple share of the final energy generation, it is important to be aware of which reference quantity the loss refers to. In PVSyst’s Loss Diagram, each loss is sometimes displayed as a ratio relative to the energy amount at the preceding stage. For example, even if the IAM Loss is a few percent, it may be shown as a loss relative to the irradiance at that stage. When explaining the impact on the final annual energy yield, it should be viewed as the cumulative result together with the other losses.

IAM Loss is an item often compared near Near Shadings and Soiling Loss. Near Shadings represents the solar irradiance that does not reach the module surface due to surrounding objects or inter-row shading, and Soiling Loss represents the effect of reduced transmitted light caused by dirt. By contrast, IAM Loss is the effect where light that has reached the module surface is reflected away because of the angle of incidence. All are optical losses occurring before power generation, but their causes are different.

In practical work, when reading a loss diagram, it is more effective to first confirm the entire flow related to solar irradiance rather than looking at IAM Loss alone. Irradiance on the horizontal plane is converted to irradiance on the tilted plane, then shading, reflections, soiling, and the like are subtracted, and finally this yields the amount of irradiance that the module can effectively use. Following this flow makes it easier to explain where IAM Loss affects the overall power generation.

For example, if IAM Loss is shown relatively large in a report, the first things to check are the azimuth and tilt angle of the mounting structure. If the azimuth deviates significantly from true south, or if the design is east-west oriented, the angular relationship with the sun changes, and the way IAM Loss manifests will also change. Also, in low-tilt installations the seasonal distribution of incidence angles differs from that of a standard south-facing tilted installation, so the effect of IAM also changes.

Next, check the module's IAM settings. PVSyst models the incident-angle characteristics of the module and the glass. Whether a standard glass model is used, an IAM curve based on measured data is used, or characteristics such as anti-reflective coatings are taken into account will change the IAM Loss results. When comparing with other companies' reports, even for the same module model, differences in IAM settings can lead to differences in losses.

Furthermore, for bifacial modules attention must be paid not only to the front-side irradiance but also to how irradiance on the rear side is treated. Light entering the rear from ground reflection has a different angular distribution than the usual direct irradiance on the front surface. Therefore, depending on the modeling assumptions for reflected irradiance and rear-side reception, the IAM-related corrections can have a different impact on energy yield. In bifacial projects, these factors need to be considered together with Albedo, ground surface conditions, mounting height, row spacing, and rear-side shading.

In a Loss Diagram, the relationships with the items before and after IAM Loss are also important. For example, in projects with large Near Shading, the direct irradiance reaching the module is reduced by shading in the first place. In this case, looking at IAM Loss alone does not accurately capture the meaning of the overall optical losses. It is necessary to lay them out in the order of shading, IAM, and soiling to see how much each factor reduces the effective irradiance.

On the other hand, a small IAM Loss does not necessarily mean that the design is superior. IAM Loss is a loss related to the angle of incidence, and maximizing annual energy production also depends on factors such as irradiance on the tilted surface, shading, temperature, land-use efficiency, racking cost, and constructability. For example, even if IAM Loss is slightly reduced at a certain tilt angle, the total energy yield can be disadvantaged if the annual irradiance on the tilted surface or land-use efficiency worsens. IAM Loss is one part of design decision-making and is not a standalone optimization metric.

When reviewing PVSyst reports internally, organizing IAM Loss as a "correction on the irradiance side" makes explanations more consistent. Specifically, classify the reasons for reduced energy production into insufficient irradiance, optical losses, module temperature, electrical losses, PCS conversion losses, and grid-side losses, and place IAM Loss under optical losses. With this classification in place, you can clearly answer customers when they ask, "Why did this loss occur?"

How to read 3: Interpret the magnitude of the values based on the mounting angle and module characteristics

When looking at PVSyst's IAM Loss values, it is important not to evaluate them solely by their magnitude. IAM Loss varies depending on the plant's installation conditions and the optical characteristics of the modules, so it is risky to simply judge that smaller values are better and larger values are worse. The first things to check are the mounting angle, azimuth, tracking method, and the module surface IAM model.

On fixed-tilt racking, the angle of the module surface is fixed relative to the sun’s movement. As a result, the angle of incidence changes during the day—morning, around solar noon, and evening—and also varies by season. For south-facing fixed-tilt racking, the angle of incidence is relatively small around solar noon and larger in the mornings and evenings. In winter, because the solar altitude is low, the way IAM Loss manifests depends on the chosen tilt angle. Thus, the distribution of incidence angles throughout the year is reflected in the IAM Loss for fixed racking.

When the mounting tilt is low, it can be relatively advantageous for the high sun in summer, but it may be more susceptible to the effects of oblique incidence for the low sun in winter and during morning and evening. Conversely, when the mounting tilt is high, it more easily captures winter solar radiation, while the incidence conditions change in summer and during certain time periods. PVSyst calculates these time-dependent relationships between sun position and the module surface and aggregates them as the annual IAM Loss.

Azimuth is also important. Designs that are close to true south tend to receive direct sunlight during the day more efficiently. On the other hand, designs oriented east–west or shifted toward southeast or southwest change the generation characteristics in the morning or afternoon and alter the distribution of incidence angles. In east–west configured plants, the design intent is sometimes to broaden morning and evening generation while suppressing the peak, but the way IAM Loss is interpreted differs from that for standard south-facing fixed-tilt racking.

In the case of tracking mounts, because the module plane follows the sun, the pattern of IAM Loss differs from that of fixed mounts. Theoretically, because sunlight is received more directly, incidence-angle losses for direct irradiance tend to be reduced. However, the overall loss composition changes depending on the tracking range, backtracking, terrain, inter-row shading, and the treatment of diffuse irradiance. For tracking projects, it is necessary to consider not only IAM Loss but also Shading and the increase or decrease of available irradiance.

Module characteristics also strongly affect IAM Loss. The surface of a solar module is covered with glass, and the reflectance changes depending on the angle at which light enters. Glass with an anti-reflective coating, textured glass, and ordinary glass have different transmittance characteristics at oblique incidence. It is important to verify how well the module data and IAM settings in PVSyst match the actual product specifications.

If IAM Loss differs between another company's report and our own analysis, the first thing to suspect is differences in these settings. Even if the site, capacity, and azimuth appear the same, differences in the module's IAM curve, glass settings, anti-reflective characteristics, bifacial settings, and ground reflectance settings will change IAM Loss. Therefore, when creating a comparison table, you should list not only the IAM Loss values but also the differences in the module database and optical settings used, which will make the explanation more persuasive.

If the IAM Loss value is larger than expected, first check the design conditions in order. Verify that the azimuth is not significantly off, that the tilt angle is set to the intended value, that the terrain and surface orientations are entered correctly, that the tracking settings are configured correctly, and that the module data is correct. In particular, input mistakes for tilt angle or azimuth should be checked early, because they affect not only the IAM but also the irradiance on tilted surfaces and shading assessment.

On the other hand, caution is needed when the IAM Loss is smaller than expected. A low reflection loss is not inherently bad, but the settings may be overly favorable. For example, this can occur when an overly favorable IAM curve that does not match the actual module specifications is used, when tracker settings or surface azimuth differ from reality, or when the treatment of diffuse irradiance and ground-reflected irradiance differs from the comparison. Even when the numbers look good, it is important in practice to verify the underlying rationale.

Also, because IAM Loss is displayed as an annual average loss, its monthly or time-of-day impacts can be difficult to see. Even if it appears to be a small loss on an annual basis, it can have a relatively large effect on generation in winter or during morning and evening periods. For projects where the feed-in price varies by time of day or where battery storage is installed and generation is more valuable in the morning and evening, looking at the generation profile by time of day as well as the annual value will give a deeper understanding of the significance of IAM.

When using PVSyst results for design review, rather than minimizing IAM Loss alone, we make a holistic judgment that includes annual energy production, PR, Specific Yield, Capacity Factor, land use, constructability, and maintainability. For example, changing the tilt angle may slightly improve IAM Loss, but if it widens row spacing and reduces installed capacity, it could be unfavorable from a business perspective. Conversely, even if IAM Loss increases slightly, installed capacity and land use efficiency might improve, leading to higher annual electricity sales.

When explaining to customers, saying, "The IAM Loss value varies depending on the racking angle and the module surface characteristics. Rather than judging it as good or bad on its own, it is an item to check whether it is within a natural range for the design conditions and whether it is consistent with other losses," helps avoid discussions that focus excessively on overly detailed numerical values.

Reading 4: Read as factors contributing to PR differences without confusing them with other losses

When reading the IAM Loss in PVSyst in practice, the most important thing is not to confuse it with other losses. In solar power simulation results, many losses are presented in similar percentage terms. As a result, IAM Loss, Soiling Loss, Shading Loss, Mismatch Loss, Module Quality Loss, Ohmic Loss, Inverter Loss, and so on are sometimes treated as the same kind of loss. However, IAM Loss is an optical incidence-angle loss, and its cause is different from electrical losses or equipment degradation.

Soiling Loss is the loss caused by light being blocked by dirt on the module surface. It is influenced by the power plant’s environmental conditions, such as dust and sand, pollen, bird droppings, snow, volcanic ash, and agricultural dust. On the other hand, IAM Loss is the reflection loss that occurs when light strikes the module at an oblique angle, even if the module is clean. In other words, Soiling Loss can be improved by cleaning, whereas IAM Loss is not fundamentally resolved by cleaning. It is a loss that depends on the design angle and module characteristics.

Shading Loss is the loss that occurs when solar irradiance is blocked by surrounding obstacles or inter-row shading. Mountains, trees, buildings, utility poles, racking rows, and terrain can cause it. When shaded, solar irradiance does not reach the module surface in the first place. IAM Loss is the loss where irradiance does reach the surface but is reflected due to the angle of incidence. Understanding this difference prevents confusion when explaining optical losses on a Loss Diagram.

Mismatch Loss is the loss that occurs due to electrical variations among modules or strings. Because the current–voltage characteristics of each module do not match perfectly, the portion by which the string as a whole cannot operate optimally becomes a loss.

IAM Loss is a loss related to the incident light conditions rather than electrical variations. Therefore, while Mismatch Loss can be reduced by module selection and string design, IAM Loss needs to be checked from a different design perspective.

Ohmic Loss is the loss caused by wiring resistance. When current flows through DC cables or AC cables, energy is lost as heat due to the resistance. This is a loss that occurs after electricity has been generated. On the other hand, IAM Loss occurs during the solar irradiance capture stage before power generation. In PVSyst's Loss Diagram, IAM Loss is positioned in an earlier stage and Ohmic Loss in a later stage. By understanding this order, you can correctly organize the causes of losses and the countermeasures.

Inverter Loss refers to the loss that occurs in the PCS when converting DC to AC. It varies depending on conversion efficiency, load factor, input voltage range, output limits, and so on. This is also different in nature from IAM Loss. IAM Loss is an optical loss that occurs before entering the PCS, even before DC power is generated. Therefore, you should not describe IAM Loss as a PCS efficiency issue.

When analyzing PR differences, IAM Loss can appear as a small differential factor. For example, if PR differs between a third-party report and our in-house PVSyst analysis, IAM Loss should be compared alongside temperature loss, wiring loss, PCS loss, transformer loss, auxiliary losses, Soiling, Albedo, and Shading. Even if the IAM Loss difference is small, multiple loss differences can accumulate to explain the final PR difference.

When comparing IAM Loss, it is important to confirm that the values are being viewed under the same criteria. If the PVSyst version, the report display items, module data, IAM model, or design conditions differ, a simple comparison becomes difficult. When creating a comparison table, include, in addition to the IAM Loss figures, the azimuth, tilt angle, module model, IAM settings, and whether bifacial settings are enabled so that the reasons for any differences can be more easily explained.

Also, IAM Loss can sometimes appear to be related to differences in meteorological data. In practice, changes in the temporal distribution of irradiance data and the direct-to-diffuse ratio alter how incident-angle corrections are applied, so IAM Loss may vary slightly even under identical design conditions. For example, datasets with a high proportion of direct irradiance versus those with a high proportion of diffuse irradiance will show different apparent effects of the incident-angle correction. Therefore, when comparing results that use different meteorological data, it is important to check not only IAM Loss but also how GlobHor, DiffHor, GlobInc, and Effective irradiation behave.

When explaining IAM Loss, I am sometimes asked, "Can this loss be reduced to zero?" In practice, it is usually not possible to make it completely zero. The sun is not in the same position all day, and its altitude changes with the seasons. As long as the mounting is fixed, oblique incidence will occur depending on the time of day and season. It may be possible to reduce it by using tracking mounts or modules with high anti-reflective performance, but it is necessary to consider the balance with project economics and equipment costs.

When aiming to improve IAM Loss, possible approaches include reviewing racking tilt and azimuth, adopting tracking systems, selecting modules with superior anti-reflective properties, and accurately setting module data. However, all of these affect the overall plant design. Changing the racking tilt solely to reduce IAM Loss can alter inter-row shading, installed capacity, wind loads, and construction costs. Therefore, any improvement study must always be evaluated based on annual energy yield and project economics.

In PR explanations, treating IAM Loss as a "minor correction item" fails to convey its importance to customers. Conversely, overemphasizing it can lead to the mistaken impression that it is the primary cause of differences in power generation. IAM Loss is an item that indicates how effectively solar irradiance can be captured, and it is one component of PR together with other losses. Even if it is less conspicuous than temperature loss or PCS loss, it should be properly checked when reading PVSyst.

For internal reviews, it is efficient to decide in advance when to check IAM Loss. First, confirm the azimuth and tilt angles in the design conditions. Next, check the Loss Diagram to see whether the IAM Loss is in a natural position and has a reasonable value. After that, compare with other companies' reports and past projects, and if there is an obvious difference, verify the module data and IAM settings. Reviewing in this order makes it easier to judge the meaning of the numbers in a short time.

PVSyst assumptions to confirm when reading IAM Loss

When reading IAM Loss in PVSyst, it is essential to check the assumptions before looking at the numbers. The first thing to check is the power plant's Orientation settings. This is where the module surface tilt and azimuth angles are defined. Because IAM Loss is related to the angle of incidence, if this setting is incorrect the interpretation of the loss will change. The first step is to verify that the azimuth on the drawings, the on-site layout, and the PVSyst input values are consistent.

Next, review the module data settings. PVSyst’s module database may include settings related not only to electrical performance but also to optical corrections. Check whether the IAM model is a standard one or manufacturer-derived data, and whether the characteristics of anti-reflective glass have been taken into account. Even modules with the same rated capacity can exhibit different IAM losses if surface texture or glass specifications differ.

If you have a bifacial configuration, you also need to check the conditions for rear-side irradiance. In bifacial power generation, light reaches the rear side from ground-reflected and surrounding reflections. Ground reflectance, mounting structure height, row spacing, the type of ground surface, and rear-side shading all affect power generation. Even when examining IAM Loss itself, you need to be aware of how the optical treatment of the front and rear surfaces is reflected.

Also check the Near Shading settings. If shading is present, the time distribution of direct solar irradiance changes. Because irradiance blocked by shading is reduced before it can be treated as subject to the IAM, it is necessary to separate the effects of Shading and IAM. In particular, for projects with mountain shadows, slopes on developed land, surrounding trees, buildings, or inter-row shading, looking at IAM Loss alone does not reveal the full picture of optical losses.

The setting of Soiling Loss is also important. Soiling loss is a loss separate from IAM, but both are treated as optical losses occurring before and after solar radiation enters the module. In regions with heavy snowfall or frequent dust, Soiling and Snow Loss can have a large impact on energy generation. In such projects, assumptions about soiling and snow can sometimes affect PR differences more than explanations of IAM Loss.

Checking the direct-to-diffuse ratio in the meteorological data also deepens your understanding of IAM Loss. In PVSyst, tilted-plane irradiance is calculated from meteorological data such as horizontal-plane irradiance, diffuse irradiance, and temperature, and various losses are then applied. In regions with a high proportion of direct irradiance, the effects of sun position and incidence angle become more apparent. In regions with a high proportion of diffuse irradiance, the way light arriving from the whole sky is treated becomes relatively more important. This difference also affects how IAM Loss should be interpreted.

When comparing reports, also check the PVSyst version and the configuration templates. PVSyst has many configuration items, and even reports that look similar can yield different results due to subtle differences in models and data. When discussing differences in IAM Loss, it is important not merely to list numbers but to document and clarify the meteorological data used, the modules, the azimuth and tilt, shading settings, Soiling, and the IAM model.

Possible causes of high IAM Loss

If you find the IAM Loss in PVSyst larger than expected, the first thing to suspect is inconsistencies in the input conditions. If the tilt angle or azimuth differs from the actual design, the distribution of incidence angles will change and the IAM Loss will vary accordingly. Even if the drawings intend the array to face south, the azimuth may have been entered with an offset in PVSyst. Misunderstandings of the sign convention for angles or the reference for azimuth can also cause a surface to face an unintended direction.

Next, the module's IAM model may differ from what you expect. Whether a standard glass model is used or antireflective glass properties are applied will change the losses at oblique incidence. Even if the manufacturer's documentation indicates the use of low-reflectance glass, that characteristic may not be reflected in the module data on PVSyst. Conversely, the configuration may be set to an overly favorable level.

Even in projects with atypical racking angles, IAM Loss tends to differ from that of standard projects. Low-tilt installations, east–west layouts, profiled metal roofs, carports, vertical installations, agrivoltaic systems, and sloped installations change the incidence angle conditions compared with standard ground-mounted, south-facing racking. For such projects, it is necessary to judge whether the IAM Loss is reasonable for that specific design form, rather than comparing it to a general expectation of losses.

In projects where peripheral shading and inter-row shading are significant, attention should be paid to how IAM Loss appears. During periods with heavy shading, direct solar irradiance is blocked, making the relationship with reflection loss due to IAM complex. If shading losses are large, rather than evaluating the design by looking only at the IAM Loss value, it is necessary to review the flow of Shading Loss and Effective irradiation together.

When meteorological data differ, differences in IAM Loss may occur. Not only the total solar irradiance, but also the ratio of direct to diffuse irradiance, the time-of-day distribution of irradiance, and seasonal irradiance patterns change how the incidence-angle correction works. Therefore, when comparing multiple meteorological datasets, you need to distinguish whether differences in IAM Loss are due to the meteorological data or to design/configuration settings, not just by annual energy yield or PR.

Things to Watch Out For When IAM Loss Is Low

When IAM Loss is small, many people consider it a good result. Indeed, a small reflection loss due to the angle of incidence is advantageous in terms of solar irradiance capture efficiency. However, in practice it is necessary to verify even when the numerical value is small. This is because the settings may be more favorable than the actual situation.

For example, if the module's IAM model assumes glass properties that are more performant than they are in reality, the IAM Loss may be shown as smaller than it should be. Also, if the azimuth or tilt differs from the actual design so that the incident conditions are calculated more favorably by chance, the loss can appear reduced. If tracking is mistakenly enabled in the settings, or if a fixed system is entered with conditions close to tracking, such input errors can make the losses appear unnaturally small.

If the IAM Loss is smaller than in another company's report, do not simply conclude that your analysis is superior; instead check for differences in the settings. Verify whether module data, the IAM model, meteorological data, azimuth and tilt, and the treatment of diffuse irradiance are consistent. In particular, when comparing PR or assessing energy production for financial institutions, using overly favorable assumptions can be difficult to justify later.

Even if IAM Loss is small, the overall energy production is not necessarily large. IAM Loss is, after all, a loss due to the angle of incidence. If the irradiance on the tilted surface itself is low, temperature losses are large, PCS output limits are significant, wiring losses are high, or transformer losses are large, the final energy production will not increase even if IAM Loss is small. Therefore, IAM Loss needs to be read together with other items.

Alternative Phrasing When Explaining to Clients or Internal Teams

The IAM Loss in PVSyst is a concept that's hard to convey if explained using technical jargon. When explaining to a project owner or non-technical internal members, it is easier to understand if you rephrase it as: "When sunlight hits the module at an angle, more of it is reflected off the surface, so this loss accounts for that reflected portion."

If you want to explain it more briefly, you can say, "It is the reflection loss that occurs when sunlight enters at an oblique angle, such as in the mornings and evenings or during winter." This explanation will be understood even by someone who does not know the English acronym IAM. If you want to explain in more detail, it is helpful to add, "PVSyst corrects for the portion of the solar irradiance that reaches the module surface but is reflected by the glass and does not reach the cells."

For internal reviews, it's helpful to state "IAM Loss is an optical loss on the irradiance side and is separate from wiring or PCS losses," as this makes discussions easier. If you classify loss items by cause and explain them as IAM = optical, Mismatch = electrical variability, Ohmic = wiring resistance, and Inverter = conversion efficiency, the interpretation of the entire PVSyst report becomes more consistent.

When explaining PR differences, you can say that "differences in IAM loss may arise from azimuth and tilt, module surface characteristics, and the ratio of direct to diffuse irradiance in the meteorological data." This allows them to be explained logically as differences in input conditions or models, rather than merely calculation errors.

Workflow for verifying IAM Loss in practice

When reviewing a PVSyst report in practice, start by checking the plant's design conditions. Examine the tilt angle, azimuth, racking type, whether tracking is present, and the module type. Next, confirm the position and values of the IAM Loss in the Loss Diagram. Then compare it with Shading, Soiling, Albedo, temperature loss, wiring loss, and PCS loss, and organize the impacts on PR and annual energy production.

When comparing with another company's report, rather than extracting only the IAM Loss, lay out the entire flow of solar-radiation-related items. By checking, in order, horizontal-plane irradiance, tilted-plane irradiance, shading loss, IAM Loss, Soiling Loss, and effective irradiance, it becomes easier to trace the causes of differences. If there is a discrepancy in IAM Loss, check the azimuth and tilt, module IAM settings, and the direct-to-diffuse ratio in the meteorological data.

When creating explanatory materials for a project, it is more natural to organize IAM Loss as one item among solar radiation-related losses rather than allocating a large standalone page to it. For example, under "Adjustment from irradiance to effective irradiance", explain shading, soiling, and incidence-angle reflection together. Within that grouping, position IAM Loss as the reflection loss caused by oblique incidence.

When considering design improvements, it is important not to judge based only on IAM Loss. Measures such as changing the rack tilt angle, switching modules, or adopting trackers affect not only energy generation but also cost, constructability, maintainability, and land-use efficiency. PVSyst simulations are a tool for comparing these conditions, and IAM Loss is one of the factors to consider.

Summary for reading IAM Loss in PVSyst

The IAM Loss in PVSyst is a loss that represents the effect of sunlight entering the module surface at an angle, being reflected at the glass surface, and reducing the light available for power generation. This should be understood not as an electrical loss like wiring or PCS, but as an optical loss occurring before and after the solar radiation is captured by the module.

The first interpretation is to read it as reflection losses caused by the angle of incidence. During periods when sunlight strikes at an oblique angle, such as mornings and evenings or in winter, the amount of reflection at the module surface increases. IAM Loss is PVSyst’s annual aggregation of this effect.

The second way to read it is as a loss on the irradiance side within the Loss Diagram. The IAM Loss is reflected at the stage where horizontal-plane irradiance and tilted-plane irradiance are converted into effective irradiance. Understanding it as an item that comes before temperature loss, wiring loss, and PCS loss makes the overall flow of PVSyst easier to see.

The third way to interpret it is to read the magnitude of the numbers in terms of the racking angle and module characteristics. IAM Loss varies with azimuth, tilt, tracking method, the glass properties of the module surface, and the IAM model. Rather than simply judging that smaller is good and larger is bad, you need to check whether the value is reasonable for the design conditions.

The fourth interpretation is to read it as a factor contributing to PR differences, rather than confusing it with other losses. IAM Loss has different causes from Soiling Loss, Shading Loss, Mismatch Loss, Ohmic Loss, and Inverter Loss. When comparing with other companies' reports or explaining PR differences, it is important to treat IAM Loss as an optical correction term and explain it together with differences in configuration and settings.

If you can correctly read the IAM Loss in PVSyst, it becomes easier to explain differences in energy production. You can more logically explain why the energy production took a particular value, why the PR differs from other reports, and which losses arise from design conditions. Although IAM Loss may seem like a minor item at first glance, it is an important correction factor linking irradiance to energy production when interpreting PVSyst reports in practice.

Also, for on-site inspections of solar power plants and post-construction management, it is important to verify design drawings and simulation conditions against actual site conditions. Accurately identifying the racking orientation, module layout, local terrain, surrounding obstructions, and survey point locations enables more reliable validation of PVSyst’s assumptions. Using a system that records site positions with high precision by leveraging an iPhone and GNSS—such as LRTK—makes it easier to link drawings, positioning, and field checks, and supports design verification and post-construction management of the plant. Reading PVSyst’s numbers not merely as desk calculations but by cross-checking them against on-site conditions leads to power generation assessments that are trusted in practice.