How to Set Temperature Losses in PVSyst｜5 Commonly Overlooked Items

Table of Contents

• Why temperature losses matter in PVSyst

• Basic concepts of temperature losses

• Information to check before setting temperature losses in PVSyst

• 5 easily overlooked items in temperature loss settings

• Item 1: Check the module temperature coefficient

• Item 2: Accurately reflect mounting type and ventilation conditions

• Item 3: Consider installation height and roof surface heat buildup

• Item 4: Verify ambient air temperature in meteorological data and local conditions

• Item 5: Align temperature conditions when comparing design proposals

• Practical steps to set temperature losses

• How to check temperature losses in simulation results

• Improvement points when temperature losses are large

• Points to note when explaining temperature losses in submission documents

• Linking PVSyst temperature loss settings to on-site verification

• Summary

Why Temperature Losses Are Important in PVSyst

When calculating energy production in PVSyst, temperature losses are an important loss term that directly affects the actual output of solar panels. Solar panels generate electricity from solar irradiance, but the module temperature also rises at the same time. In general, as module temperature increases, the output voltage decreases, and as a result the power output falls. In other words, although energy production tends to increase during periods of strong irradiance, those periods are also more prone to output reductions due to temperature rise.

In practice, when looking only at the size of annual power generation, the importance of temperature losses can be hard to see. Compared with differences in irradiance, the presence or absence of shading, PCS capacity, oversizing ratio, azimuth, and tilt angle, temperature loss tends to be treated as an item buried deep in the settings screen. However, even with the same module capacity, the same azimuth, and the same irradiance conditions, temperature losses vary depending on whether the installation is rooftop or ground-mounted, whether there is rear ventilation, and whether heat is trapped around the modules.

When evaluating power generation, especially during periods of high temperatures, setting temperature-loss parameters is important. Although summer gives the impression of higher generation due to increased solar irradiance, output can also drop as module temperatures rise. Looking at monthly generation, even in months with high irradiance the generation may not reach expected levels, and temperature losses can be one of the contributing factors.

Also, in PVSyst reports temperature-related losses are reflected in the loss diagram and key indicators. In internal briefings and explanations for clients, questions may arise such as “Why is the generation for this design proposal at this level?”, “Why doesn’t it increase as much as expected in summer?”, and “How much difference would changing the racking conditions make?”. If you cannot explain the rationale behind the temperature-loss settings at that time, the overall credibility of the simulation will be weakened.

Temperature loss is not merely a correction value. It is a parameter that indicates how closely the installation conditions are approximated to reality. If you want to learn how to use PVSyst in practice, you should not leave the temperature loss at its default value; you need to verify that it matches the equipment and on-site conditions.

Basic Concept of Temperature Loss

To understand temperature losses, the first point to note is that a solar panel’s output is not determined solely by standard conditions. The output listed in catalogs is a value evaluated under fixed test conditions, but in real outdoor settings the module temperature changes depending on ambient air temperature, wind speed, solar irradiance, installation angle, racking type, roofing material, and the surrounding environment. As a result, the output actually obtained from the same panel in the field can vary.

In PVSyst, the approach of estimating module temperature is used to calculate temperature-related losses. Module temperature is not determined solely by ambient temperature and solar irradiance. Of the solar radiation incident on the module, the portion that is not converted into electricity remains as heat. How much of that heat can escape depends on rear ventilation and mounting conditions. When ground-mounted with the rear exposed, heat can escape more easily, whereas installations close to the roof surface or conditions similar to building-integrated configurations tend to trap heat.

When setting temperature losses, it is important to consider separately both the characteristics of the module itself and the installation environment. As a characteristic of the module, there is the temperature coefficient that indicates how much the output decreases when the temperature rises by 1 °C. On the other hand, regarding the installation environment, there are thermal conditions that show how readily the module temperature tends to rise. These two factors combine to determine the final temperature losses.

A common misconception is assuming that temperature losses can be set correctly by checking only the temperature coefficient. The temperature coefficient is certainly important, but in practice the mounting/racking conditions also have a large influence. For example, even modules with the same temperature coefficient will see different rates of module temperature rise in a well‑ventilated ground‑mounted installation versus an installation mounted flush to a roof surface. As a result, annual energy yield and summer generation can differ.

It is important to view temperature loss in relation to other settings rather than evaluating it alone. Increasing the oversizing ratio raises DC output on sunny days, but it also makes temperature effects during high irradiance more apparent. When performing shadow analysis or configuring shading, changes in how sunlight is incident will affect estimates of module temperature. If you change the meteorological data, ambient temperature conditions will also change, which impacts comparisons of temperature loss.

In other words, temperature loss is not something you finish by simply entering the temperature field; it is a setting connected to the overall system conditions. To create simulations in PVSyst that are usable in practice, it is important to treat temperature loss as part of the equipment conditions.

Information to check before setting temperature losses in PVSyst

Before setting the temperature loss, there is information you should check first. If you try to decide the values solely by looking at the input screen, you can easily end up with settings that deviate from actual site conditions, so it is important to organize design documents and field information before proceeding.

The first thing to check is the specifications of the solar panels to be used. Because temperature coefficients differ by module, if the model is decided, confirm the values listed in the datasheet. In particular, the temperature coefficient of maximum power, the temperature coefficient of voltage, and the temperature coefficient of current relate to string design and generation calculations. For energy yield assessment, the temperature coefficient of maximum power is important, but the voltage-side temperature characteristics are also related to verifying the maximum voltage at low temperatures, so they must not be overlooked in the overall system design.

Next, check the mounting arrangement. Whether it is ground-mounted or roof-mounted, whether there is sufficient distance from the roof surface, whether the rear is open, and whether there are structures nearby that block the wind will affect the temperature conditions. Because PVSyst’s temperature loss settings reflect an approach close to the module’s cooling conditions, it is necessary to correctly understand the mounting conditions.

In the case of roof-mounted installations, the roofing material and installation height are also important. Metal roofs, flat roofs, roofs with waterproofing layers, trapezoidal (ribbed) roofs, and pitched roofs all retain heat differently. Check whether there is a gap behind the modules, whether they are close to the roof surface, and whether the structure allows airflow. If on-site photos, elevation drawings, or racking drawings are available, judging the temperature conditions while referring to them will improve accuracy.

Weather data should also be checked. In PVSyst, location settings and the import of meteorological data are used in the simulation not only for irradiance but also for ambient temperature. If the location is not set correctly or meteorological data from a site distant from the actual planned site is used, the evaluation of temperature losses can also be off. In particular, conditions such as elevation, whether the site is coastal or inland, and whether it is urban or mountainous affect ambient temperature and wind conditions.

Furthermore, when conducting design comparisons, you need to decide in advance which conditions to hold constant for the comparison. Depending on whether you are comparing module types, mounting systems, or azimuth and tilt angles, you will need to decide whether to align the temperature-loss settings or deliberately change them. If you change the temperature settings without first organizing the comparison conditions, it becomes difficult to understand the reasons for differences in power output.

What matters in using PVSyst is not the data-entry process itself but clarifying the assumptions before entering data. Temperature losses are a parameter that is easily influenced by those assumptions, so it is important to confirm module specifications, mounting-frame conditions, ventilation, meteorological data, and the purpose of the comparison before setting them.

5 Items Easily Overlooked in Temperature Loss Settings

There are five main items that are easy to overlook when setting temperature losses in PVSyst. First, whether the module’s temperature coefficient is aligned with the specifications. Even if you select a module from the database, you need to confirm that it matches the specific model and specifications you plan to use. Choosing a module only because it is in a similar power range can still result in different temperature characteristics.

Secondly, the mounting method and ventilation conditions. This is particularly important for practical settings of temperature loss. For ground-mounted, roof-mounted, and installations that are nearly back-sealed, the way module temperature rises differs. Even for installations that appear to have the same capacity, whether heat can escape from the rear affects the amount of power generated.

Third, the installation height and heat buildup on the roof surface. In rooftop projects, the space behind the modules can be small, and the roof surface itself can become quite hot. When mounting close to the roof surface, you need to consider not only the ambient air temperature but also the thermal effects from the roofing materials. While PVSyst cannot reproduce all the detailed local phenomena, you should consciously choose settings that approximate the actual conditions.

Fourth is the outdoor air temperature in the meteorological data. If you select meteorological data based solely on the plausibility of solar radiation, you may overlook the outdoor air temperature conditions. If the outdoor air temperature is lower than the actual local conditions, the temperature loss tends to be smaller, and conversely, if it is higher, the temperature loss tends to be larger. Special care is needed, particularly when there are multiple candidate sites or when there are elevation differences.

Fifth, standardize the conditions when comparing design proposals. Comparing a proposal that changes the temperature loss setting with one that does not will mix the causes of differences in power output. It is important to clarify whether you want to compare modules, mounting systems, or installation angles, and to align any conditions that are not necessary for the comparison.

These five items may not be prominently displayed on the input screen. However, in practical use they can significantly affect the reliability of simulation results. Temperature loss may seem like a minor setting, but because it directly impacts annual power generation and loss charts, it is important to make a habit of always checking it.

Item 1: Check the module's temperature coefficient

In the temperature loss settings, the first thing to check is the module’s temperature coefficient. Solar panels see their output drop as temperature rises, but the magnitude of that drop varies with the module’s specifications. When you select a module in PVSyst, its electrical characteristics and temperature behavior are reflected in the settings, but you need to verify that the data you selected matches the actual model you will use.

In practice, simulations are sometimes started before the final module model has been decided. In such cases, a module with a similar capacity range or similar specifications is provisionally selected. However, if you proceed to the submission materials with these provisional settings, the projected energy production may change once the model is finalized. This is especially true if the temperature coefficient differs, since that affects not only annual energy production but also output assessments during the summer.

When looking at temperature coefficients, focus primarily on the temperature coefficient of maximum power. This is the value that indicates how much the maximum power changes when the module temperature rises from the reference temperature. It is typically expressed as a negative value, meaning that output decreases as temperature increases. The larger the absolute value of the number, the greater the decrease in output due to temperature rise.

However, it is not appropriate to judge a module's performance solely by its temperature coefficient. Actual power generation is determined by many factors, such as module efficiency, capacity, irradiance conditions, installation conditions, the combination with the PCS, shading, and wiring losses. The temperature coefficient is an important factor, but rather than drawing conclusions from it alone, it is important to perform comparisons under identical conditions in PVSyst and verify the results by checking the annual energy yield and the loss diagram.

Also, you need to be aware of its relationship with string design. Voltage decreases at high temperatures and increases at low temperatures. In the context of temperature losses, attention tends to focus on the output reduction at high temperatures, but in the overall system design the maximum voltage at low temperatures and the PCS input range are also important. When using PVSyst, you should not only configure the temperature loss settings but also check how module temperature affects the electrical design.

Even when using modules selected from a database, we recommend cross-checking the specifications with the values in PVSyst. Check capacity, open-circuit voltage, short-circuit current, maximum power operating voltage, maximum power operating current, temperature coefficients, etc., and look for any obvious discrepancies. Using data for different model numbers or different generations can affect not only temperature losses but the overall energy yield.

Checking temperature coefficients is an unglamorous task, but it is the basis of simulation. If you leave this ambiguous and then finely adjust mounting or meteorological conditions, the reliability of the results will not improve if the underlying module characteristics are incorrect. Accurately reflecting the module specifications is the first step in configuring temperature loss settings.

Item 2: Accurately reflect the mounting system and ventilation conditions

The mounting method and ventilation conditions are the factors that most often cause the greatest practical differences in temperature losses. Solar panels heat up when exposed to sunlight, but how much of that heat escapes depends on the installation method. If there is sufficient space at the rear and the installation allows airflow, module temperatures are less likely to rise. Conversely, installations close to the roof surface where rear air circulation is limited tend to trap heat and exhibit larger temperature losses.

In PVSyst, the temperature-related settings deal with conditions that represent the thermal behavior of the modules. What practitioners should be aware of is choosing ventilation conditions that are close to the actual installation environment. Treating systems with an open rear, such as ground-mounted installations, and systems close to the roof surface under the same conditions can cause the evaluation of temperature losses to deviate from reality.

For ground-mounted installations, there is often a space behind the modules, resulting in a design that allows wind to pass through easily. In this case, heat generated by the modules can escape relatively easily, and temperature rise may be suppressed compared with roof-mounted installations. However, even for ground-mounted installations, airflow can be restricted if there are windbreaks, slopes, vegetation, equipment, or adjacent arrays nearby. When the racking height is low or the array spacing is narrow, it may also be inappropriate to simply assume open conditions.

For rooftop-mounted installations, rear ventilation is a major point to check. Verify whether there is sufficient distance between the roof surface and the module, whether the structure allows air to escape, and whether heat from the roof surface is likely to affect the module’s rear. Especially when installed close to the roof on low mounts, the increase in roofing material temperature and the small rear space create conditions that make module temperatures more likely to rise. Even if it appears to be a typical rooftop installation, temperature losses will vary depending on whether ventilation is ensured.

Also, although building rooftops may give the impression of being windy, in reality the wind flow is made complex by parapets, equipment foundations, air-conditioning equipment, adjacent buildings, and so on. Even if the rooftop as a whole feels windy, there is no guarantee that there is sufficient ventilation behind the modules. Since it is difficult to directly reproduce such detailed wind distributions in PVSyst settings, it may be necessary to adopt conservative settings depending on the installation conditions.

Setting ventilation conditions is not a matter of choosing convenient parameters simply to make the projected power output look larger. Rather, it is important to select values that can be justified by the installation conditions. If the system is ground-mounted with the rear exposed, you should be able to explain that. If it is roof-mounted with limited ventilation, you should be able to explain the potential for increased temperature-related losses. In submission materials, not only the power generation figures but also the assumptions that support those figures are important.

When you're still unfamiliar with PVSyst, you may mistakenly believe that simply entering the racking conditions will automatically and fully reflect the temperature conditions. However, if you proceed without understanding the concept of temperature losses, differences in installation methods may not be adequately reflected. Always check the racking conditions together with the ventilation conditions, and review whether the settings closely match the actual installation.

Item 3: Consider installation height and heat buildup on the roof surface

When considering temperature losses, installation height and heat buildup on the roof surface are points that are easily overlooked. In particular, for roof-mounted or rooftop installations, the way heat escapes changes depending on how much space there is behind the module. Because classification by racking type alone may not be sufficient to make a judgment, it is important to verify the actual arrangement using drawings and on-site photographs.

When installed close to the roof surface, heat tends to accumulate on the rear side of the module. The roofing material exposed to solar radiation also heats up, and that heat warms the air around the module. If the air at the rear is not easily exchanged, the module temperature rises further and thermal losses increase. This effect is difficult to grasp by looking only at the ambient air temperature.

On the other hand, installations where there is sufficient space behind the modules and air can flow allow heat to dissipate more easily. Ground-mounted installations tend to meet this condition, but roof-mounted installations can also achieve relatively good ventilation depending on racking height and layout. Conversely, even for ground-mounted installations, if the distance to the ground is small, the surroundings are enclosed, or array spacing is narrow, heat is more likely to become trapped.

When checking the installation height, it's practical not just to look at the height of the module's lower edge but to consider whether there are air inlets and outlets. Even if there is space behind the module, the cooling effect is limited if the air cannot escape. Roof pitch, array orientation, nearby obstructions, parapet height, and the placement of mechanical equipment also affect ventilation.

Also, the color and material of the roof surface affect the thermal environment. Dark-colored roofs and materials that retain heat tend to cause the roof surface temperature to rise more during the day. It can be difficult to model these details in PVSyst, but they are important background information when assessing temperature conditions. Especially when installing on the roof of an existing building, checking the condition of the roof surface and the surrounding thermal environment will make it easier to explain the simulation results.

When configuring temperature losses, it is important not only to pursue numerical precision but also to check whether your assumptions are unrealistic for the actual site conditions. For example, if an installation is very close to the roof surface yet is modeled on the assumption that the rear is adequately cooled, the estimated power output may be overly high. Conversely, if a well-ventilated ground-mounted installation is modeled under heat-trapping conditions, the estimated power output may be underestimated.

Installation height and heat buildup cannot always be determined from drawings alone. If possible, review on-site photos, detailed drawings of the mounting frames, cross-sections, and roof plans to understand how the equipment will be accommodated. Especially at the detailed design or proposal-comparison stage, leaving notes on the assumptions for temperature loss will make later explanations and reviews easier.

When setting temperature losses in PVSyst, it is important not to judge solely by broad categories like "roof-mounted" or "ground-mounted", but to also be aware of the installation height and the way heat dissipates. Simply adopting this perspective greatly increases the realism of the simulation results.

Item 4: Check the outdoor air temperature in meteorological data and the on-site conditions

PVSyst's temperature losses are also influenced by the ambient air temperature conditions in the meteorological data. Many practitioners prioritize solar irradiance when selecting meteorological data. Annual solar irradiance, monthly solar irradiance, and tilted-plane irradiance are naturally important because they directly affect energy production. However, from the perspective of temperature losses, the validity of the ambient air temperature should be checked in the same way.

Using weather data with relatively low ambient temperatures tends to produce lower estimated module temperatures and may result in smaller temperature losses. Conversely, using data with relatively high ambient temperatures may lead to larger temperature losses. Even if the solar irradiance alone appears reasonable, if the ambient temperature differs significantly from local conditions, it will still affect the evaluation of temperature losses.

Particular attention should be paid to the distance and topographic differences between the planned site and the meteorological data location. Ambient air temperature and wind tendencies differ in plains, mountainous areas, coastal areas, inland areas, and urban areas. When elevations differ, temperature conditions also change. Even if you believe you are using data close to the candidate site, in reality elevation differences and topographic conditions may be substantially different.

Additionally, in urban areas the thermal environment can be influenced by surrounding buildings and paved surfaces. Around large factory roofs, logistics centers, commercial facilities, and parking areas, the thermal conditions of ground and roof surfaces can affect module temperature. It is difficult to fully represent these effects using only PVSyst's meteorological data, but it is important to be aware of the differences between ambient air temperature conditions and the installation environment.

When reviewing monthly results, be mindful of the influence of ambient temperature. If solar irradiance is high in summer but the increase in power generation is smaller than expected, temperature-related losses may be involved. Conversely, in winter, even with lower irradiance, the low temperatures can create conditions that tend to raise module efficiency. It is important to confirm seasonal variations that are hard to see from annual generation alone by combining monthly generation data with loss diagrams.

When replacing meteorological data, also check for changes in temperature losses. Changes in power generation due to differences in insolation may include changes in temperature losses caused by differences in ambient temperature. When comparing multiple meteorological datasets, looking not only at annual insolation but also at monthly ambient temperatures and conditions during high-temperature periods will make the results easier to interpret.

In submission materials, rather than cluttering the main text with meteorological data source names and overly detailed explanations, it is important to explain how appropriate the data used are for the project site and how the assumptions about solar radiation and outdoor air temperature are reflected in the results. To correctly handle temperature losses, meteorological data should not be treated merely as input values for solar radiation but as simulation assumptions that include outdoor air temperature.

Item 5: Keep temperature conditions consistent when comparing design proposals

When comparing multiple design proposals in PVSyst, care must be taken in how temperature conditions are handled. In design comparisons, you may change module capacity, azimuth, tilt angle, mounting system, PCS capacity, overloading ratio, shading conditions, etc., to observe differences in energy production. If temperature conditions unintentionally vary in the process, it becomes difficult to determine the reasons for the differences in energy production.

For example, if you want to compare only the azimuth, it is fundamental to keep the temperature loss settings the same. Similarly, if you want to compare only the tilt angle, you also need to keep the module, racking, and ventilation conditions consistent. If conditions unrelated to the comparison purpose differ, it becomes difficult to determine whether the difference in energy output is due to azimuth or to temperature losses.

On the other hand, if you want to compare mounting configurations, you should deliberately vary the temperature conditions. When comparing a ground-mounted option with a rooftop option, ventilation conditions and installation height differ, so temperature-related losses should be adjusted to reflect the actual situation. In that case, explain that part of the difference in power output is due to differences in temperature conditions. The important point is that whether to fix or vary the temperature conditions depends on the purpose of the comparison.

The same applies to module comparisons. When comparing different modules, differences in temperature coefficients are reflected as part of the performance differences. However, if you change the racking conditions or the meteorological data, you will not be able to compare module differences correctly. If you want to compare the temperature characteristics of modules, keep the installation conditions the same and reflect the differences in temperature coefficients by model to make the comparison clearer.

When comparing design proposals, it is important to use case names and notes to record which conditions were changed. If you modify the temperature loss settings, make them clear for later review with entries such as "changed ventilation conditions for rooftop mounting", "rear-open condition for ground-mounted installation", or "temperature coefficient changed due to module model change". When handling multiple cases, it is easy to forget which settings were altered, so keeping records is essential.

When sharing comparison results internally, we check not only the difference in annual energy yield but also the temperature loss component in the loss diagram. Even if the difference in energy yield is small, the breakdown of temperature losses can differ significantly. Conversely, even if there appears to be a difference in energy yield, that difference can shrink when temperature conditions are aligned. To read PVSyst results correctly, it is essential to check the loss factors as well as the total energy yield.

Aligning temperature conditions is fundamental to maintaining the fairness of simulations. By clarifying the purpose of the comparison and separating the conditions that will change from those that will not, PVSyst design comparisons become more practical for real-world use.

Practical Procedure for Setting Temperature Loss

The practical procedure for setting temperature losses in PVSyst is easier to follow if you proceed in this order: clarifying prerequisites, checking the modules, checking the mounting/racking conditions, verifying the meteorological data, running the simulation, and reviewing the results. Rather than opening only the temperature settings screen from the start, organizing the necessary information before entering it helps reduce configuration errors.

First, confirm the project's basic conditions. Organize the planned site, system capacity, module type, PCS configuration, racking method, whether it is roof-mounted or ground-mounted, azimuth, tilt angle, shading conditions, and so on. Temperature losses are not an item independent from these but are linked to the installation conditions, so it is important to understand the overall conditions.

Next, check the module data. Verify that the module selected in PVSyst matches the model intended for actual use. If the model is undecided, clearly indicate that it is a provisional setting and ensure it can be updated later. Confirm that the temperature coefficient matches the specifications, and review the data as necessary.

Next, confirm the mounting arrangement and ventilation conditions. Check whether the installation is ground-mounted with the rear exposed, roof-mounted close to the roof surface, or on a rooftop where surrounding structures obstruct the wind. If mounting drawings or cross-sections are available, verify the space behind the modules and how the wind passes through. Reflect the conditions determined here in PVSyst’s temperature-related settings.

Next, review the meteorological data. Verify that the site settings match the planned location and that the outdoor air temperature conditions in the meteorological data show no anomalies. Checking monthly temperature trends as well as solar radiation will make it easier to interpret the temperature loss results. If there is a discrepancy between the candidate site and the meteorological data location, consider how that discrepancy affects the results.

When the setup is complete, run the simulation. After running it, do not judge solely by the annual energy production; check the loss diagram, monthly energy production, PR, and temperature loss parameters. If the temperature loss is larger than expected, recheck the racking conditions, ventilation, ambient air temperature, and module temperature coefficient. Conversely, if the temperature loss is unnaturally small, also verify that you are not overestimating the cooling conditions.

When comparing design proposals, it is safer to create a baseline case and then change one condition at a time. Changing multiple conditions simultaneously makes it difficult to discern the effect of temperature losses. Separating cases by purpose—such as module-change cases, mounting-structure-change cases, and tilt-angle-change cases—makes it easier to explain the comparison results.

Finally, concisely record the assumptions for temperature loss in the report for submission and in internal shared materials. You do not need to describe every detailed input screen, but make sure you can explain that module specifications, installation conditions, ventilation conditions, and assumptions about meteorological data are reflected in the results—this will make reviews and responses to questions go more smoothly.

By making this procedure a habit, the temperature-loss settings in PVSyst become not merely a data-entry task but a step for verifying design quality. In practice, especially because multiple cases are often created in a short time, establishing a verification order helps prevent mistakes.

How to Check Temperature Loss in Simulation Results

Temperature losses are not something you set and forget. After running a simulation in PVSyst, you need to check how they are reflected in the results screens and reports. It's important not only to see whether the energy yield matches expectations, but also to verify how much temperature affects the breakdown of losses.

The first thing to check is the temperature-related losses displayed in the loss diagram. PVSyst's loss diagram shows the conversion process from solar irradiance to the final energy output. Within that, check how much loss is occurring due to temperature. If temperature losses are large, it may indicate conditions that cause the module temperature to be high.

Next, we check the monthly power generation. Temperature losses do not occur uniformly over the year; they tend to be larger during high-temperature periods. If power generation increases less than expected despite high summer solar irradiance, temperature losses may be affecting performance. By looking at the monthly results, you can grasp seasonal characteristics that are not apparent from annual values alone.

When evaluating PR, also take into account the impact of temperature losses. PR is an important metric for assessing system performance, but in regions or installation conditions with severe temperature constraints, PR can appear low because of temperature losses. Rather than simply concluding that a design is poor because PR is low, it is important to check the weather conditions and the breakdown of temperature losses.

When comparing design options, check the temperature losses for each case side by side. Even a case with high annual power generation can have large temperature losses. Conversely, a case with small temperature losses may still fail to achieve high total generation because of solar irradiation conditions or shading. To interpret the results correctly, you need to evaluate power generation, loss diagrams, and monthly values together.

Also, caution is necessary when the temperature loss is unnaturally small. It may be due to overestimating ventilation conditions, using weather data with an ambient air temperature that is too low, or module data that differ from reality. Simulation results are not necessarily correct just because they produce good numbers. It is important to compare them with actual site conditions to confirm whether the results are reasonable.

Checking temperature loss is a basic task when reviewing PVSyst results. Especially before submission, check the temperature loss item on the loss diagram to ensure it does not conflict with the mounting/racking settings and the meteorological conditions you specified. This will allow you to explain, with supporting evidence, why the energy production turned out the way it did if asked later.

Improvement Points When Temperature Loss Is Large

When temperature losses in PVSyst results are large, the first thing to do is check for configuration errors. Review whether the module temperature coefficient is correct, whether the mounting structure conditions are set more conservatively than the actual situation, and whether the ambient air temperature in the meteorological data is appropriate for the planned site. A large temperature loss does not necessarily mean that a design change is immediately required. It is important to first confirm that the assumptions are correct.

If the settings are appropriate but temperature loss remains large, consider potential design improvements. A typical measure is improving ventilation. By creating a structure that allows air to flow more easily behind the module, it may be possible to suppress rises in module temperature. For roof-mounted installations, consider ensuring a distance from the roof surface, arranging the layout thoughtfully, or using details that make it harder for heat to become trapped.

Even for ground-mounted installations, racking height and array spacing affect ventilation. Of course, changing racking height or spacing also impacts land use, shading, structure, safety, constructability, cost, and so on. Therefore, changes should not be made solely for the sake of temperature-related losses; a comprehensive assessment is necessary. In PVSyst, you can create cases with altered conditions and check the differences in energy production to quantitatively compare the improvement effects.

Module selection should also be considered. By comparing modules with different temperature coefficients, you can confirm differences in power generation under high-temperature conditions. However, module selection involves many factors—such as output, dimensions, weight, electrical characteristics, availability, warranty, and ease of installation—so you should avoid judging solely by the temperature coefficient. In PVSyst, swapping modules under the same installation conditions for comparison makes it easier to see the impact of temperature characteristics.

When the thermal environment on the roof surface is severe, reviewing the layout plan can also be effective. Check whether you can avoid locations where heat tends to accumulate, areas with poor airflow, or spots enclosed by surrounding equipment. Including the effects of building services and parapets, it is more practical to make decisions by considering both shadow analysis and temperature conditions.

If the sole objective is to minimize temperature loss, you may upset the balance with other conditions. For example, widening the spacing between arrays to improve ventilation can reduce installed capacity. Raising the racking height may affect structural requirements, wind loads, and constructability. Therefore, improvements in temperature loss must be evaluated taking into account annual power generation, installed capacity, constructability, and maintainability.

The role of PVSyst is to make such design decisions easier to compare numerically. When temperature losses are large, breaking down the causes and comparing each changeable condition one by one makes it easier to find realistic improvement measures.

Points to note when explaining temperature loss in submission materials

When sharing PVSyst results with internal stakeholders or the client, it is important that explanations of temperature losses be concise yet convey the underlying rationale. You do not need to list every technical equation and detailed input value, but you should be able to explain what installation conditions were assumed and how temperature losses are reflected in the energy production.

First, explain that temperature loss is due to the general characteristics of solar panels. Because solar panels’ output decreases as temperature rises, losses occur depending on ambient temperature and installation conditions. This explanation conveys that temperature loss is not a design mistake but a physical effect expected in actual operation.

Next, describe the installation conditions. Clarify whether the system is ground-mounted with rear ventilation, roof-mounted under conditions prone to heat buildup, and what racking configuration is assumed. Because temperature loss depends on installation conditions, it is insufficient to simply state “temperature loss is X%.” Make sure you can explain why the loss is at that level.

In comparative documents, unifying the conditions is important. When comparing design proposal A and design proposal B, clearly state whether the temperature conditions were kept the same or whether the temperature conditions were changed as a result of differences in the mounting system. If this is left ambiguous, the reasons for differences in power generation may be misunderstood. In particular, for comparisons that include module changes or mounting changes, explain that differences in temperature losses are also reflected in the results.

Also, it's important not to overemphasize temperature losses alone. Power generation is determined by many factors such as solar irradiance, shading, azimuth angle, tilt angle, PCS capacity, wiring losses, soiling, and degradation. Temperature losses are important, but indicating where they fit within the overall loss structure will make the explanation more balanced.

Before submission, check that the report's settings and explanatory text do not contradict each other. If the main text describes a rooftop installation but the simulation conditions assume a ground-mounted setup with good rear ventilation, reviewers may point that out. Conversely, it is also unnatural to describe a ground-mounted installation as if it were under conditions that cause heat buildup.

When explaining temperature losses, not only the accuracy of detailed numerical values but also the consistency of the assumptions is important. To use PVSyst results in practice, confirm that input values, installation conditions, and explanatory documentation are consistent.

Connecting PVSyst temperature loss settings to on-site verification

PVSyst's temperature loss settings cannot be completed by desk-based simulation alone. In actual power generation facilities, module temperature conditions change depending on the local terrain, roof shape, surrounding structures, wind flow, and the as-built conditions after construction. Therefore, it is important to link the assumptions set in the simulation to on-site verification.

During the on-site inspection, we first check the surrounding environment of the installation site. For ground-mounted installations, we look for structures that might block wind, whether array spacing is sufficient, and the height from the ground surface to the modules. For roof-mounted installations, we check the distance to the roof surface, the height of parapets, nearby equipment, and airflow escape routes. This information is used to assess the appropriateness of the temperature loss settings.

Also, it is important to accurately record on-site positioning and documentation. Recording installation locations, mounting layout, rooftop equipment, obstructions, and inspection access routes will make it easier to reconcile the assumptions created in PVSyst with the actual site conditions. Especially for large-scale installations or complex roof geometries, drawings alone may not fully capture the site's thermal environment and ventilation conditions.

In such on-site inspections, using an LRTK, a GNSS high-precision positioning device that attaches to an iPhone, can streamline recording installation locations and site conditions. By recording rack/mounting locations, areas around equipment, obstacles, and inspection points based on the high-precision position data obtained on-site, it becomes easier to compare the conditions assumed in PVSyst with the actual site conditions. Managing photos, point clouds, and location data together helps with checking simulation assumptions, comparing before and after construction, and verifying during operation and maintenance.

Temperature losses are more accurately assessed when considered together with the on-site installation environment rather than judged solely by the on-screen numbers. By establishing a workflow that evaluates generation with PVSyst and records on-site location information and conditions with LRTK, it becomes easier to conduct practical photovoltaic system assessments that connect desk-based design with on-site verification.

Summary

To understand how to set temperature losses in PVSyst, simply memorizing the input fields on the settings screen is not enough. Temperature losses are closely related to the module temperature coefficient, the mounting system, ventilation conditions, installation height, the thermal environment of the roof surface, the ambient air temperature in the meteorological data, and the condition settings for comparing design options. Organizing and entering these factors will increase the reliability of the simulation results.

As five commonly overlooked items, it is important to check the module temperature coefficient, the racking type and ventilation conditions, the installation height and heat buildup on the roof surface, the ambient air temperature in the meteorological data, and the consistency of conditions when comparing design proposals. In particular, because heat dissipation differs between rooftop and ground-mounted installations, temperature losses can vary even for systems of the same capacity. The less familiar you are with using PVSyst, the more necessary it is to avoid proceeding with the default values and to make it a habit to verify that they match the site conditions.

After the simulation, check not only the annual energy production but also the loss diagram, monthly energy production, PR, and the breakdown of temperature losses. If temperature loss is large, start by checking for configuration errors and, as needed, compare ventilation, racking height, layout, module selection, and so on. When comparing design proposals, clearly define what you want to compare and organize whether to fix the temperature conditions or to reflect them as design differences.

In the submitted materials, explain that temperature losses are based on the characteristics of the solar panels and the installation conditions, and ensure that the input conditions and the explanations in the main text do not contradict each other. PVSyst results become practically usable only when you can explain not only the numbers themselves but also their underlying assumptions.

Furthermore, the assumptions about temperature losses are closely linked to on-site verification. If the positions of the mounting structures, distances to the roof surface, surrounding obstacles, and wind flow can be accurately recorded on site, it becomes easier to validate the simulation conditions. By utilizing LRTK, an iPhone-mounted high-precision GNSS positioning device, you can record and combine on-site location data, photographs, and point clouds, and link and manage the design conditions assumed in PVSyst with the actual site conditions. For photovoltaic system energy yield assessments, integrating desktop simulations and on-site verification—rather than treating them separately—leads to more convincing designs and explanations.