5 Basic Temperature Losses to Understand in the PVSyst Manual

Table of Contents

• The significance of checking temperature losses in PVSyst

• Temperature loss basics 1: Solar panels are rated at a reference temperature of 25℃

• Temperature loss basics 2: evaluate losses using array temperature, not ambient temperature

• Temperature Loss Basics 3: Uc and Uv are important parameters that indicate how easily heat is dissipated

• The Four Basics of Temperature Loss: Results Vary with Mounting Structures, Roofs, and Ventilation Conditions

• Temperature Loss Basics 5: Confirm in Reports the Impact on Power Generation

• Practical considerations when setting temperature loss

• Summary

The significance of checking temperature losses in PVSyst

When running generation simulations in PVSyst, temperature losses are easy to overlook, yet they play a major role in evaluating annual energy yield. There is a common impression that PV output increases as irradiance increases, but in reality module temperature also tends to rise as irradiance strengthens. As module temperature increases, the output of the solar cells decreases. Therefore, even in regions with ample irradiance, if temperature conditions are not properly accounted for, the expected energy yield can be overestimated.

PVSyst's manual explains that the nominal performance of PV modules is specified at 25℃, whereas in actual operation array temperatures are often higher, and that difference affects power output as a temperature-related loss. As a rule of thumb for losses due to temperature increase, PVSyst's documentation indicates a range of approximately −0.2 to −0.4% per 1℃. This is not determined by a simple coefficient alone, but it is an important premise for understanding that temperature conditions directly impact energy yield assessments.

Temperature losses cannot be understood simply as “greater because the region is hot.” In reality, they are determined by a combination of ambient temperature, solar irradiance, wind speed, module mounting method, rear ventilation, distance to the roof, racking structure, and module efficiency. Even in the same region with the same modules and the same capacity, heat dissipation conditions—and therefore the resulting temperature losses—will differ between ground-mounted and roof-flush-mounted installations. Therefore, when reading the PVSyst manual, it is important to understand temperature loss not as a standalone input field but as an important bridging parameter for reflecting design conditions into the energy yield.

In practice, the temperature-loss setting is sometimes left at its default value. Using the default value is not necessarily wrong in itself, but if the installation configuration differs from the assumptions behind the default, the simulation results can deviate from actual field conditions. For example, a ground-mounted installation with sufficient rear ventilation and a project where modules are installed close to the roof surface have different module backside cooling conditions. Ignoring this difference can alter the estimated array temperature under the same irradiance conditions and ultimately lead to differences in the evaluated power generation.

When using PVSyst not only for energy yield estimation but also for design comparisons, project feasibility assessments, materials for financial institutions, EPC proposals, and O&M planning, interpreting temperature losses becomes even more important. This is because temperature losses are not a one-time setting; they must be evaluated comprehensively together with azimuth angle, tilt angle, shading, wiring losses, inverter capacity, weather data, and so on. This article breaks down the fundamentals to keep in mind when reviewing temperature losses in the PVSyst manual into five points.

Temperature Loss Basics 1: Solar panels are evaluated using 25℃ as the reference

The first step in understanding temperature losses is to know under what conditions a solar panel’s catalog performance is reported. In general, the nominal power of PV modules is specified assuming standard test conditions. A representative temperature reference for those conditions is a cell temperature of 25℃. In other words, the output shown in catalogs should be regarded as a reference value for comparison under defined test conditions, not a value that will always be obtained outdoors as-is.

On site, cell temperature and module temperature are often not maintained at 25℃. Under sunny conditions, modules exposed to solar radiation can heat up and reach temperatures substantially higher than the ambient air temperature. Module temperatures are especially prone to rise in summer, at low wind speeds, when installed close to the roof surface, or when rear-side ventilation is poor. PVSyst takes this operational temperature increase into account and calculates the power reduction due to the difference from the 25℃ reference as a temperature loss. PVSyst’s documentation also explains that PV arrays in real operation tend to reach temperatures above 25℃, in which case output losses occur.

What’s important here is that temperature losses are not simply a matter of “it drops by a fixed percentage just because the ambient temperature is high.” Ambient temperature is certainly important, but what actually affects output is the module or cell temperature. For example, even at the same ambient temperature of 30℃, modules on racking with strong winds and good rear ventilation will cool more easily, whereas installations with weak wind and a small gap to the roof surface tend to trap heat. Therefore, when interpreting PVSyst’s temperature losses, you should not judge by ambient temperature alone; you need to look at how the array temperature is being estimated.

Also, temperature losses are not constant throughout the year. They tend to be larger in summer and under high solar irradiance, and smaller in winter and at low temperatures. Under certain conditions, there can be periods of operation at temperatures lower than 25 ℃ that are advantageous from a temperature standpoint. The older PVSyst help also explains that when the array temperature is above 25 ℃ it represents a loss, and when it is below it can represent a gain.

If you don’t understand this mechanism, then when you look at the results of a generation simulation it becomes difficult to explain why energy output doesn’t increase as much as expected even in months with high solar irradiance, and why the performance ratio tends to decrease in high-temperature regions. When reading the PVSyst manual, the starting point should be to grasp the basic fact that solar panels are evaluated at a reference temperature of 25℃ and that in actual operation output falls as temperature rises.

Temperature Loss Basics 2: Evaluate Losses Using Array Temperature Rather Than Air Temperature

To correctly interpret temperature losses, you need to understand which temperature PVSyst is using. In everyday conversation people sometimes say "power output decreases because the ambient temperature is high," but what matters in PVSyst's calculations is not the ambient air temperature itself but the temperature at which the PV array or the cells are actually operating.

In the current PVSyst documentation, the temperature model is treated in two stages. First, a thermal balance model calculates the steady-state temperature, represented by the variable TArrSS. Next, a thermal inertia model calculates the actual transient array temperature, using the variable TArray. In other words, PVSyst does not simply input ambient air temperature alone to determine temperature losses; rather, it adopts an approach that estimates the array temperature taking into account irradiance, efficiency, heat dissipation conditions, wind speed, and so on.

This point is very important when reading the PVSyst manual in a practical context. For example, even if the ambient temperature in the meteorological data is the same, modules will heat up more when solar irradiance is high. Conversely, if wind speed is high and the installation allows good rear ventilation, heat will dissipate more easily from the modules. Therefore, when looking at temperature losses, you need to check not only the temperature data but also what heat dissipation conditions PVSyst assumes when calculating the array temperature.

The PVSyst steady-state thermal balance model evaluates temperature by using the relationship between the absorbed solar energy, the heat released to the surroundings, and the energy extracted as electrical power. The official documentation presents a heat balance equation that includes incident solar irradiance, absorptance, module efficiency, Uc, Uv, wind speed, ambient temperature, and other factors. This allows estimating not simply "what the ambient temperature is" but "how hot the PV array will get under those conditions."

In practice, when viewing temperature-related figures on PVSyst’s result screens or reports, it is important not to look at TArray or temperature loss values in isolation, but also to check monthly trends and their relationships with other loss items. It is natural for temperature losses to increase in summer, but if they are larger than expected there may be room to review the mounting configuration, ventilation conditions, U-value settings, and the validity of the meteorological data. Conversely, if temperature losses are too small, you should also check whether overly optimistic heat dissipation conditions have been assumed.

Especially for roof-mounted installations, there can be heat buildup that cannot be judged from ambient temperature alone. The type of roofing material, the distance to the roof surface, the airflow beneath the modules, surrounding obstructions, and reduced ventilation due to continuous arrangements all affect the actual module temperature. When using PVSyst results in explanatory materials, rather than assuming “temperature losses are already accounted for because ambient temperature conditions were entered,” it is important to verify whether the conditions used to estimate array temperature match the actual site.

Heat Loss Basics 3: Uc and Uv are important indicators of how easily heat is lost

When reading temperature losses in the PVSyst manual, it is essential to understand Uc and Uv. These are heat transfer coefficients that describe how easily a PV array loses heat to its surroundings. Simply put, Uc is the basic heat-loss component that does not depend on wind speed, while Uv is the heat-loss component that varies with wind speed.

The PVSyst documentation explains that thermal exchange is split into a constant component and a component proportional to wind speed, and that Uc and Uv are the primary input parameters defining the thermal behavior of a PV array. In the formulation, the value combining Uc and Uv with wind speed is used as the heat transfer coefficient. Uc and Uv are said to need to be determined empirically to match real system data, and should be handled carefully according to installation conditions rather than treated as mere fixed values.

If the understanding of Uc and Uv is insufficient, it is easy to make mistakes when setting temperature loss. For example, in ground-mounted installations with adequate back ventilation, heat can escape easily from both the front and back of the modules. On the other hand, installations close to the roof or where the rear is nearly insulated make it harder for heat to escape. The easier heat can escape, the less the array temperature rises and the smaller the temperature loss. The more difficult it is for heat to escape, the higher the array temperature and the greater the temperature loss.

In PVSyst’s help, as suggested values when reliable measured data are not available, Uc is given as 29 for open, ground-mounted installations, 15 for cases where the rear is fully insulated, and around 20 for intermediate cases. Values for other installation conditions, such as dome-shaped installations, are also provided as examples. These are not values to be applied unconditionally to every project, but they are useful for understanding that heat dissipation conditions can vary greatly depending on the installation configuration.

One thing to be careful about is not to treat initial or suggested values as "correct." PVSyst's documentation also explains that U-values should, in principle, be determined to match measurement data from the actual system. It presents the idea of evaluating them using long-term on-site measurements, solar irradiance, ambient air temperature, module backside temperature, and, in some cases, wind speed.

However, it is not always possible to determine Uc and Uv based on on-site measurements for every project. In the early design stage, the estimating stage, and the feasibility assessment stage, simulations are often carried out without measurement data. In such cases, it is important to choose values that closely match the installation type and to be able to explain why those values were used. For example, for a large ground-mounted project where rear ventilation is sufficiently ensured, an approach close to an open type is likely to be appropriate. For rooftop projects with weak ventilation, it is necessary to consider intermediate values or conditions closer to insulated.

Care is also needed with Uv. Using wind-speed-dependent coefficients may appear to allow the cooling effect during windy periods to be reflected. However, if the measurement height or measurement environment of the wind speed data differs from the actual wind around the module, the results can become unrealistic. PVSyst's documentation explains that wind speed is usually measured at a height of 10 m (32.8 ft), while wind speeds obtained from PV system monitoring are under different conditions and the wind speed at the collector level may be lower. Therefore, Uv should be evaluated together with the conditions under which the wind speed data were obtained.

Four Fundamentals of Temperature Loss: How Mounting Racks, Roofs, and Ventilation Conditions Affect Results

The difference in installation configuration is where the greatest practical variability in setting temperature losses occurs. The PVSyst manual shows that temperature losses depend not only on the characteristics of the module itself but also strongly on the racking and the installation environment. In particular, whether air flows behind the module has a large impact on estimating the array temperature.

In ground-mounted, open-field installations, heat exchange tends to occur on both the front and rear of the modules. If there is sufficient spacing between rows and surrounding ventilation is ensured, the modules tend to remain relatively cool.

By contrast, on rooftop installations, heat is more likely to become trapped when the distance to the roof surface is small or when air beneath the modules cannot escape easily. PVSyst documentation also explains that the U-value varies depending on the installation type—fixed tilt, rooftop, façade, trackers, floating, ground conditions, and so on.

What requires particular attention for rooftop installations is that simply saying "mounted on the roof" does not fully describe the conditions. Corrugated metal roofs, flat roofs, pitched roofs, rack height, the thermal properties of the roofing material, the gap beneath the modules, row spacing, and the presence or absence of surrounding upstands or parapets all change how air flows. PVSyst’s older help explains that air circulation on pitched roofs arises from temperature differences, but the driving force is weak so the upper-row modules may not have sufficient heat exchange, and that detailed modeling of airflow paths is beyond the scope of PVSyst.

Therefore, when handling temperature loss in rooftop projects, it is important not to proceed with the initial default values but to explicitly describe the on-site ventilation conditions. For example, if there is sufficient clearance from the roof surface and a structure that allows air to escape from under the modules, the conditions cannot be regarded as close to complete insulation. Conversely, if the gap between the modules and the roof surface is small and air tends to stagnate, treating it the same as an open, ground-mounted installation may lead to underestimating temperature loss.

Also, even for the same rooftop installation, as the system size increases the ventilation conditions within the array can become less uniform. Edge modules tend to receive more wind, while central modules can be more prone to heat buildup. Because PVSyst treats temperature conditions as averaged values, it cannot fully reproduce all localized overheating or non-uniformities. Therefore, when reading the results, it is important to understand that there can be a gap between the modeling assumptions and the complexity of the actual site.

Floating solar, facade-mounted, carport, agrivoltaic, and building-integrated installations are all configurations where the approach to temperature losses tends to vary. In some cases a cooling effect can be expected over water surfaces, while in others humidity, wind conditions, or airflow obstruction by structural members can have an impact. For facades and building-integrated systems, rear-side heat dissipation is easily restricted, and conditions that tend to result in higher temperatures can occur. Even if PVSyst input values look simple, they embody an interpretation of the installation type, so it is important to retain the rationale for the temperature loss settings.

Temperature Loss — Basic Principle 5: Ensure Reports Verify the Impact on Power Generation

The purpose of checking temperature losses in PVSyst is not simply to enter numbers in the settings screen. Ultimately, what matters is seeing how the temperature conditions are reflected in the annual energy yield, monthly energy yields, the performance ratio, and the loss diagram. Temperature loss is one of PVSyst’s loss items that reflects both meteorological conditions and design conditions. Therefore, in the results report you should be able to explain not just the percentage but why that value occurred.

In PVSyst, detailed hourly simulations are performed as part of project design, and as a result you can review many variables and loss diagrams. PVSyst's documentation also explains that the simulation results include monthly, daily, and hourly values, and that the Loss Diagram is useful for identifying design weaknesses.

When assessing temperature losses, it is practical to first check the annual values and then examine monthly variations. Even if annual temperature losses fall within a certain range, extreme losses in summer may require design measures or explanations. Conversely, even if annual losses appear small, they can have a significant impact during specific high-temperature periods due to output limitations or their relationship with demand peaks. For projects aimed at self-consumption or peak shaving, it is important to check not only simple annual generation but also time-of-day output reductions.

Also, it is important not to consider temperature losses separately from other loss terms. For example, during periods when shading has a large effect, incident solar irradiance decreases and module temperature also changes. Overall energy output is determined by the combined effect of wiring losses, mismatch losses, inverter losses, clipping, IAM, soiling, and other factors. Even if you over-optimize temperature losses alone, if other losses are large, the overall improvement will be limited.

When reviewing a results report, it is important not only to look at the temperature loss figures but also to check their consistency with the input conditions. If the settings assume open heat-dissipation conditions but the actual design is close to flush-mounted to the roof, the reported power generation may be optimistic. Conversely, if a well-ventilated ground-mounted installation is modeled using near-insulated conditions, the estimated power generation may be overly conservative. It is not a matter of right or wrong; what matters is setting reasonable assumptions according to the purpose and being able to explain those assumptions.

When submitting a PVSyst report to financial institutions or project owners, compiling the justification for the temperature loss settings into supplementary materials increases the credibility of the explanation. Organizing the meteorological data used, the installation configuration, the distance to the roof, racking conditions, ventilation conditions, the approach to the U-value, and the reasons for any changes from the initial values will make it easier to respond to reviews later. Understanding the rationale for temperature losses in the PVSyst manual is not merely operational knowledge; it also helps improve the ability to explain the design.

Practical considerations when setting temperature loss

When setting temperature losses, the first thing you should check is the validity of the meteorological data. If ambient air temperature, solar irradiance, and wind speed differ significantly from local conditions, the estimated array temperature will become unstable no matter how carefully you set the U-value. In particular, when using wind speed you need to confirm the measurement height and the measurement environment. The PVSyst documentation explains that wind speed is often measured at a height of 10 m (32.8 ft) and may differ from the wind actually experienced near the modules of a PV system.

Next, it is important not to settle for abstract terms when describing installation types. Classifying them only as "roof-mounted", "ground-mounted", or "agrivoltaic" does not sufficiently describe the heat dissipation conditions. For roof-mounted installations, check the distance to the roof surface, the rack height, airflow beneath the modules, row orientation, tilt, and surrounding obstructions. For ground-mounted installations, check the minimum ground clearance, row spacing, condition of the ground surface, and surrounding ventilation. Based on this information, judge whether the temperature loss settings in PVSyst are close to the actual site conditions.

Also, the required accuracy of the temperature loss settings varies depending on the design stage. In the initial study stage, the focus is on comparing multiple options using standard values. At this stage, it is more important not to overlook differences between installation configurations than to pursue fine numerical precision. In the detailed design stage, as racking and module layouts become more concrete, it is necessary to revise the settings to be closer to actual site conditions. In post‑commissioning performance evaluations and O&M analyses, you can verify whether the simulation assumptions were appropriate by comparing them with measured data.

A common mistake with temperature loss in PVSyst is failing to document why the initial values were changed. Even if the person who changed the numbers remembers the reason at the time, when the report is reviewed months later it can be unclear why those values were chosen. In projects involving multiple designers and reviewers, sharing the rationale for settings is more important than the input values themselves. Organizing PVSyst file names, versions, input conditions, change history, and comparison cases will make reviews and recalculations smoother.

Furthermore, it is important not to make temperature losses appear excessively small. If, in an effort to make the project's profitability look better, you assume better heat dissipation than the site conditions actually allow, the discrepancy between actual performance after start-up and the simulation will be large. Conversely, if you persistently use overly conservative settings, you may undervalue projects that could in fact be economically viable. When working with the PVSyst manual, the required approach is to accurately reflect site conditions rather than adjust figures to be favorable.

Care must also be taken when handling NOCT. A module's catalogue may list NOCT, but PVSyst's documentation states that NOCT is a value for a bare module under free-ventilation conditions and, because it does not include information about the installation configuration, is not suitable for assessing module temperature during simulations. In PVSyst, you should understand the concept of handling temperature behavior via the U-value and installation conditions, rather than relying solely on NOCT.

In practice, it is more realistic to treat temperature losses not as something to be optimized on their own but as a candidate for design improvement. From the results of temperature losses, you can consider design improvements such as ensuring clearance from the roof surface, avoiding arrangements of components that impede ventilation, reviewing inter-row spacing and mounting height, and checking the relationship with inverter operation and clipping at high temperatures.

Of course, this must be balanced with racking cost, wind-resistance design, constructability, roof loading, and maintainability. PVSyst simulations are effective as material for such comparative evaluations.

Summary

The most important thing in understanding temperature losses in the PVSyst manual is not to treat temperature loss as a mere input value or as an item on a report. The nominal performance of PV modules is evaluated at a reference temperature of 25℃, but in actual operation the array temperature rises due to solar irradiation, and that temperature rise manifests as a reduction in output. Therefore, temperature loss is an important factor for reflecting meteorological conditions and installation conditions in the power generation.

There are five basic points to keep in mind. First, solar panels are rated at 25℃, so deviations in operating temperature result in losses. Second, the temperature to consider is not the ambient air temperature but the array temperature estimated by PVSyst. Third, Uc and Uv are important inputs that indicate how easily heat is dissipated and should be considered according to installation conditions. Fourth, temperature losses change depending on the racking, roof, rear ventilation, mounting height, and so on. Fifth, in reports it is important to check not only the figures for temperature losses but also the annual energy yield, monthly trends, and the relationship with other loss items.

In PVSyst power output simulations, the results can change noticeably based on the setting for temperature losses alone. Especially for projects with complex operating conditions—such as rooftop installations, high-temperature regions, sites with poor ventilation, special racking, floating systems, agrivoltaics, and self-consumption systems—you should verify consistency with the actual site conditions before using the default initial values as-is. If you change the settings, record why you chose those values and keep comparison cases; this will make later reviews and explanations easier.

If you correctly understand temperature losses, a PVSyst report becomes more than just a set of energy production figures; it becomes a document for validating the appropriateness of design conditions. By interpreting solar irradiance, ambient temperature, wind speed, mounting conditions, roof conditions, and module characteristics together, the credibility of the simulation increases. When consulting the PVSyst manual, covering not only the operating procedures but also the concepts behind temperature losses will enable you to produce energy-yield assessments that are more robust in practice.