4 Ways to Read PVSyst Temperature Losses｜Organizing the Effects During High Temperatures

In solar PV design and energy-yield forecasting, attention tends to focus on visually obvious factors such as shading and orientation, while temperature losses are often overlooked. However, in PVSyst, temperature is treated as a fundamental factor that determines array output, and cell temperature is regarded as one of the basic inputs to the one-diode model. In other words, temperature losses are not a minor adjustment applied afterward, but part of the fundamental assumptions of energy production.

Especially for practitioners searching for information on "how to read PVSyst", it can easily become unclear how to interpret temperature losses. Understanding only that losses increase because the ambient temperature is high is not sufficient. PVSyst’s thermal model calculates array temperature using the irradiance incident on the module surface, module efficiency, ambient temperature, and heat‑dissipation conditions, and it evaluates how that temperature affects output. In other words, what you should look at is not "air temperature" alone but "under which conditions the cell temperature rises to what level, and as a result how much the output decreases."

Also, PVSyst's official documentation explains that while a module's nominal performance is specified at 25℃, in actual operation the array temperature becomes higher, and that difference constitutes the temperature loss. The overview indicates that the power decrease due to temperature is roughly in the range of -0.2〜-0.4%/℃, and even from that alone it is clear that in summer or in installations with poor ventilation it is difficult to ignore.

This article explains how to interpret PVSyst's temperature losses in practice, organized into four perspectives. First, read them as array temperature rather than ambient air temperature; next, confirm the assumptions about Uc/Uv and the installation method; further, interpret the impact at high temperatures using monthly results and TempLoss; and finally, separate the temperature coefficient from the assumptions of the thermal model. It also summarizes common points of misunderstanding and how the accuracy of on-site condition assessment affects confidence in the temperature loss estimates.

Table of Contents

• What to know before reading temperature loss

• Reading method 1｜Read as array temperature rather than ambient temperature

• Reading method 2｜Read based on the assumptions of Uc/Uv and the mounting configuration

• Reading method 3｜Read the impact of high temperatures using monthly results and TempLoss

• Reading method 4｜Read by separating the temperature coefficient and the loss rate

• Common misconceptions

• The accuracy of on-site conditions affects confidence in temperature loss

• Summary

What you should know before reading about temperature loss

When reading temperature losses in PVSyst, the first thing to note is that temperature losses are not losses that occur simply because the ambient air temperature is high. In the official help, as a thermal equilibrium concept, cell temperature is calculated from the incident irradiance, absorptance, module efficiency, and heat transfer coefficient. Written in equation form, the cell temperature is the temperature at which incoming energy and heat dissipation by cooling balance. PVSyst states that, for simplification, it uses GlobInc as the incident irradiance, while explaining that in reality the effective irradiance GlobEff is involved.

In other words, even at the same ambient temperature, the actual array temperature will vary depending on whether it’s a period of strong solar radiation, whether wind can pass through, or whether the mounting method leaves the rear open. Conversely, even if the ambient temperature isn’t that high, if solar radiation is strong and heat dissipation is poor, cell temperature can rise significantly. If you don’t understand this mechanism, you may be tempted to dismiss large summer losses as simply “because it’s hot,” and easily overlook opportunities to improve installation and heat-dissipation conditions.

It is also important that PVSyst’s temperature model has a two-layer structure. First, a steady-state temperature TArrSS is obtained from a heat balance model, and then the actual transient temperature TArray is calculated with an inertia model that takes heat capacity into account. Thus, temperature losses are not a simple static correction but the result of a model that temporally follows changes in meteorological conditions. When assessing temperature losses in practice, understanding them as outputs of these temperature calculations rather than as mere coefficients reduces misinterpretation.

Furthermore, the result variable TempLoss is provided independently as "PV loss due to temperature". In other words, PVSyst explicitly treats temperature loss as one of the major losses alongside shading and wiring. By checking how much this value influences the Loss Diagram and the simulation variables, it becomes easier to quantitatively track the impact during high-temperature conditions.

Reading 1｜Read as array temperature, not ambient temperature

The initial reading is to treat temperature losses as the array temperature rather than the ambient temperature. In PVSyst’s thermal balance equation, the cell temperature is expressed as the ambient temperature Tamb plus an increase determined by irradiance and heat-dissipation conditions. In short, ambient temperature is the starting point, not the final value that directly determines losses. Actual losses occur depending on how much Tcell or TArray exceed 25 ℃.

This interpretation is important in practice because temperature loss can differ substantially even for projects with similar ambient temperatures. For example, even two projects in the same area can have different array temperatures—one might be a free-standing installation with good airflow under the racking, while the other might have limited rear ventilation—so even with the same external air temperature conditions the magnitude of TempLoss will differ. If you look only at the summer results and conclude "this site is hot so it can't be helped," you'll miss differences that originate from installation conditions.

Also, because PVSyst allows you to view temperature losses by month and by hour, you can notice that months with high ambient temperatures do not necessarily coincide exactly with months that have the largest losses. When strong solar irradiance, weak ventilation, and high ambient temperature occur together, losses tend to be larger; conversely, even when ambient temperature is high, during periods or times with weak irradiance losses may not be as large as expected. That is why it is important to track array temperature results together with TempLoss, rather than relying solely on a time series of ambient temperature.

A practical tip is that when you look at temperature loss, first ask whether the loss is due to the ambient temperature itself or whether poor heat dissipation is also contributing. Simply by asking this one question, it becomes easier to determine whether there is room for the design side to address it or whether it should be accepted purely as a climatic condition. Temperature loss is not a figure that reflects only natural conditions; it also mirrors installation conditions.

How to Read 2 | Interpreting from the Assumptions of Uc/Uv and Installation Method

A second way to interpret this is to check the assumptions about Uc/Uv and the mounting configuration before looking at the loss rate itself. In PVSyst’s thermal model, ease of heat dissipation is expressed as U = Uc + Uv × WindVel. Uc is the basic heat transfer coefficient that does not depend on wind speed, and Uv is the contribution to heat transfer that increases with wind speed. The official help notes that using Uv is difficult and meteorological wind speed data are often not sufficiently reliable, so in practice it is recommended to set Uv = 0 and include the effect of average wind in Uc.

What is important here is that the value of Uc strongly depends on the mounting method. In the PVSyst tutorial, example proposals are given: Uc=29 W/m²K for fully free ventilation, Uc=20 W/m²K for a semi-embedded mounting with an air layer at the rear, and Uc=15 W/m²K for an integrated mounting close to rear insulation. In other words, when looking at temperature loss you should not just look at "what percentage TempLoss is," but check "under what heat dissipation assumptions that TempLoss was calculated," otherwise you will misjudge the significance of the numbers.

This point is very easily misunderstood in practice. For example, even if TempLoss looks large in a given case, if Uc is set relatively low because the rear is installed in a fairly enclosed condition, that may be a natural result given the design assumptions. Conversely, if Uc is set excessively low for a case that should be close to free ventilation, the temperature loss may be overestimated. In other words, temperature loss should be read not just as a number but together with the assumptions of the thermal model.

Furthermore, the official tutorial states that NOCT is presented only for comparison and reference and does not have great direct relevance to actual simulations. In practice, what you should really look at is not the apparent NOCT but the consistency between the Uc/Uv values adopted within PVSyst and their installation conditions. When evaluating temperature losses, it is important not to be overly pulled by catalog NOCT values and to return to the assumptions of PVSyst’s thermal model.

How to Read 3｜Interpreting Monthly Results and High-Temperature Effects with TempLoss

The third way to interpret it is to read TempLoss not as a single annual rate but in conjunction with the monthly results. PVSyst’s result variables include TempLoss, and it can be checked on a monthly basis as well. Viewed on an annual basis it appears as a single consolidated value, but in reality the effect varies considerably by season. If you want to clarify the impact during high-temperature periods, you must check not only the annual value but also which months show stronger effects.

One of the most typical practical cases is when power generation increases from spring to early summer, but in midsummer the increase is subdued relative to the solar irradiance conditions. If you look only at annual generation, the discrepancy is hard to spot, but when you look at monthly TempLoss it becomes clear that temperature losses expand during the period when high temperatures coincide with strong solar irradiance. In other words, instead of misinterpreting the summer slowdown as a "lack of solar irradiance," you can read it as "losing out because of temperature."

Also, by looking at monthly results, it becomes easier to question whether the assumptions about the installation method are reasonable. For example, if a case that should be close to naturally ventilated shows an extreme TempLoss only in midsummer, you would want to recheck the Uc settings and how the wind conditions were represented. Conversely, if the installation method has restrictive rear-side conditions, it is reasonable to expect TempLoss to be somewhat large during the summer. Monthly results are useful for cross-checking the thermal model’s assumptions and outcomes.

A practical way to look at this is to first examine the peaks and troughs of monthly power generation, then overlay TempLoss to confirm that months with higher solar irradiance do not necessarily produce more power. Being able to read it this way makes it easier to explain per-project high-temperature risk. It is easier to organize the impact of high temperatures if you interpret temperature loss not only as "what % over a year" but also in terms of "how it affects summer."

Reading 4 | Separate Temperature Coefficient and Loss Factor

The fourth way to read it is to separate the module temperature coefficient from the temperature loss rate that appears in PVSyst. In the official documentation, muPmpp is described as the temperature-dependent slope of the maximum power point Pmpp, and is explained as being close to the value commonly called Gamma. PVSyst applies a correction called muGamma as needed, adjusting muPmpp to be closer to actual measurements. In other words, the temperature coefficient is a concept close to the module-specific "power slope per 1℃".

On the other hand, PVSyst’s TempLoss is not determined solely by that coefficient. It manifests as the accumulated result of how high the actual array temperature rose, how long it operated in that temperature range, and how much solar irradiance was present. Therefore, even modules with similar temperature coefficients will have different TempLoss if the mounting method or ventilation conditions differ. Conversely, using modules with different temperature coefficients in the same mounting arrangement will change the way the loss rate appears. Confusing the two makes it easy to choose the wrong improvement measures.

In practice, people sometimes simplistically assume that a module with a small temperature coefficient will therefore have small temperature losses. However, that alone is insufficient. The temperature coefficient is the module's sensitivity, and TempLoss is the result of that sensitivity applied to the actual operating temperature. In other words, you need to separate and examine both the "strength of the module" and the "severity of the installation conditions." Once you can read these separately, you can avoid mixing discussions of module selection and installation design.

Also, PVSyst’s one-diode model handles temperature dependence internally, and the way muPmpp is treated affects both sizing and energy production calculations. For that reason, rather than splitting them into “look at the temperature coefficient because it’s a catalog value” and “look at TempLoss because it’s a result,” it’s easier to understand if you organize them as the temperature coefficient being on the input side and TempLoss on the result/output side. If you want to clarify the effects at high temperatures, this separation is very important.

Common Misunderstandings

A common misunderstanding about temperature losses is to simply think, "a month with high ambient temperatures = a month with large temperature losses." In reality, PVSyst's thermal model determines cell temperature using solar irradiance, efficiency, and the heat transfer coefficient together. Therefore, losses are not determined by ambient temperature alone. With high irradiance and poor ventilation, temperature losses tend to be larger, and even if ambient temperature is high, if irradiance is weak the losses may not increase as much as expected.

Another common misconception is to take the Uc/Uv settings lightly as "they're advanced settings so they won't have much effect." In the official tutorial, thermal parameters are listed as a representative checkpoint of detailed losses, and recommended values are even provided. In other words, this is not merely decorative; it is a primary factor in determining TempLoss. If you enter a Uc that does not match the installation method, both annual and monthly results may appear skewed.

Moreover, treating NOCT as if it were the most important indicator is a common misconception. The PVSyst tutorial explains that NOCT is a value defined under very specific surrounding and operating conditions and has little real relevance to actual simulations. In practice, what you should look at is not the NOCT number itself but the thermal model assumptions adopted by PVSyst and the TempLoss that results from them.

The accuracy of on-site conditions influences the credibility of temperature loss

To correctly interpret temperature loss in a desktop evaluation, accurate knowledge of on-site conditions is critically important. The thermal model depends heavily on assumptions about the installation method — for example, whether it is freely ventilated, semi-embedded, or whether the rear is close to insulated. In other words, if site information such as the clearance between the roof and the module, how the rear is ventilated, the distance to surrounding equipment, and wind paths is unclear, there will be uncertainty in how to set Uc/Uv, which in turn weakens confidence in TempLoss.

In practice, even if drawings show free ventilation, roof upstands, nearby equipment, cable racks, parapets, and the like can actually alter airflow. Conversely, projects that were thought to be nearly integrated with the roof may unexpectedly have rear clearance. These differences can change the expected temperature loss. In other words, temperature loss is not only a Meteo issue but also an issue of how the installation interfaces with the site.

In that sense, having a means to accurately grasp on-site positional relationships is also effective for refining the assumptions behind temperature losses. If equipment layout, distances to obstacles, rear clearance, and orientation can be organized more precisely, confidence in the thermal model increases. Naturally related here is LRTK, an iPhone-mounted GNSS high-precision positioning device used to capture on-site positional relationships with high accuracy. By making it easier to carry out high-precision on-site position checks and to clarify relationships between equipment and obstacles, it becomes easier to tighten the assumptions for PVSyst’s temperature losses. In practical work where you want to avoid leaving the effects of high temperatures as desk-calculated numbers and instead translate them into site-based decisions, such high-precision on-site understanding is helpful.

Summary

When reading PVSyst temperature losses, first think in terms of array temperature rather than ambient temperature, then verify the assumptions for Uc/Uv and the mounting configuration, after that trace the effects at high temperatures using the monthly TempLoss, and finally distinguish the module temperature coefficient from the loss rate. Simply mastering these four ways of reading makes temperature loss appear not just as a “loss that increases in summer” but as an indicator reflecting the quality of the design and installation conditions.

The important point is not to stop at viewing TempLoss as a single annual rate. PVSyst’s temperature loss is built on multiple assumptions—thermal equilibrium model, thermal inertia model, Uc/Uv, muPmpp, and so on. That is why, when reading the numbers, you need to go back to the assumptions and then return to the monthly breakdown after confirming those assumptions. Once you can make that back-and-forth, the accuracy of design reviews, customer explanations, and comparative evaluations improves considerably.

And to make that interpretation even more certain, it is essential to grasp on-site positional relationships with high precision. If you want to organize equipment layout, back-side conditions, and relationships with obstacles more accurately, it can be useful to leverage LRTK, an iPhone-mounted GNSS high-precision positioning device. By combining the ability to correctly read PVSyst temperature losses with the ability to accurately capture the site, it becomes easier to arrive at more convincing high-temperature evaluations and design decisions.