6 Loss Settings in the PVSyst Manual That Are Easy to Overlook

Table of Contents

• Why you should check the loss settings in the PVSyst manual

• Easily overlooked loss setting 1: Fouling loss

• Easily Overlooked Loss Setting 2: Temperature Loss

• Easily overlooked loss setting 3: Wiring loss

• Easily overlooked loss setting 4: mismatch loss

• Often overlooked loss setting 5: angle-of-incidence loss

• Easily overlooked loss setting 6: Losses related to stoppages and operating rate

• Practical procedure for confirming loss settings

• Key points to keep in mind when reviewing analysis results

• To master the PVSyst manual for practical use

Why you should check the loss settings in the PVSyst manual

In simulations of photovoltaic power generation systems, the energy output is not determined solely by solar irradiance and installed capacity. Actual generation is affected by various factors such as module soiling, temperature rise, voltage drops in cables, component variability, shading, and downtime. The purpose of using PVSyst is not simply to produce a generation figure, but to organize which conditions affect the energy output and by how much, and to create a basis that can be used for design and project planning.

However, even when working while consulting the PVSyst manual, the loss settings are prone to being overlooked. This is because the loss settings are spread across multiple screens and calculations will proceed even with the default values. Since no errors are shown on the screen, users may assume they have entered values or may produce reports without fully checking the meaning of the configured settings.

Where beginners particularly tend to stumble is that loss settings can look like "minor adjustment items." In reality, loss settings are a crucial element that determines the reliability of simulation results. For example, if soiling losses are underestimated, the annual energy production will come out higher. If temperature losses are not properly reflected, the drop in output during summer will be underestimated. If wiring losses are set smaller than they actually are, there is a risk of overestimating the efficiency of the entire system.

When reading the PVSyst manual, you need to be aware not only of how to operate the software but also of what each setting means and which conditions it affects. If you focus only on filling in the input fields on the screen, you may overlook the underlying assumptions that influence energy production. Conversely, understanding the rationale behind loss settings makes it easier to explain simulation results and increases your persuasiveness when making design changes or conducting project feasibility assessments.

Easily Overlooked Loss Setting 1: Soiling Loss

Soiling loss is a setting that represents the reduction in incident irradiance caused by dust, pollen, yellow sand (Asian dust), bird droppings, fallen leaves, exhaust-derived dirt, and other contaminants adhering to the surface of photovoltaic modules. When progressing through the settings while referring to the PVSyst manual, this item is easy to enter, so it is common to proceed with the default value without much thought. However, the impact of soiling can vary significantly depending on the installation environment.

At ground-mounted power plants, having unpaved roads, farmland, development sites, quarries, factories, logistics facilities, and similar surroundings makes them more susceptible to the effects of dust. Even for rooftop installations, modules can accumulate dirt in areas with heavy local traffic or in environments where exhaust tends to stagnate. In addition, modules with a low tilt angle are less effectively cleaned by rainwater, so dirt tends to remain.

The point to note about soiling losses is whether they should be considered as a uniform annual value or allowed to vary seasonally. During the rainy season and periods of heavy rainfall, dirt is more likely to be washed away, whereas in dry seasons and periods with high levels of pollen or yellow sand, soiling tends to accumulate. In regions with snowfall, there can be a cleaning effect from snow, but after the snow melts mud and dust may remain. In this way, soiling losses need to take into account both regional characteristics and maintenance planning.

When checking the PVSyst manual, pay attention not only to the soiling loss input field but also to how those values are determined. Organizing the local environment, cleaning frequency, nearby dust sources, module tilt angle, and rainfall conditions before setting them makes it easier to explain the results. For simulations used in business planning, it is safer to create and compare not only optimistic values but also conservative cases.

If you overlook soiling losses, it will become difficult to explain why actual energy production is lower than the simulation. Especially for projects intended for long-term operation, you need to consider not only the first year but also soiling and maintenance regimes several years down the line. To make PVSyst results predictions that are close to real-world operation rather than mere theoretical values, it is important not to underestimate soiling losses.

Often Overlooked Loss Setting 2: Temperature Loss

Temperature loss is a setting that represents the effect of reduced output caused by the temperature rise of a solar module. In solar power generation, it is commonly perceived that stronger sunlight leads to more power generation, but when module temperature increases, output decreases. Especially in summer and in high-temperature regions, even under good irradiance conditions, temperature loss can be large and the generated power may not increase as much as expected.

Reading the PVSyst manual makes it clear that temperature-related settings are linked to the system’s installation configuration and heat dissipation conditions. When a module is mounted on a ground-mounted rack with good rear ventilation versus mounted close to a roof surface where heat tends to accumulate, the way the module temperature rises is different. In other words, even in the same region, with the same module and the same solar irradiance, temperature losses vary depending on the installation method.

An often-overlooked issue is the ventilation conditions for rooftop installations. When the clearance from the roofing material is small, air has difficulty flowing behind the modules, and module temperatures tend to be higher. The thermal environment varies depending on the mounting method—trapezoidal metal roofs, slate roofs, pitched roofs, flat-roof racking, etc. Simply selecting the standard temperature conditions in PVSyst may not adequately reflect the actual installation state.

When verifying temperature losses, the temperature conditions in the meteorological data are also important. If the meteorological data used are not representative, deviations will occur in the predicted annual power generation. Urban areas can be affected by the heat island effect, and their temperature trends differ from those of coastal or mountainous areas. It is necessary to confirm that you have selected meteorological conditions close to the site and are not using data from locations that are extremely distant.

Temperature-related losses also affect system comparisons. If modules have different temperature coefficients, the degree of output reduction at high temperatures will differ. Simply comparing rated output alone can lead to misjudging the actual difference in annual energy production. When comparing multiple scenarios in PVSyst, confirm that the temperature conditions are assumed to be the same and that the module characteristics are correctly reflected.

Temperature loss is a very basic cause of reduced power generation, yet it is an item that is easily overlooked in configuration screens. In particular, for rooftop installations, self-consumption systems, high-temperature regions, and installation conditions with poor ventilation, it is important to always check how temperature loss is treated and how it affects the results.

Commonly Overlooked Loss Setting 3: Wiring Loss

Wiring losses are the losses that occur in the process of transmitting power from the solar modules to the power conditioner, junction boxes, collection equipment, and the like. Because cables have electrical resistance, when current flows a voltage drop and heat generation occur, reducing the amount of power that can be effectively utilized. When configuring a system while consulting the PVSyst manual, wiring losses may be present as an input item, but they are often left as approximate values in the early stages of design.

The key factors affecting wiring losses are cable length, conductor cross-sectional area, current value, and circuit configuration. As the scale of a power plant increases, the distance from the strings to the collection equipment tends to become longer, and wiring losses become non-negligible. Even for rooftop installations, the location of the power conditioner is influenced by the positions of the distribution board and incoming power equipment, so cables can end up longer than anticipated.

What is easy to overlook is the difference between the initial layout proposal in the early design stage and the final construction drawings. Even if PVSyst inputs assume short cable lengths, actual cable lengths can increase when detours in wiring or changes to conduit routing occur during detailed design. This is especially true for roofs of existing buildings, factories designed for on-site consumption, or projects spanning multiple buildings, where wiring routes tend to become complex and the gap from the initial assumptions can be large.

Wiring losses must be considered on both the DC and AC sides. On the DC side, the distance to the strings and to the combiner/junction boxes affects the losses, while on the AC side the distance from the power conditioner (inverter) to the point of connection (point of common coupling) has an impact. Focusing on only one side will lead to underestimating the total system losses. When consulting the PVSyst manual, it is important to understand which range of the system the wiring loss input refers to.

Also, when setting wiring loss, estimating it too low tends to overstate generation, while estimating it too high can lead to an overly strict evaluation of the project's viability. In practice, it is desirable to use approximate values during the basic design phase and to recalculate later to reflect cable routes and conductor types as detailed design progresses.

Wiring losses are closely linked to construction conditions and layout planning. Rather than leaving them solely to the simulation engineer, comparing them with information from electrical design, construction planning, and on-site surveys makes it possible to set values that are closer to reality. It is important not to look only at the numbers from PVSyst but to check the wiring plan behind them.

Commonly Overlooked Loss Setting 4: Mismatch Loss

Mismatch loss is the loss that occurs due to performance differences among modules or strings. Even photovoltaic modules of the same model do not have identical output characteristics because of manufacturing variations. In addition, if irradiance, temperature conditions, or shading patterns differ between strings, the overall optimal output may not be achieved.

When progressing through settings while reading the PVSyst manual, mismatch losses can seem technical and are often left at their default values. However, they are a factor that affects actual energy production, and particular care is needed for projects that combine complex roof shapes or surfaces with different orientations. Treating east–west faces, south-facing surfaces, roofs with different tilt angles, or some strings affected by shading as if they share the same conditions can fail to adequately reflect the impact of mismatch.

What is easy to overlook about mismatch losses is that it’s not just the variability of the modules. Differences in string length, differences in connection points, variations in solar irradiance conditions, partial shading, and variability in aging-related degradation also contribute. Even if you think you have neatly aligned the string configuration at the design stage, in reality conditions may not be uniform due to the effects of roof obstacles, equipment, adjacent buildings, railings, chimneys, outdoor air-conditioning units, and similar influences.

When checking mismatch losses in PVSyst, it is important to treat the layout and the electrical connection relationships separately. Even if the visual arrangement looks tidy, if strings connected to the same MPPT have different conditions, power generation efficiency will be affected. Especially when dealing with roofs of multiple orientations, grouping them under the same input conditions can produce results that appear better than reality.

Another caveat is to avoid treating the effects of partial shading solely as mismatch losses. Shading influences performance not just by shaded area ratio, but also by which module is shaded, at what times, and in what manner. If shading settings and mismatch loss settings are considered separately, they can end up being double-counted or incomplete. You should consult the PVSyst manual and clarify the relationship between shade analysis, string configuration, and mismatch losses.

Mismatch losses may appear to be a small percentage if you only look at the input values. However, in complex projects they can show up as differences in annual energy generation and monthly generation. Especially for rooftop installations and sites with many constraints, it is important not to underestimate mismatch losses.

Frequently Overlooked Loss Component 5: Angle-of-Incidence Loss

Incidence angle loss describes the effect in which sunlight strikes the module surface at an oblique angle, increasing reflection and reducing the amount of energy that can be received. When sunlight strikes the module at an angle close to perpendicular it can be received efficiently, but in the morning and evening, during winter, or when the tilt angle and azimuth are not optimal, losses due to the angle of incidence become large.

In the PVSyst manual, losses related to the angle of incidence are treated as a specialized topic, so they can be difficult for beginners to understand. In many cases they are calculated using the standard model and default values, but installation conditions can affect the results. It is particularly important to check incidence angle losses for special installations such as east–west mounting, low-tilt mounting, near-vertical mounting, or building-integrated installations.

What is easy to overlook with incidence angle losses is the combination of azimuth and tilt. For common south-facing installations with an appropriate tilt, the treatment of losses can be considered relatively standard. However, when roof shape or site conditions necessitate using east- or west-facing surfaces, the share of generation in the morning and evening increases and the impact of the incidence angle changes. On low-tilt flat roofs, the effect of solar altitude differs between summer and winter, and this also affects the annual generation curve.

Also, incidence angle losses need to be considered together with the effects of soiling and shading. At low solar elevations in the morning and evening, shadows tend to lengthen and the incidence angle becomes more severe. In winter, because the solar elevation is lower, the way losses appear changes seasonally even for the same installation. When reviewing PVSyst results, it is important not only to look at annual energy yield but also to check monthly generation and loss diagrams to understand when losses are greatest.

Incidence angle loss is an item that can be difficult to explain to on-site personnel and sales staff. Simply saying "loss due to angle" doesn't convey the idea well, so explaining it in terms such as when sunlight strikes at an oblique angle reflection increases; it tends to have a greater impact in the mornings, evenings, and during winter; and it varies depending on the installation azimuth and tilt angle makes it easier to understand.

When using the PVSyst manual to verify settings, do not treat incidence-angle losses as an isolated, standalone parameter; instead, check them together with the installation azimuth, tilt angle, seasonal generation trends, and shading effects. This will enable you to explain the validity of the simulation results more concretely.

Easily Overlooked Loss Setting 6: Losses Related to Downtime and Operating Rate

Losses related to downtime and availability are settings to reflect that equipment does not always operate ideally. Solar power generation systems automatically generate power when there is sunlight, but in practice there may be periods when they cannot generate power due to inspections, failures, communication anomalies, power conditioner shutdowns, grid-side constraints, output control, maintenance work, etc.

When creating a simulation while consulting the PVSyst manual, this type of loss tends to be deferred, because it appears to be an operational factor compared with system capacity, module layout, and irradiance conditions. However, for project planning and revenue forecasting, consideration of downtime and availability is extremely important. If you do not take into account not only how much the system can theoretically generate but also how long it can actually continue operating, your forecasts will not be realistic.

One thing that is easy to overlook is treating downtime losses as a single small value. Shortly after commissioning, initial equipment defects may occur, and during long-term operation equipment will deteriorate, require replacement, or need inspection work. If the remote monitoring system is weak, it can take longer to detect failures, which may increase downtime losses. The risk of downtime varies depending on the size of the power plant, the maintenance regime, the equipment configuration, and site accessibility.

In projects where output curtailment is anticipated, generation may be curtailed by grid-side constraints in addition to simple equipment outages. How this is handled varies by project, but it is at least a factor that cannot be ignored in business viability assessments. When using PVSyst simulation results, it is necessary to clearly define which parts are treated as technical losses and which are managed separately as business conditions.

Losses related to downtime and availability may not stand out prominently in reports. However, they have a cumulative effect on long-term power generation forecasts. Even a few days of downtime per year can affect revenue from electricity sales and the benefits of self-consumption. In particular, for materials prepared for financial institutions, investment decision documents, and internal approval documents, it is important to be able to explain the assumptions regarding availability.

If you use the PVSyst manual in practice, you should not treat downtime losses as mere input items; instead, you need to consider them in conjunction with maintenance planning, monitoring arrangements, fault-response procedures, equipment warranties, and replacement planning. Making loss settings that reflect real operational conditions improves the reliability of the simulation results.

Practical steps for checking loss settings

When checking loss settings in PVSyst, it is important not to start by immediately entering numbers on the screen; instead, proceed after organizing the project conditions. The items to verify first are the installation site, installation method, surrounding environment, system capacity, module layout, power conditioner configuration, wiring plan, and maintenance arrangements. If you perform loss settings while this information remains unclear, the basis for the figures will be weak.

In practice, the assumptions can change between the basic design stage, the detailed design stage, before construction, and after completion. During the basic design stage, it is acceptable to use approximate values, but it is necessary to record that those values are estimates. As the detailed design progresses, it is desirable to update the simulations to reflect wiring lengths, equipment layouts, shading conditions, maintenance plans, and so on.

When checking loss settings, it is important not only to look at each item individually but also to verify that there are no overlaps or omissions. For example, even if you set the effects of shading in detail, if another loss item also adds elements related to shading you may end up double-counting the loss. Conversely, if operational factors such as soiling or downtime losses are not considered at all, the generation estimate will be overly optimistic.

Also, when you change loss settings, always verify their impact on the results. Don’t just enter numbers and stop; it’s important to check how annual energy production, monthly energy production, loss diagrams, performance ratio, and so on have changed. If a small change to a particular loss item causes large changes in the results, that item is an important assumption for the project. In explanatory materials, you should clearly state why that value was adopted.

When reading the PVSyst manual, it becomes easier to understand if you not only follow the step-by-step on-screen procedures but also take notes on the meaning of the settings as you go. Soiling losses are related to the local environment, temperature losses to the mounting method, wiring losses to the electrical design, mismatch losses to the string configuration, incidence-angle losses to orientation and tilt angle, and stoppage losses to the operational regime. By linking each loss to practical, on-site information in this way, the simulation becomes a tool for design decisions rather than merely software operation.

Points to note when reviewing analysis results

After running a simulation in PVSyst, you need to verify not just the final annual energy production but also which losses are impacting it and by how much. Annual energy production is an easy-to-understand indicator, but that figure alone cannot determine the validity of the settings. What matters is whether the breakdown of losses aligns with the project conditions.

For example, if temperature losses are too small despite installation in a high-temperature area or close to roof contact, the installation conditions may not have been correctly reflected. If soiling losses are expected to be negligible in a dusty region, the forecast may be more optimistic than actual operation. Likewise, in large-scale projects with long cable routes, if wiring losses are too small, you should verify consistency with the design conditions.

Checking monthly energy production is also important. Even if the annual figures look reasonable, the monthly breakdown can reveal unnatural patterns in winter or summer. If generation is too high in winter when shading and incidence angle effects should be significant, or if temperature losses in summer are smaller than expected, those inconsistencies are a cue to revisit the settings. PVSyst results should not be accepted at face value; it is important to interpret them against the actual site conditions.

The performance ratio is another metric you should check. The performance ratio is an indicator of how efficiently a system is generating power relative to solar irradiance conditions. If the performance ratio is extremely high, the loss settings may be insufficient. Conversely, if it is extremely low, you need to check for shading, temperature, wiring, equipment selection, setting errors, and so on. Because the performance ratio is also used for project comparisons, be prepared to explain the background behind the value.

Also, before submitting the report, we recommend organizing the configuration assumptions separately. The PVSyst output report alone may not sufficiently communicate why those loss values were chosen. For internal review and client explanations, briefly summarizing the basis for soiling losses, the rationale for temperature conditions, the assumptions regarding wiring losses, and the treatment of downtime losses will also be helpful when reviewing the report later.

When reviewing analysis results, it is important not to aim to produce favorable numbers. Simulations are not work to make a project look good, but work to visualize risks and assumptions. By carefully checking loss settings, the reliability of power generation forecasts increases, and this also leads to improvements in design and operation.

Mastering the PVSyst Manual for Practical Use

The loss settings in the PVSyst manual that are easy to overlook are six: soiling loss, temperature loss, wiring loss, mismatch loss, incidence-angle loss, and losses related to downtime and availability. Each of these may seem like a small item on its own, but when multiple losses accumulate they can have a significant impact on annual energy production and project viability assessments.

To master PVSyst, it's important to use the manual not merely as an operating instruction, but as a guide for checking design conditions. You need to understand not only what to enter on each screen, but also which site conditions those values represent and which results they affect.

In practice, it is especially important that simulation engineers, design engineers, construction personnel, and maintenance personnel share information. Soiling loss is related to the local environment and cleaning schedule, temperature loss is related to the mounting method and ventilation conditions. Wiring loss is determined by the electrical design and cable routing during installation, and mismatch loss is affected by string configuration and shading. Incidence-angle loss is related to orientation and tilt angle, and downtime loss is linked to maintenance arrangements and monitoring systems.

When viewed this way, PVSyst's loss settings are not simply numbers entered into the software; they are checkpoints to verify the design quality of a photovoltaic power generation system. Carefully setting losses is not meant to underestimate the energy output. Rather, it is work to reflect conditions close to reality and to produce a simulation that can be explained later.

For beginners, it's recommended to first create a simulation using standard settings and then check the loss items one by one. If you try to understand everything perfectly from the start, you can easily become confused by the many screens and terms. By checking from the six perspectives of soiling, temperature, wiring, mismatch, angle of incidence, and shutdown, you can reduce important oversights.

Ultimately, aim to be able to explain from PVSyst results not only "how much electricity is generated" but also "why that amount was produced." By organizing the rationale for the loss settings and cross-checking them against the project conditions, the reliability of the simulation is greatly increased. If you use the PVSyst manual, it is important not only to memorize the operating procedures but also to understand the meaning of the loss settings and interpret the results in a form that can be used for practical decision-making.