Six Easily Overlooked Points in PVSyst Loss Settings

Table of Contents

• Why PVSyst loss settings affect the reliability of generation forecasts

• Point 1 Treating weather conditions and loss settings separately

• Point 2 Estimating temperature-related losses uniformly

• Point 3 Underestimating soiling losses compared to on-site reality

• Point 4 Underestimating wiring and electrical losses

• Point 5 Not fully reflecting shading losses and the impact of layout conditions

• Point 6 Reusing loss settings without reviewing them for each project

• An approach to linking PVSyst loss settings to on-site accuracy

Why PVSyst loss settings affect the reliability of generation forecasts

When running generation simulations with PVSyst, many practitioners first look at annual or monthly generation figures. However, what determines how close those numbers are to reality is not only the system capacity or orientation. In practice, the approach to loss settings has a major influence on the overall credibility of the results. Even projects that appear to have favorable irradiance conditions and a tidy layout tend to produce optimistic forecasts if loss assumptions are too lenient.

PVSyst allows simulations to be built by carefully organizing conditions, but that also means the modeller’s assumptions are readily reflected in the numbers. In other words, the same project can produce markedly different impressions depending on how realistically loss settings are set. In practice, it is more important to produce numbers that are easy to justify later and useful for design and business decisions than to make attractive-looking figures. In that sense, loss settings are not mere correction values but part of the backbone of the simulation.

Loss settings cannot be corrected by adjusting a single item. Temperature, soiling, wiring, shading, equipment variability, and operational conditions all accumulate to reduce final generation. The troublesome part is that, although each factor may seem small, together they can create differences that are not negligible. Therefore, when handling loss settings in PVSyst, you should not finish by tweaking a single value but instead check whether the overall approach to losses is appropriate for the entire project.

Many readers searching for PVSyst are probably those who want to improve simulation accuracy or produce figures that can withstand internal review. For them, knowing in advance the commonly overlooked points in loss settings is very practical. Below, I summarize six points that are particularly easy to miss in practice, explain why they are overlooked, and discuss how to think about them.

Point 1 Treating weather conditions and loss settings separately

A common oversight in loss settings is treating weather conditions and loss settings as separate things. In PVSyst, users often set the site, irradiance conditions, and temperature conditions first and then consider losses, which can mentally separate “weather” from “losses.” In reality, the two are strongly linked. The environment in which the equipment operates affects both the magnitude and the nature of expected losses.

For example, even with the same system configuration, temperature-related impacts differ in locations with different temperature conditions, and differences in seasonal irradiance distribution change how monthly losses appear. Nevertheless, applying loss settings used in one project directly to another site creates a mismatch in simulation assumptions. A common mistake in PVSyst is changing only the site conditions per project while leaving the loss settings from a previous project unchanged. That produces superficially neat numbers but undermines practical reliability.

The countermeasure is to treat loss settings not as a fixed set of values but as items to be adjusted according to the site environment. After inputting weather conditions, always consider how those conditions relate to temperature, soiling, and operational impacts; doing so will improve the quality of loss settings. Especially when comparing multiple candidate sites, if you do not confirm that loss assumptions are appropriate for differences in weather, you may end up comparing assumption differences rather than design differences.

When reviewing results, check not only annual generation but also monthly trends and the loss breakdown to verify the consistency between weather conditions and loss settings. PVSyst loss settings do not stand alone. They become meaningful only in the context of the site’s characteristics, so thinking of weather and losses as a single flow is the first step to avoiding oversights.

Point 2 Estimating temperature-related losses uniformly

Temperature-related losses are one of the loss items most easily overlooked. The reason is simple: temperature effects are not visually obvious and can differ slightly from the field engineer’s intuition. While differences in site layout or orientation are easy to imagine, how much temperature affects results is hard to grasp without organizing the numbers. As a result, PVSyst users sometimes leave temperature settings at their defaults or fail to reflect project-specific differences.

In reality, temperature-related losses vary with the installation environment. Airflow, surrounding environment, equipment packing density, and ground conditions all influence how much heat the system experiences. Even within the same region, different installation methods can change the character of results. When comparing outputs in PVSyst, attention tends to focus on orientation and shading differences, but the way temperature is considered can quietly influence comparison results.

To avoid this oversight, do not treat temperature loss as a uniform fixed value. Imagine the local conditions for each project and revise the settings based on the environment in which the equipment will operate. When handling several similar projects, it is tempting to reuse previous settings, but if installation environments differ, the approach to temperature-related losses should also be re-evaluated. Temperature losses may not look like a large difference on their own, but accumulated they affect the reliability of annual results.

Revising temperature settings also strengthens your explanations. You can explain underperformance not only by irradiance or shading but also by installation environment. To master PVSyst loss settings in practice, treat temperature as an important factor that reflects installation conditions rather than as a mere supplementary item.

Point 3 Underestimating soiling losses compared to on-site reality

Soiling losses are another typical point easily overlooked in PVSyst. Although they appear as a single input item, they are closely linked to site conditions and maintenance policy. Under time pressure, users tend to set soiling losses conservatively low or reuse the same values from previous projects. The result can be an annual generation estimate that is higher than reality and difficult to justify later.

Soiling impacts vary considerably by site: ground conditions nearby, the tendency to generate dust, rainfall patterns, and ease of maintenance all play roles. In short, soiling loss is not suitable for a simple uniform setting. Nevertheless, initial project stages often treat it lightly because the number is only a rough estimate. Yet initial estimates frequently become the baseline for later comparisons and internal explanations, so optimistic assumptions here can skew overall decisions.

As a countermeasure, consider maintenance and management when entering soiling losses. Think about how quickly soiling accumulates at the site and how often maintenance is expected; this will make your assumptions more realistic. PVSyst loss settings are not only about equipment performance calculations but also about how much operational assumptions are reflected. Carefully setting soiling losses brings desk calculations closer to on-site reality.

Soiling is often underrated compared to other loss items but can subtly affect results depending on project characteristics. When evaluating proposals with small generation differences, differences in soiling assumptions can have a larger impact than expected. Therefore, when setting soiling in PVSyst, avoid using vague experiential values and base assumptions on the project’s location and operation.

Point 4 Underestimating wiring and electrical losses

Wiring and electrical losses may seem mundane, but overlooking them can undermine simulation reliability. Many PVSyst users focus first on irradiance conditions, shading, orientation, and system configuration, while wiring and electrical losses feel like a detail and are left at default values. These losses may be inconspicuous, but they reliably affect the final results.

In practice, wiring conditions change with site extent and layout. When the layout changes, distances change and the expected electrical losses can differ. Yet it is not uncommon to revise installation conditions substantially while leaving wiring and electrical loss settings unchanged. This is a frequent oversight in PVSyst and is easy to miss because it is not visually obvious.

The remedy is to make it a habit to review electrical losses whenever system configuration or layout changes. In comparative scenarios, even when you think you only changed orientation or capacity, wiring conditions may have changed as well. Leaving loss settings unchanged then makes it impossible to read differences correctly. To improve comparison accuracy in PVSyst, treat these subtle losses as part of the conditions and handle them carefully.

Appropriate wiring and electrical loss settings also strengthen explanations of final results. When explaining why generation is at a certain level, you can show that you incorporated practical total losses, not just the prominent factors. PVSyst loss credibility is often determined by the accumulation of such modest items rather than flashy components. Not underestimating wiring and electrical losses leads to more robust simulations.

Point 5 Not fully reflecting shading losses and the impact of layout conditions

Although many practitioners are aware of shading losses, they sometimes fail to fully reflect the relationship with layout conditions. Shading treatment in PVSyst strongly affects results, so checking only the presence or absence of shading and assuming that is sufficient is risky. What matters is whether shading settings consider how the shading arises from layout choices and which times of day or seasons are affected.

In practice, reducing spacing between arrays can make better use of the site but increase shading impacts. Conversely, increasing spacing to avoid shading may constrain capacity planning. In other words, shading losses are not independent but tied to layout and design choices. Treating shading lightly in PVSyst may lead to wrong conclusions not only about generation but also about the feasibility of the design itself.

The countermeasure is to regard shading not merely as a loss but as a consequence of layout choices. Enter inputs while considering why the shading occurs and how it relates to spacing and orientation. This makes loss settings easier to link to design decisions. When creating comparative proposals, check whether one option has excessively lenient shading assumptions or whether shading treatments are consistent across options. Fairness of conditions is a prerequisite for PVSyst comparisons, and discrepancies in shading handling will skew the evaluation.

Also, when reviewing results, pay attention to monthly dips and differences among proposals; these patterns make lenient shading settings easier to spot. Shading losses are easy to miss if you only look at annual totals. Correctly handling shading in PVSyst requires integrating layout and loss settings, treating them as one design decision.

Point 6 Reusing loss settings without reviewing them for each project

A very common error is reusing loss settings across projects without reviewing them. As users become more comfortable with PVSyst, they increasingly reference past project settings, which can be efficient. The problem arises when those reused settings are applied without confirming their appropriateness for the current project. This can leave loss assumptions from a previous project unchanged even though site conditions and layout have changed.

Care is especially needed when projects are similar. If site size or capacity range is close, you naturally want to reuse previous settings. However, installation environment, maintenance conditions, shading patterns, and the specifics you want to compare may differ slightly, and those differences change the validity of loss settings. While PVSyst loss settings appear easy to reuse at a glance, they actually make sense only in the context of the project background.

As a remedy, even when using past settings, always set review items for each project. Decide on minimum checkpoints—consistency with weather conditions, approaches to temperature and soiling, wiring and shading assumptions, and fairness across comparison options—to reduce overlook risks when reusing settings. Streamlining PVSyst operation and fixing loss settings are different things. Efficiency is important, but it must be paired with a habit of review.

Mistakes from reusing loss settings are especially hard to detect because the results often look plausible. They are not obvious input errors but credible numbers that can be used in internal reports as is. That is why, when handling loss settings in PVSyst, you should ask whether there is a valid reason to use the same settings as before. The correct order in practice is not to use the same settings unexamined but to review them and then intentionally decide to keep them if appropriate.

An approach to linking PVSyst loss settings to on-site accuracy

What the six points above have in common is that loss settings are not stand-alone numbers but are deeply tied to site, weather, layout, operational assumptions, and the purpose of comparison. Individual items like temperature, soiling, wiring, and shading are important, but what truly matters is whether they are consistent within the overall project. Mastering PVSyst loss settings enables you to not only produce generation figures but also to explain the basis for those figures.

For practitioners, the aim of loss settings is neither to lower nor to raise numbers. The goal is to set reasonable assumptions close to on-site reality and produce simulations that are stable and resistant to later changes. If loss settings are too optimistic, initial numbers may look attractive but often require revision as design proceeds. Conversely, if practical losses are properly incorporated from the start, the results become stable materials for internal explanation and comparative evaluation.

To truly connect PVSyst loss settings to practical accuracy, it is also important not to rely solely on desk-based condition organization. If you cannot accurately grasp site status, surrounding topography, site usage, shading potential, and maintainability, loss settings themselves will be vague. In short, the quality of loss settings depends on the quality of on-site information.

In that sense, when you want to efficiently confirm locations or acquire coordinates on site, using an iPhone-mounted high-precision GNSS positioning device such as LRTK is an effective approach. If you can easily organize on-site position information and site conditions, you can more readily align PVSyst layout, shading, and loss assumptions with reality. Creating a workflow that improves desk simulation accuracy with PVSyst and supports on-site understanding with LRTK reduces discrepancies between design and the field. Correctly setting losses is not merely being cautious with calculations; it is fundamentally about improving judgment accuracy based on the site.