8 Items to Check When Setting Loss Factors in PVSyst

Table of Contents

• Why setting loss factors in PVSyst becomes important

• Item 1 Treat loss factors as design premises, not as after-the-fact adjustments

• Item 2 Adopt a loss approach that fits the site and meteorological conditions

• Item 3 Consider temperature-related losses separately and carefully

• Item 4 Do not confuse losses of different natures such as soiling and shading

• Item 5 Do not underestimate wiring and electrical losses

• Item 6 Verify consistency with modules, PCS, and string configuration

• Item 7 For comparative simulations, align conditions other than loss factors

• Item 8 Back-check loss factors from the results screen

• How to translate PVSyst loss factor settings into practical outcomes

Why setting loss factors in PVSyst becomes important

For practitioners running generation simulations with PVSyst, setting loss factors is not a mere fine-tuning exercise. Settings that are visually obvious—module capacity, azimuth, tilt, PCS conditions—naturally attract attention, but what truly makes a difference in getting final generation close to reality is how much loss you expect relative to these ideal conditions. In other words, loss factors should not be an item you tweak at the end; they must be treated as one of the premises that support the overall credibility of the simulation.

In practice, people tend to want scenarios that show as high generation as possible. That makes modest loss factors more visually appealing, and there is a temptation toward optimistic assumptions. However, if you proceed to internal reviews or comparative assessments with overly lenient loss factors, you will likely see generation drop when conditions are tightened later, weakening the overall persuasiveness of the design. Conversely, setting loss factors overly conservatively can prematurely rule out layouts that would have been feasible. The important thing is neither to show higher nor lower generation for appearance’s sake, but to align assumptions with what is reasonable for the project.

Also, although “loss factors” may look like a single concept, in reality they comprise elements of different natures: temperature, soiling, wiring, variability, shading, electrical mismatches, and so on. If you only look at the aggregated total, it becomes hard to tell which elements are contributing and to what extent. When using PVSyst for practical simulations, you should not treat loss factors as a single consolidated number; rather, assign meaning to each contributing factor. If those components are well organized, it becomes easier to decide what to revise when results feel off.

Furthermore, loss factor settings directly affect comparative evaluations. Even with the same equipment configuration, the perceived performance can change depending on how losses are modeled, and rankings between alternatives can reverse. What matters in practice is not just the magnitude of the numbers but being able to explain why you anticipated those losses and why the results came out as they did. Carefully setting loss factors in PVSyst improves not only the accuracy of generation forecasts but also the explanatory power of the design proposals.

Item 1 Treat loss factors as design premises, not as after-the-fact adjustments

First, remember not to treat loss factors as after-the-fact correction values. In practice it is common to create simulations under ideal conditions first, then add loss factors later to make the results more realistic. While that workflow is natural, treating loss factors as last-minute balancing items can lead to misjudging the design proposals. When setting loss factors in PVSyst, it is important to treat them from the outset as part of the project’s assumptions.

For example, even on the same site with identical module and PCS conditions, different approaches to soiling, temperature, and wiring will change the interpretation of the final result. Nevertheless, if you only apply loss factors en masse after the layout and DC/AC ratio and string configuration are nearly fixed, those layout choices may be judged based on ideal conditions. As a result, impractical proposals can remain because they look good in terms of generation alone. Loss factors are not mere result-adjustment items; they affect the foundation on which designs are compared.

Also, treating loss factors as post-hoc corrections weakens internal justification. If you explain that you compared proposals under ideal conditions and then the numbers changed after adding losses, the rationale becomes harder to follow. If proposals are compared from the start with loss factors included, it becomes easier to explain which proposal is advantageous under what realistic conditions. For practical simulations, consistency of premises is more important than the neatness of results.

As a countermeasure, before starting comparisons in PVSyst, organize which losses should be considered and to what extent for the project. You don’t need perfect details, but at least distinguish items you will run at ideal values from those you will treat as realistic from the beginning. Treating loss factors as premises that make the design feasible—rather than as something to tack on at the end—makes proposal evaluations much more practical.

Item 2 Adopt a loss approach that fits the site and meteorological conditions

When setting loss factors, it is important to adopt a mindset that matches the site conditions and meteorological context. In practice there is a temptation to reuse loss factors from previous projects or to assume that similar regions can use the same approach. However, if site conditions differ, the same loss settings may not be appropriate. In PVSyst, differences in meteorological data and installation environment directly affect temperature, soiling, and how losses manifest.

For example, temperature-related losses can look different between a well-ventilated location and an enclosed one. Differences in the surrounding environment also change how easily soiling accumulates, and whether there are many nearby obstructions will change how you should treat shading. In short, loss factors are not a one-size-fits-all rule; they need to be adapted to each site. If you use PVSyst in practice, loss factors should be tied to project-specific assumptions.

Also, ignoring meteorological conditions when setting losses weakens the meaning of comparative results. If you change weather data or the meteorological premise but keep loss factors the same as in a previous study, it becomes unclear which differences come from weather and which from loss assumptions. In practice you may compare multiple layouts at different locations, and if you apply uniform loss assumptions across distinct sites you risk undermining fairness. In PVSyst, it is important to check that site and loss assumptions are properly linked.

As a countermeasure, before entering loss factors, organize the site and meteorological conditions for the project and consider which losses are likely to be significant. You don’t need to overcomplicate things, but at minimum confirm whether your approaches to temperature, soiling, shading, and wiring fit the location. To set loss factors correctly in PVSyst, don’t simply copy numbers from past projects—give them meaning for the current site.

Item 3 Consider temperature-related losses separately and carefully

Among loss factors, temperature-related losses are especially prone to causing differences in practice. The reason is that temperature effects are hard to perceive yet reliably affect results. Module output values and PCS capacity are easy to compare visually, and shading and access paths are easy to spot on drawings, but temperature effects are hard to judge by intuition and are often lumped together with other losses. For realistic simulations in PVSyst, you should consider temperature-related losses independently and carefully.

For example, even within the same region, ventilation, surrounding conditions, and array density change the thermal environment. Ground conditions and installation methods alter how much heat equipment is exposed to. Therefore, using a uniform fixed value for temperature-related losses can lead to misinterpreting generation differences. In practice, even if ambient temperature conditions are similar between projects, different array density or layout can change how temperature losses appear.

Also, if you lump temperature losses with other losses, it becomes difficult to see differences between proposals. You won’t know whether a proposal underperforms due to temperature or another loss factor. Because PVSyst makes it easy to compare proposals, being aware of temperature as a separate factor greatly clarifies the background of generation differences. In practice, being able to explain why proposals differ matters more than the raw numbers themselves.

As a countermeasure, do not treat temperature-related losses as part of a general lump; evaluate them individually relative to the project’s installation environment. Developing a habit of reviewing temperature losses carefully in PVSyst will allow you to assess equipment selection and array design from a more practical perspective. The difference in loss factor settings comes not from simply knowing a number, but from understanding independently which conditions produce that loss.

Item 4 Do not confuse losses of different natures such as soiling and shading

When setting loss factors, mixing losses of different natures makes result interpretation much more difficult. In practice, people sometimes view losses from soiling, shading, temperature, wiring, and electrical issues as a single loss block. Because PVSyst ultimately aggregates to annual generation, that tendency is easy to fall into, but if you want to drive design improvements this confusion should be avoided.

For example, if a proposal’s generation drops mainly because of strong shading, but you view it together with soiling or temperature losses, you won’t know where to start improvements. If shading is the main cause you should reconsider row spacing or access arrangements; if soiling is the main cause you may need to revise maintenance plans or re-evaluate the installation environment. Different types of losses require different mitigation actions and priorities. Separating loss types is essential to using loss factors practically in PVSyst.

Also, when reading comparative proposals, distinguishing the nature of losses clarifies what differences mean. One option might show low shading loss but harsh temperature conditions; another might have shading but be easier to maintain and thus favorable for soiling. If you summarize everything into a single loss rate, you lose sight of which proposal is strong or weak in what aspect. To use PVSyst comparisons effectively, you must read not only the final figure but also the nature of the losses.

As a countermeasure, when setting loss factors, at minimum separate major loss types—temperature, soiling, shading, wiring, etc. The goal is not to create many fine categories but to make sure that after reviewing results you can identify what to revise. Being able to translate loss factors into design actions is the real point; PVSyst’s loss factor settings matter less for whether you can input values and more for whether you can link those losses to concrete design steps.

Item 5 Do not underestimate wiring and electrical losses

A commonly overlooked aspect when setting loss factors in practice is wiring and electrical losses. Because they are less conspicuous than modules or PCS, they tend to be deferred or copied from previous projects, or set conservatively low. However, if you want PVSyst simulations to reflect reality, these subtle losses deserve careful treatment. While they may look small on an annual total basis, accumulated effects can change the ranking of proposals.

For example, on a large site with dispersed arrays, how wiring is routed changes the perceived wiring conditions. If the layout changes, distances change, and as a result the apparent electrical losses change. If you overhaul the layout but keep wiring loss assumptions from a previous premise, the meaning of the comparison weakens. In practice, differences you thought were due to equipment configuration can actually be strongly influenced by wiring conditions.

Also, underestimating wiring and electrical losses not only makes generation look slightly better, it can obscure the influence of other losses. If you want to see differences in shading or temperature but your electrical loss assumptions are sloppy, the overall differences will be muddled. In PVSyst comparisons, it is important to clarify what each proposal’s differences actually stem from. That requires treating subtle losses with a reasonable level of precision.

As a countermeasure, when setting loss factors, review the array spread, partitioning, and electrical grouping to confirm you are not underestimating wiring and electrical losses. It’s not only the numerical accuracy of small values that matters, but whether they are consistent with the project’s layout. When handling loss factors in PVSyst, differences can arise not only from dramatic losses but also from how meticulously you account for these understated premises.

Item 6 Verify consistency with modules, PCS, and string configuration

Loss factors should not be set in isolation; they must be considered in the context of module, PCS, and string configuration consistency. In practice the workflow often sets equipment configuration first and then uses loss factors to bring results closer to reality. However, how losses manifest depends on the equipment configuration, and the naturalness of a configuration changes what the losses imply. If you want PVSyst simulations that mirror practice, read loss factors within the whole system configuration.

For example, increasing module count or changing the DC/AC ratio alters output constraints and how the PCS handles input, which changes how losses appear. If string configurations are poorly grouped, shading or electrical losses may present differently across proposals. Adjusting loss factors without looking at these conditions may make numbers look neat while sacrificing design plausibility. In PVSyst what matters is reading losses within the consistency of the configuration, not merely tuning them.

This perspective also helps you understand differences between proposals. If a loss looks large in one case, the reason may be not just equipment differences but how strings are cut or how the PCS accepts input. Conversely, a slightly larger loss may be balanced by better site fit or maintainability. In practice, understanding which configuration conditions produced a loss figure is more important than the loss number itself.

As a countermeasure, when setting loss factors in PVSyst, check for contradictions with module conditions, PCS settings, and string configuration. Rather than optimizing loss numbers individually, confirm that the entire system forms a natural, cohesive arrangement—this yields stronger practical results. Treat loss factor settings as part of the system configuration consistency check rather than as standalone number tuning.

Item 7 For comparative simulations, align conditions other than loss factors

When using loss factors to compare multiple proposals, align as many conditions other than loss factors as possible. In practice, even if you intend to isolate a single point of comparison, module conditions, array conditions, PCS settings, and shading conditions often change at the same time. That makes it hard to know where annual generation differences or loss gaps originate. To use PVSyst effectively for comparisons, be clear about what you change and what you keep the same.

For example, if you want to observe the effect of different loss assumptions but you simultaneously change azimuth or tilt, you cannot interpret the resulting difference as due to loss assumptions alone. Conversely, if you compare within the same equipment, array, and shading conditions while changing only the loss premises, the meaning of the difference becomes much clearer. In practice, how you align premises greatly affects the quality of comparisons. Because PVSyst makes comparisons easy, lax premise organization directly leads to misinterpretation.

Also, aligning comparison conditions makes internal justification easier. If you can show how generation changes when you alter only the loss assumptions under the same conditions, it is easier to discuss the validity of the loss factors. If conditions are mixed, you may have differences but be unable to briefly explain why they occurred. In practice, the clarity of comparisons directly speeds up decision making.

As a countermeasure, before creating comparative proposals, write one sentence that clearly states what you will change in the comparison and consciously align the other conditions. When comparing loss factors in PVSyst, organize conditions so you can confidently explain the difference as a difference in loss assumptions—not just as numbers. The meaningfulness of loss factor differences depends not only on the magnitude of values but on how well you aligned the comparison premises.

Item 8 Back-check loss factors from the results screen

Finally, it is important to back-check the validity of loss factors from the results screen. PVSyst returns results based on the loss factors you input, but entering a value does not guarantee it is appropriate. In practice, annual generation may deviate greatly from expectations, or comparative differences may be unnaturally large or small. In such cases you should question not only equipment configuration but also how loss factors were modeled.

For example, if the results show large differences that cannot be explained by layout or equipment conditions, one or more loss premises—temperature, soiling, shading, wiring—may be overly influential. Conversely, if a shaded proposal shows too small a difference, you may be underestimating shading losses. Because PVSyst organizes results clearly, you can use the outcome as a clue to revisit input premises. In practice it is important not to treat loss factors you entered as fixed constants.

Also, cultivating a habit of back-checking improves the quality of comparative proposals. Instead of stopping at numeric differences, confirm which losses produced those differences and, if necessary, adjust loss factors and rerun comparisons. This turns PVSyst comparison results from mere lists of numbers into assessments that include the validity of premises. In practice, this iteration raises the quality of design judgments.

As a countermeasure, when you view the results screen, check not only annual values but the way losses are distributed, the comparative deltas, and seasonal trends, and reassess whether the entered loss factors truly fit the project. Don’t stop at entering values—return from results to premises. Being able to question and refine entered loss factors, rather than simply inputting numbers, is central to mastering PVSyst in practice.

How to translate PVSyst loss factor settings into practical outcomes

What ties together the eight items above is the principle of not treating loss factors as mere correction values. Treat them as design premises, match them to site and meteorological conditions, consider temperature-related losses independently, avoid mixing different types of losses such as soiling and shading, do not underestimate wiring and electrical losses, ensure consistency with equipment configuration, align conditions when comparing, and finally back-check from results. When you follow this flow, setting loss factors in PVSyst becomes not an annual generation tweak but a practice that improves the quality of design judgments.

For practitioners, the goal is not to find the loss factors that produce the highest generation. The real value lies in being able to explain why you anticipated those losses for the project. If loss factors are consistent with modules, PCS, arrays, strings, site and shading conditions, and if they make comparative differences easy to explain, then simulation results become useful for internal comparisons and design review. Conversely, prioritizing appearance by underestimating losses increases the burden of explanation and revision downstream.

Also, raising the precision of loss factors requires not relying solely on desk simulations. If boundary lines, slopes, buildings, trees, access paths, and existing conditions are unclear, the premises for loss factors weaken. To leverage PVSyst numbers in practice, repeatedly iterate between field understanding and simulation to organize which losses to weight more heavily. Loss factors are not only numbers on a screen but also reflections of site conditions.

In that sense, when you want to secure position verification and coordinates on site more reliably, using iPhone-mounted GNSS high-precision positioning devices like LRTK is a useful idea. If on-site position data and site conditions are easier to organize, the assumptions for layout and shading in PVSyst become clearer when setting loss factors. If you can improve desk comparison accuracy with PVSyst and support field understanding with devices like LRTK, loss factor setting becomes closer to a site-rooted design judgment rather than mere data entry. Carefully setting loss factors not only improves generation forecast accuracy but also enhances the practical capability that links desk work and field work.