What is easily overlooked in PVSyst? 7 important configuration points

PVSyst is simulation software used to predict power generation, evaluate losses, and compare design conditions for solar power plants. It is utilized in various practical situations such as initial feasibility studies for power generation projects, design reviews, preparing explanatory materials for financial institutions and stakeholders, and organizing conditions before and after construction. However, because PVSyst produces calculation results based on the entered conditions, if the assumptions behind the settings are left vague, it can generate power generation forecasts that, despite appearing as well-structured reports, diverge from reality.

Many practitioners who search for "What is PVSyst" not only want to know what the software can do, but also want to know where mistakes are likely to occur when actually using it and which settings should be carefully checked. This article organizes seven particularly easy-to-overlook points during PVSyst setup and explains them so that even first-time users can understand the practical points they should verify.

Table of Contents

• What is PVSyst used for?

• Pitfall 1: Proceeding without verifying the assumptions of the meteorological data

• Pitfall 2: Entering the installation azimuth and tilt angle based only on drawings

• Pitfall 3: Oversimplifying shading conditions

• Pitfall 4: Overlooking the consistency between equipment specifications and system configuration

• Pitfall 5: Using standard loss rates as-is

• Pitfall 6: Failing to fully reflect land topography and on-site conditions

• Pitfall 7: Only looking at the results screen and not reviewing input conditions

• Verification workflow for using PVSyst in practice

• Summary

What is PVSyst used for?

PVSyst is simulation software for predicting annual power generation by entering the design conditions of a photovoltaic power system and using inputs such as solar irradiation, temperature, PV layout, electrical configuration, and loss conditions. It is characterized not only by calculating the power generation but also by allowing users to break down and check at which stages and what kinds of losses occur. This enables designers and project developers/operators to assess whether a plant’s plan is reasonable, whether the assumed installed capacity is realistic, and whether they can explain the basis for the expected generation to stakeholders.

The power output of a solar photovoltaic system is not determined by installed capacity alone. Even systems with the same capacity can produce very different amounts of electricity depending on the irradiation conditions at the installation site, panel orientation, tilt angle, surrounding shading, temperature rise, wiring losses, equipment conversion efficiency, soiling, snow cover, and degradation over time. PVSyst allows many of these factors to be entered as conditions and calculates annual energy production and a breakdown of losses.

However, PVSyst does not automatically and correctly understand on-site conditions. It is merely a tool that performs calculations based on the input conditions. Therefore, if the input conditions are incorrect, the output results will also be based on incorrect assumptions. In practical work, it is necessary to configure the model while cross-checking multiple sources of information, such as design drawings, equipment specifications, on-site survey data, site development plans, shading conditions, electrical design, and maintenance conditions.

When using PVSyst, what's important is not just learning how to operate the software. Rather, it's crucial to understand which input items are likely to affect the energy yield, which assumptions are still tentative, and which conditions should be verified on site. By identifying settings that are easy to overlook, you can avoid overreliance on simulation results and make them easier to use as a basis for design decisions.

Caution 1: Proceeding without verifying the assumptions underlying meteorological data

One of the first things easily overlooked in PVSyst settings is the assumptions behind the meteorological data. In calculations of solar power generation, solar irradiance and air temperature are extremely important. Because annual energy production is strongly influenced by how much sunlight is available, the choice of meteorological data greatly affects the overall results.

When selecting meteorological data, you need to confirm whether the data come from a site close to the planned power plant location, whether the data period and representativeness are adequate, and whether the assumptions used to convert horizontal-plane irradiance to tilted-plane irradiance are reasonable. Even if you think you have chosen a nearby observation site, meteorological conditions can change with only a small difference in distance in mountainous areas, coastal areas, basins, and snowy regions. Especially in areas that are sensitive to terrain effects, data from a nearby site are not necessarily reliable.

Also, meteorological data can represent an average year, be based on long-term statistics, or be estimated from satellite data, among others. Instead of simply deciding which data are “correct,” it is important to confirm whether the data fit the purpose of the simulation. For preliminary assessments, standard data may be acceptable, but when using them for business feasibility evaluations or explanations to stakeholders, you should be able to explain the rationale for the meteorological data you adopted.

What is easy to overlook is becoming complacent after choosing meteorological data just once. When generation is higher or lower than expected, people tend to review only equipment configuration or loss rates, but unless you verify whether the underlying solar irradiation conditions are reasonable, you will not achieve a substantive validation. Especially when comparing multiple candidate sites or reconciling with the actual performance of an existing plant, differences in the assumptions behind the meteorological data can be the cause of differences in results.

In practice, when you set meteorological data, it's good to record the data type, location, period, whether any corrections were applied, and the reason for adopting them. If, when you later review the report, you cannot determine why those meteorological conditions were used, your ability to explain the results will be diminished. PVSyst is both a tool for performing precise calculations and a working environment for organizing the rationale behind inputs. Verifying the meteorological data is the starting point for that.

Note 2: Entering the installation azimuth and tilt angle based solely on drawings

Another commonly overlooked factor is the orientation and tilt angle of the solar panels. Orientation and tilt directly affect how sunlight is received. Whether they face close to south, are tilted toward the east or west, or have a shallow or steep tilt will change not only the annual energy output but also the generation characteristics in the mornings and evenings and across seasons.

In practice, inputs are often entered based on the orientation and layout shown on design drawings, but the north indicated on the drawings, the actual site orientation, the post-development ground slope, and the installation angle of the racking do not necessarily match. Even if the project is designed with an ideal orientation during the planning stage, the actual layout may be adjusted because of site topography, property boundaries, drainage plans, roads, or nearby existing structures. If those changes are not reflected in PVSyst input conditions, discrepancies will arise between the simulation results and the actual system conditions.

Particular attention should be paid to the definition of the azimuth. If you do not understand which direction the software uses as the reference when entering angles and simply transcribe the angles from the drawings, you may end up with an unintended orientation setting. Definitions such as the sign of the angle, whether it is north-referenced or south-referenced, and which of east or west is considered positive are areas prone to input errors. Even a small input mistake can affect the time distribution of power generation and the evaluation of shadows.

With regard to the tilt angle, it is necessary to check not only the designed mounting angle but also its relationship with the slope of the ground surface. When installed at the same angle on flat ground and along a slope, the actual angle at which sunlight is received can differ. If the site development plan changes midway or the actual ground elevation differs from the drawings, the originally set angle may no longer be appropriate.

PVSyst is software that organizes design conditions as numerical values. Therefore, seemingly simple items such as azimuth and tilt angle should have their definitions checked before input. Rather than using values read directly from drawings, you should set values that are close to the actual installation conditions by cross-checking them with site coordinates, survey results, and design change history.

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Note 3: Oversimplifying shadow conditions

Shading settings are another important aspect that is easy to overlook in PVSyst. In solar power generation, shadows are caused by surrounding mountains, buildings, trees, utility poles, fences, adjacent rows of panels, and so on. Shadows not only reduce power generation, but because the way they occur changes with the time of day and season, they have a major impact on annual generation forecasts.

During preliminary studies, shadow conditions are sometimes simplified. While this can be necessary for work efficiency, a practical risk to be aware of is forgetting those simplified assumptions. If you proceed to detailed design or feasibility assessments with the simplified settings unchanged, simulations can overestimate energy production even though the actual impact of shadows may be substantial.

Especially in ground-mounted power plants, shading between panel rows is important. Row spacing, tilt angle, ground slope, panel height, and installation azimuth affect the conditions that cause shading. During winter, when the solar altitude is low, shadows between rows tend to extend farther. Because this can lead not only to impacts on annual energy production but also to reduced generation in the morning and evening and to output constraints during certain periods, it is necessary to confirm to what extent inter-row shading has been taken into account.

Shadows cast by nearby obstacles are also easily overlooked. If there are buildings or trees around the planned site, they may not be conspicuous during an on-site check but can cast shadows in seasons with low sun angles. In addition, there are elements not fixed at the planning stage, such as trees that will grow in the future, changes in neighboring land use, and the addition of fences or signs. It is difficult to predict everything precisely, but it is important at least not to ignore the obstacles that are currently known.

In shadow assessment, creating detailed three-dimensional geometry itself is not the objective. What matters is not missing shadows that are likely to affect power generation and being able to explain the degree of simplification used. The more carefully shadow settings are configured, the more reliable the simulation results become. Conversely, entering detailed geometry is meaningless if the original site information is inaccurate. It is essential to take an approach that brings shadow conditions closer to reality based on site surveys, photographic records, and topographic data.

Point 4: Overlooking the consistency between equipment specifications and system configuration

In PVSyst, you set photovoltaic modules, power conditioners, string configuration, number of connections, DC capacity, AC capacity, and so on. One thing that is easy to overlook is the consistency between equipment specifications and the system configuration. Even if the individual equipment specifications are correct, if they are not combined appropriately, discrepancies can arise in the power generation forecast and loss assessment.

Solar cell modules have characteristics such as rated output, temperature coefficient, voltage, and current. Power conditioners have input voltage range, maximum input current, conversion efficiency, and capacity. The number of modules connected in series and the number of strings connected in parallel must be designed to match these specifications. In particular, if the voltage rise at low temperatures and the voltage drop at high temperatures are not taken into account, the assessment of the operating range can be insufficient.

Also, the ratio between DC-side capacity and AC-side capacity is important. In designs that increase the solar array capacity and leave the AC-side equipment capacity with headroom, the output can reach its upper limit during periods of high solar irradiance, causing losses known as clipping. This is not necessarily a bad design, but if it is set without understanding how much loss will occur, the results can be misinterpreted.

A common issue in practice is that PVSyst settings are not updated after design changes. Even if the number of modules changes, the number of power conditioners changes, the string configuration is altered, or the layout plan is adjusted, if the simulation settings remain outdated, the reported energy production will not reflect the latest design. Version control of drawings, electrical design, and simulation conditions is especially important when multiple people are working.

PVSyst is not software for merely selecting equipment; it is software for checking how the overall system configuration affects power generation performance. You need to verify consistency not only by entering the values from equipment datasheets but also by including the actual connection configuration, capacity ratios, operating range, and design change history. It is important to keep in mind that just because the results screen shows generated energy does not mean the configuration is correct.

Note 5: Using the loss rate as the default value

In PVSyst settings, you enter various loss conditions. Soiling losses, wiring losses, temperature-related losses, mismatch losses, equipment conversion losses, assumptions about downtime and degradation — the factors that affect energy production are many and varied. What is easy to overlook here is simply using initial values or commonly used values as-is.

Loss rates vary depending on site conditions and design details. For example, the effects of soiling depend on the surrounding environment, frequency of rainfall, dust, distance to farmland or roads, bird damage, cleaning schedules, and so on. Wiring losses depend on cable length, cross-sectional area, current, and layout planning. Temperature-related losses depend on the mounting method, ventilation conditions, local ambient temperature, and module characteristics.

Using a standard loss rate in itself is not inherently wrong. In the initial assessment, because detailed design has not yet been decided, it is necessary to use provisional values. However, it is important to make clear whether a value is a temporary placeholder or one adopted with supporting rationale. If provisional values persist into later stages, stakeholders may mistakenly treat them as fixed conditions.

One thing to be especially careful about is adjusting loss rates to make the results look favorable. If you set losses lower to make the power output appear higher, the forecast will be more optimistic than reality. Conversely, if you increase losses excessively to appear conservative, you may unfairly underestimate the project’s viability. Simulations should not be tuned to match desired outcomes; they should reflect site conditions and design parameters as accurately as possible.

When setting losses, you need to understand the meaning of each item and confirm that you are not double-counting any losses. If a loss included under one item is also added under another, the energy production will be underestimated. Conversely, if a loss is not included in any item, the energy production will be overestimated. When reading PVSyst’s loss diagram, it’s important not simply to look at the final energy output but to check at which stage each loss is applied.

Note 6: Cannot fully account for land topography and on-site conditions

One aspect that is surprisingly easy to overlook in PVSyst settings is the reflection of landform and on-site conditions. In simulations of solar power plants, attention tends to focus on equipment and solar irradiance conditions, but actual plants are installed on land. Topography, site grading and embankment heights, slopes, drainage, surrounding structures, access roads, and site boundaries all influence layout planning and shading conditions.

If the terrain is flat, the configuration is relatively simple; however, on sloping or undulating land, panel height, orientation, row spacing, and shading can vary from place to place. Even when panels are neatly arranged on plans, differences in actual ground elevation can make shadows from the front rows fall on the rear rows more easily, or cause installation angles to vary locally. Setting uniform conditions without taking this into account can lead to significant discrepancies with the actual site conditions.

Also, caution is needed when pre-development topography data and the designed ground after earthworks are mixed. In the initial stages, studies are based on the current topography, while in detailed design the layout may be based on the post-earthworks ground. If this switch is not reflected in PVSyst settings, shadow and slope conditions may remain outdated. It is important to clarify whether the reference is on-site survey data, the earthworks plan, or the layout drawing.

Site conditions include not only shading but also factors related to operation and maintenance. In areas with abundant vegetation nearby, inadequate future vegetation management can lead to shading and soiling. In locations close to the sea, the effects of wind carrying salt should be taken into account. In snowy regions, snow-induced shading, sliding, reflection, and the conditions for maintenance work affect power generation and operations. While PVSyst cannot represent everything perfectly, settings that ignore site conditions should be avoided.

PVSyst is a software tool for organizing design conditions on paper, but it only becomes meaningful when used in combination with on-site information. Simply preparing input values without accurately understanding the shape of the land will not adequately reflect the generation decreases or construction constraints that occur in the field. During the design phase, it is important to repeatedly verify, based on survey results and on-site inspections, that the simulation conditions match the site.

Note 7: Do not review input conditions by looking only at the results screen

When you use PVSyst, annual energy production, performance ratio, loss diagrams, monthly generation, and the main configuration conditions are organized into a report. Because the presentation looks neat, you may sometimes feel that simply looking at the results screen completes the evaluation. However, the most important thing is not only the results themselves but to verify which input conditions produced those results.

Even if the results show high power generation, a good performance ratio, or low losses, they are meaningless if the input conditions are optimistic. Conversely, if the results show low power generation, the cause may be unnecessarily strict loss settings or incorrect shading settings. The results screen is a starting point for final verification and should be used as material for reassessing the validity of the input conditions.

In practice, it is important to make a habit of reviewing the key conditions before submitting a report. Check the weather data, azimuth, tilt angle, equipment capacity, equipment configuration, loss rates, shading settings, simulation period, and so on, and verify that they are consistent with the drawings and specifications. In particular, when creating a new project by duplicating a past project file, conditions from the previous project can remain. If place names, capacity, loss conditions, azimuth, equipment configuration, and the like remain partly outdated, they can lead to hard-to-detect errors.

Also, to judge the validity of results, it is necessary to look at multiple indicators together rather than a single numerical value. In addition to annual energy production, check monthly generation trends, the breakdown of losses, whether any particular loss is excessively large, and whether the results are consistent with the intended design assumptions. For example, if the area experiences significant shading in winter yet shading losses are almost nonexistent, the shading input may be insufficient. Likewise, if the ratio of DC capacity to AC capacity is high but losses due to output clipping are extremely small, the system configuration or input conditions should be reexamined.

PVSyst is not just for producing final numbers; it is a tool for verifying design conditions. When reading the results screen, you should consider not only "Is this number reasonable?" but also "Are the assumptions that led to this number correct?" By checking back and forth between the input conditions and the output results, the reliability of the simulation is improved.

Verification flow when using PVSyst in practice

When using PVSyst in practice, rather than jumping straight into detailed settings, decide on a verification flow to reduce mistakes. First, clarify the purpose of the simulation. Whether it is for an initial study, basic design, detailed design, or for stakeholder presentations will change the required level of accuracy and the granularity of supporting inputs. If you proceed with settings while the purpose is unclear, it becomes difficult to judge which parts need to be developed in detail and which parts can be provisional.

Next, organize the materials used for input. Gather meteorological data, site plans, single-line wiring diagrams, equipment specifications, survey maps, land development plans, on-site information related to shading, and so on, and confirm which version of each document will be used. In projects where design changes occur frequently, there is a risk of entering data based on outdated materials. Simply checking the document version numbers and update dates can prevent many configuration errors.

On that basis, we first set the major conditions. We lock down the basic conditions first — installation site, meteorological conditions, orientation, tilt angle, system capacity, and equipment configuration — and then verify shading, loss rates, and detailed correction factors. Even if you finalize the detailed loss settings first, if the basic conditions are incorrect the overall results cannot be trusted. It is important to proceed from the broad framework to the finer details.

After configuring, don’t just look at the results and stop; go back and check the input conditions. Verify whether the annual power generation is within the expected range, whether there are any anomalies in the monthly trends, and whether the breakdown in the loss diagram matches the local conditions. If there are any anomalous results, do not adjust the numbers; instead, search for the input conditions that caused them. By repeating this verification process, the simulation becomes design validation rather than mere operational work.

Finally, when submitting or sharing the report, it is important to be prepared to explain the key assumptions. Being able to state which meteorological data were used, to what extent shading was considered, what assumptions underlie the loss rates, and which design revision the equipment configuration is based on will help reduce gaps in understanding among stakeholders. PVSyst’s output reports are useful, but the report alone does not necessarily convey all the assumptions. Practitioners need to consciously manage the report figures together with the underlying input data.

Summary

PVSyst is a powerful simulation software for predicting the energy yield and assessing losses of solar power plants. It helps organize design conditions and explain the basis for the estimated energy yield, but if the input conditions are not correct, the output results cannot be trusted. In particular, meteorological data, azimuth and tilt angles, shading, equipment configuration, loss rates, land topography, and methods for verifying results are important items that are easy to overlook when configuring the settings.

When using PVSyst in a professional setting, it is important not to make filling in the interface the goal, but to verify how accurately local conditions and design conditions are being reflected. Simulation results are an aggregation of the assumptions entered. Rather than looking only at the energy production numbers, reviewing which meteorological conditions, layout conditions, shading conditions, and loss conditions led to those numbers will increase the explanatory power of the results.

Also, when evaluating a solar power plant, not only desk-based design but also the accuracy of on-site positional and topographic information is important. To correctly understand orientation, tilt, extent of site development, equipment layout, and shading factors, the reliability of coordinates and positioning data obtained on site is indispensable. To improve simulation accuracy, it is necessary not only to carefully configure settings in PVSyst but also to accurately assemble the on-site data that form the basis for those settings.

If you want to streamline site inspections, surveying, and equipment layout planning, using LRTK (an iPhone-mounted GNSS high-precision positioning device) can also be effective. By mounting it on an iPhone you can obtain high-precision location information, which is useful for confirming planned sites for solar power plants, layout planning, recording existing site conditions, and verifying positions before and after construction. Combining PVSyst simulations with the high-precision location data collected on site reduces discrepancies between desktop studies and actual field conditions, making it easier to reach better-founded design decisions.