7 Points to Note When Proceeding with Rooftop Installation Projects Using the PVSyst Manual

Table of Contents

• Basic approach to using the PVSyst manual for rooftop installation projects

• Note 1: Do not overestimate the roof shape and the available installation area

• Note 2: Properly organize the azimuth and tilt angles for each roof

• Note 3 Carefully reflect shadows cast by surrounding buildings and rooftop equipment

• Note 4: Adapt module layout and string design to real-world conditions

• Note 5: Do not overlook temperature conditions and roof ventilation conditions

• Note 6: Do not leave loss settings at their default values

• Note 7 Be prepared to explain the assumptions before outputting the report

• Practical points when using the PVSyst manual for rooftop installation projects

• Summary

Basic approach for applying the PVSyst manual to rooftop installation projects

When progressing with photovoltaic simulations while referring to the PVSyst manual, the points to focus on differ greatly between ground-mounted and rooftop projects. For ground-mounted installations, mounting structures can be placed on relatively consolidated sites and the azimuth and tilt are easier for the designers to adjust, whereas rooftop installations are heavily constrained by the existing building shape, roof pitch, obstacles, equipment, waterproofing layers, load conditions, and maintenance access routes. Therefore, even if PVSyst shows an attractive power generation estimate, if that differs from layouts that can actually be constructed or from operable site conditions, major rework can occur in later stages.

The purpose of reading the PVSyst manual is not merely to learn how to operate the interface. It is important to understand which input values affect energy production and which assumptions should be handled carefully. In particular, for rooftop installation projects the installation surfaces are often divided into multiple areas, and south-facing, east-facing, west-facing, corrugated metal roofs, flat roofs, and pitched roofs may coexist. If these are simply aggregated into a single condition, the resulting energy production and loss assessments can easily differ from reality.

Also, roof-mounted installation projects are a field in which simulation results are often used by owners and designers as material for judging investment decisions, the benefits of self-consumption, reductions in electricity bills, and the appropriateness of utilizing the roof. Therefore, it is necessary not only to output a PVSyst report, but also to be in a position to explain under what assumptions those figures were calculated, to what extent the effects of shading were anticipated, and how much the constraints on the roof were reflected.

For rooftop installation projects, when using the PVSyst manual it is essential not to simply fill in the input screens in order, but to consciously translate the site conditions into the simulation settings. The basis of a trustworthy simulation is to configure it to capture the power generation characteristics during realistic operation, not to use settings designed to make the energy output look higher.

Note 1: Do not overestimate roof shape and installable area

The first thing to watch for in rooftop installation projects is that the roof area and the area where modules can actually be installed are not the same. Before starting a simulation in PVSyst, you need to identify which roof surfaces are candidates for installation based on building drawings, aerial photographs, and on-site survey results. If you estimate module capacity by looking only at the total roof area, items such as lightning protection equipment, skylights, HVAC outdoor units, ducts, drainage routes, maintenance walkways, and clearances around parapets may later become issues and can greatly reduce the number of modules that can be placed.

In the PVSyst manual, input items are organized by step, such as system design, array settings, and near-shading settings. However, before entering capacity or module counts into the software's input fields, it is important to realistically confirm on site the area that can actually be used for installation. Drawings that place arrays right up to the edge of the roof may at first glance seem to increase energy generation, but they can be inappropriate when considering safety during construction and future inspections.

On flat roofs in particular, parapet height, the condition of the waterproofing layer, foundation placement, and measures against wind loads affect the layout. On standing-seam metal roofs, seam direction, the condition of the roofing material, compatibility of the fastening hardware, the position of the purlins, and interference with existing equipment are important. On pitched roofs, the orientation and slope of each roof plane, ridges and valleys, snow guards, and the conditions around the eaves influence the layout. The system capacity entered into PVSyst should be determined based on a feasible layout that takes these conditions into account.

If the installable area is overestimated, the annual power generation will also be overestimated. Furthermore, in self-consumption projects, overestimating generation directly affects the expected electricity savings and the outlook for investment payback. If capacity is later reduced, it may become necessary to reevaluate equipment costs, the amount of electricity bill reductions, subsidy requirements, contract terms, and so on. Setting a conservative and realistic installable area at the initial stage is an important task for improving the overall accuracy of the project.

On PVSyst, it is important not to treat the roof as a mere flat surface but to distinguish between areas where installation is possible and areas where it is not. Capacity settings that ignore rooftop obstacles and clearance requirements reduce the reliability of the simulation results. As a premise for operating in accordance with the manual, correctly interpreting the site conditions is the first step in any roof installation project.

Note 2: Correctly organize the azimuth and tilt angles for each roof

In rooftop installation projects, the azimuth and tilt angle settings have a major impact on energy production. While ground-mounted systems can sometimes be designed to face close to south or to an optimal tilt, rooftop installations are generally adjusted to match the existing roof’s orientation and pitch. Therefore, when entering azimuth and tilt values while consulting the PVSyst manual, you need to decide whether it is acceptable to treat the entire roof as a single representative value or whether each roof plane should be evaluated separately.

For example, when installing modules on a pitched roof divided into east and west faces, the times of generation differ between the east and west sides. The east face tends to produce more in the morning, while the west face tends to produce more in the afternoon. If this is simply treated as a single south-facing plane, both the annual generation and the time-of-day generation pattern will deviate from reality. In self-consumption projects, whether the times of generation align with the facility’s power demand is important, so this difference is particularly significant.

Care must also be taken when installing low-tilt racking on a flat roof. Increasing the tilt angle may appear to improve generation efficiency, but it can widen the spacing between racking rows, reduce the number of modules that can be installed, and require stricter measures against wind loads. Conversely, reducing the tilt angle makes it easier to place more modules, but you need to consider soiling, drainage, snow accumulation, and reflection conditions. When simulating with PVSyst, it is important not simply to look for the angle that maximizes annual energy yield, but to set a reasonable angle that also accounts for installation and maintenance.

Also, when handling multiple roof surfaces within a single project, how you separate sub-arrays and orientation conditions in PVSyst becomes important. If the azimuth and tilt of each roof surface differ significantly, grouping them under the same input conditions can lead to results that differ from the actual power generation behavior. In particular, when considering inverter and MPPT assignments, you must carefully verify whether it is acceptable to mix strings with different orientations on the same electrical system.

Azimuth and tilt angles are items that can be entered as numerical values on PVSyst’s screen, but behind them are the roof geometry, installation conditions, electrical design, and power generation patterns. For roof-mounted projects, you should organize the conditions for each roof based on information obtained from on-site surveys and drawing checks, and reflect them in the simulation as closely as possible to the actual situation.

Note 3 Carefully reflect shadows cast by surrounding buildings and rooftop equipment

One of the factors that greatly affects power generation in rooftop installations is shading. The PVSyst manual includes settings related to near-shading and shading, but in practice the reliability of simulation results varies significantly depending on how carefully these settings are configured. Even when a roof appears open at first glance, there are actually many shading contributors such as parapets, outdoor air-conditioning units, electrical cubicles, ducts, piping, antennas, adjacent buildings, rooftop structures, chimneys, and railings.

One point to note is that the effect of shadows is not constant throughout the year. In winter, when the sun’s altitude is lower, long shadows are more likely to occur even for short periods. During the morning and evening, shadows from adjacent buildings and rooftop equipment can extend farther than expected. Even shadows that seem minor when looking only at annual power generation can, if concentrated in specific times of day or seasons, affect the effectiveness of self-consumption and power generation during peak hours.

When dealing with shading in PVSyst, the objective is not to recreate shadows perfectly but to avoid overlooking the primary obstructions that affect energy yield. Trying to reproduce every small pipe or minor protrusion can make model creation take too long while their impact on energy yield may be limited. Conversely, omitting elements that have a large shading impact—such as parapets, rooftop penthouses, or large air-conditioning units—will make the results overly optimistic. Assessing the importance of shading factors is indispensable for rooftop installation projects.

Shading on a roof affects not only individual modules but also generation at the string and MPPT levels. Even if only some modules are shaded, depending on the wiring configuration, the shading can affect the output of the entire string. When checking PVSyst’s shading assessment, you should not simply look at the percentage of shading losses; you need to understand during which times, over what range, and to what extent the shading occurs.

In rooftop installation projects, overlooking shading can cause actual power generation to fall short of expectations. For simulations submitted to clients, it is also effective to organize them in an easily explainable way, such as by comparing a case that accounts for shading with a case where shading conditions are minimal. Rather than just reading the PVSyst manual as operational procedures, adopting the perspective of evaluating shading conditions as on-site risks will make the analysis more practically useful.

Note 4: Align module placement and string design with real-world conditions

When evaluating a rooftop installation project in PVSyst, it is important to match the string design to real-world conditions, not just the module capacity and count. After confirming how many modules can be placed on the roof, you need to consider how to connect those modules in series and which inverter(s) or MPPT(s) they will be connected to. The PVSyst manual provides a workflow for entering module and inverter selection, number of strings, number of modules in series, and so on, but if the site layout and electrical design are misaligned at this stage, the system may not be feasible in practice.

In rooftop installation projects, roof surfaces are often dispersed. Sometimes modules can be grouped together on the south-facing side, but in other cases they must be installed across east- and west-facing surfaces, on the roofs of multiple buildings, on multi-level (stepped) roofs, or divided between areas that are prone to shading and areas that are not. Under such conditions, it is fundamental not to mix modules with different power generation characteristics within the same string. If modules with different orientations, tilts, or shading conditions are combined in the same string, power generation efficiency decreases and interpreting simulation results becomes more difficult.

Also, when setting the number of modules in series, it is necessary to consider the module voltage characteristics and temperature conditions. Because the open-circuit voltage rises at low temperatures and the operating voltage falls at high temperatures, it is essential to verify whether the voltages will fall within the inverter’s input voltage range. In addition to checking that PVSyst does not issue warnings, it is important to evaluate whether the design provides adequate margin by taking into account actual rooftop temperature conditions and local characteristics.

In roof-mounted installations, cable routes and the positions of junction boxes also affect string design. A connection that appears ideal from a power-generation perspective alone can, in practice, lead to excessively long cables, make waterproofing difficult, or become hard to access during inspections. PVSyst input values are useful for confirming electrical feasibility, but you must always verify that they are consistent with the construction drawings and the electrical wiring plan.

Furthermore, from the perspective of future maintenance, it is also important to clearly identify which area of which roof surface corresponds to which string. To make it easier to pinpoint the source on site when a drop in power generation or an abnormality occurs, the layout plan and string design need to be organized in a clear and easy-to-understand manner. If the assumptions in the PVSyst report, construction drawings, and electrical drawings are consistent, the documentation will be easier for the owner, the contractor, and the maintenance personnel to use.

Point 5 Do not overlook temperature conditions and rooftop ventilation conditions

The power output of a solar photovoltaic system is not determined by irradiance alone. Module temperature also has a significant impact on generation performance. For rooftop installation projects, the airflow conditions behind the modules can differ from those of ground-mounted systems, so losses due to temperature rise need to be properly accounted for. When reviewing the temperature model and thermal loss sections in the PVSyst manual, it is important to be aware of what kind of ventilation conditions the rooftop mounting configuration will produce.

For example, when attaching modules to a corrugated metal roof with low-profile brackets, the distance between the back of the module and the roofing material can be small, causing heat to be trapped. Even when using racks on flat roofs, the rack height, module spacing, placement of rooftop equipment, and the height of surrounding walls affect airflow. Under poorly ventilated conditions, module temperatures are more likely to rise, which can reduce power generation efficiency.

In PVSyst you can set temperature-related coefficients and thermal loss conditions, but it is not always appropriate to use the default values as-is. You should choose conditions that are close to reality, taking into account the type of roofing material, installation height, racking configuration, local meteorological conditions, and the wind environment around the building. In particular, on the roofs of factories, warehouses, and commercial facilities, roof surface temperatures tend to be higher in summer, and temperature losses may be greater during periods of high solar irradiance.

Overlooking temperature conditions can lead to an overestimation of annual energy generation. This is particularly important for rooftop, self-consumption projects, since summer daytime generation often determines the effectiveness of electricity savings; therefore, properly accounting for temperature-related generation declines is crucial. In PVSyst's results screen, you should check how much temperature loss is occurring and verify whether it is within a reasonable range compared with other loss components.

Also, rooftop ventilation conditions affect not only energy production but also the long-term reliability of the equipment. In environments where excessive temperature rise persists, modules and ancillary equipment may be subjected to increased stress. Simulations are intended to calculate energy yield, but in practice it is necessary to consider design lifetime and maintenance as well. When using the PVSyst manual, do not treat temperature settings as mere input fields; confirm them as an important factor that reflects the rooftop-specific environmental conditions.

Note 6: Do not leave loss settings at their default values

In simulations using PVSyst, various loss components are reflected in the energy production. In rooftop installation projects, multiple factors—shading losses, temperature losses, wiring losses, mismatch losses, soiling losses, inverter losses, angle-of-incidence losses, etc.—overlap to determine the final energy output. When progressing through the settings while reading the PVSyst manual, be careful not to adopt initial or default values as-is, but to verify that they match the conditions of each project.

In rooftop installation projects, wiring distances can vary greatly depending on the building’s structure and the location of the incoming transformer and switchgear. When the route from the roof to the power conditioner, junction boxes, the distribution board, and the incoming power equipment is long, it is necessary to appropriately allow for wiring losses. Conversely, when equipment is located close together and wiring can be done efficiently, assuming an excessively large wiring loss can lead to an overly conservative result. The important thing is to provide a rationale for the numbers.

Losses due to soiling also vary depending on conditions in rooftop installation projects. In industrial areas, along busy roads, coastal locations, facilities prone to dust generation, or roofs where bird damage is anticipated, the impact of soiling can be significant. Conversely, where there is sufficient tilt and rain can easily wash away dirt, losses may be comparatively small. In PVSyst settings, it is important to consider realistic values based on the region, facility use, and cleaning schedule.

Regarding mismatch losses, caution is required for projects where the roof surface is divided into multiple sections or where shading conditions vary. If the generation conditions of individual modules are not uniform, actual generation can differ even if the design capacity is the same. How detailed you reflect this in PVSyst settings depends on the project scale and purpose, but at a minimum you should be able to explain the conditions that are likely to affect the results.

Loss settings are not a convenient means for adjusting power output. They are intended to quantify, as reasonably as possible, the on-site factors that can reduce generation. Even when using standard values, it is desirable to verify whether those values are appropriate for the current rooftop installation project and, if necessary, document them as notes. If, when reviewing simulation results later, it is unclear why a particular loss value was chosen, decisions will vary with each design change or reexamination.

Note 7: Be able to explain the assumptions before outputting the report

When you run a simulation with PVSyst, you can organize the breakdown of energy production and losses into a report. However, what truly matters for rooftop installation projects is not producing the report itself but being able to explain its contents to stakeholders. Depending on the project, the people who will look at the report vary: the client, designers, contractors, the chief electrical engineer, financial institutions, and personnel involved in subsidy reviews, among others. You must be prepared to clearly explain the key assumptions to any of these parties.

Before generating the report, first confirm that the installed capacity, module model, inverter model, azimuth, tilt angle, division of roof surfaces, shading conditions, loss settings, meteorological data, and temperature conditions match the current design. It is not uncommon for PVSyst settings to remain outdated even though the number of modules or the layout changed during the design process. For rooftop installation projects, because the layout often changes after the site survey, a consistency check before the final report is indispensable.

Next, check not only the generation results but also the breakdown of losses. If the annual generation is lower than expected, identifying whether shading losses, temperature losses, or wiring losses are large will reveal directions for design improvement. Conversely, if the generation appears higher than expected, you should confirm whether that result is due to optimistic settings. In particular, be aware that lenient settings for shading, soiling, or temperature can easily lead to an artificially high apparent generation.

Also, for rooftop installation projects, it is important not to present power generation figures alone, but to convey together how much is installed on which roof surfaces and under what assumptions the calculations were made. For example, in projects where installations are distributed across east- and west-facing surfaces, providing not only annual generation but also morning and evening generation trends and how they align with self-consumption will make the materials easier for the client to evaluate. When choosing a low-tilt installation on a flat roof, it is desirable to be able to explain not only generation efficiency but also the balance among the number of panels installed, wind loads, and constructability.

PVSyst reports can be persuasive documents, but their reliability decreases if the input conditions are unclear. Before final submission, it is important to organize the rationale for the input values, the assumed constraints, conditions that were not incorporated, and items that may change as a result of future design modifications. The purpose of using the PVSyst manual is not merely to complete the report, but to create simulations that can be used as decision-making material for rooftop installation projects.

Practical Points When Using the PVSyst Manual for Rooftop Installation Projects

When using the PVSyst manual for rooftop installation projects, it is important not to consider the simulation work as a standalone step. Treating it as part of a continuous series of evaluations linked to roof survey, structural verification, layout planning, electrical design, construction planning, and maintenance planning makes the PVSyst results easier to apply in practice.

First, before running the simulation, organize on-site information as thoroughly as possible. Having the roof plan, elevations, equipment layout, single-line wiring diagram, location of incoming power equipment, power usage data, and information about surrounding buildings makes it easier to determine the conditions to input into PVSyst. If drawings are old or rooftop equipment has been updated, prioritize on-site verification. For roof-mounted projects, areas that appear vacant on drawings often actually contain equipment, so it is risky to proceed with the simulation without confirming the real site conditions.

Next, comparing multiple proposals can also be effective. For example, by comparing several options in PVSyst—such as a south-facing-centered layout, an east-west-utilizing layout, a layout that maximizes capacity, and a layout that reduces capacity to avoid shading—you can more easily grasp not only the energy production but also the design's advantages and disadvantages. For roof-mounted projects, maximum capacity is not always the optimal solution. Rather than forcing installation in heavily shaded areas, it can be more rational in the long term to slightly reduce capacity to improve generation efficiency and maintainability.

In rooftop projects for self-consumption, it is important to be aware not only of annual power generation but also of generation trends by time of day. Factories, warehouses, commercial facilities, schools, and hospitals have different timing of electricity demand. The orientation of the roof and the dispersion of installation surfaces change how generation is produced—morning-peaking, afternoon-peaking, or daytime-centered patterns. By viewing PVSyst results in combination with electricity usage data, it becomes easier to move from a simple generation-volume evaluation to an examination of self-consumption effects.

Also, PVSyst input conditions need to be updated in line with project progress. At the stages of initial proposal, basic design, detailed design, and final pre-construction checks, the accuracy of available information differs. In the early stages, simulations may be run using approximate conditions, but in such cases it must be clearly stated that the conditions are approximate, and it should be assumed that they will be updated in later phases to reflect actual site conditions. If the PVSyst data created at the beginning are reused until the end, design changes will not be reflected and the documents may become inconsistent with reality.

Furthermore, it is important to share assumptions among stakeholders. If only the person operating PVSyst understands the settings, design changes may not be fully reflected. If the layout engineer, the electrical designer, the construction manager, and the sales representative at least share the main assumptions, it becomes easier to align understanding of the report’s figures. In roof-mounted projects, because architectural and electrical conditions are intertwined, the precision of information sharing also affects the accuracy of the simulation.

The PVSyst manual is useful as a reference for checking how to operate the software, but in practice you need the ability to apply it in the context of site conditions. Judging how extensively to configure each item, which conditions can be simplified, and which risks require separate explanation makes it possible to produce simulations appropriate for rooftop installation projects.

Summary

When progressing a rooftop installation project using the PVSyst manual, merely memorizing the sequence of screen operations is insufficient. Rooftop installations are more strongly influenced by building conditions than ground-mounted systems. Roof shape, usable installation area, azimuth, tilt angle, shading, string design, temperature conditions, loss settings, and report assumptions all affect the final energy production and project decision-making.

What's particularly important is to relate the numbers entered into PVSyst to actual on-site conditions. Even if a roof area appears large, there are portions where modules cannot be installed because of equipment, clearances, and inspection walkways. If orientations or slopes are split across multiple roof surfaces, you need to organize the generation characteristics for each roof surface. The impact of shading changes with season and time of day and also affects string design and MPPT configuration. Overlooking temperature and ventilation conditions can lead to an overestimation of energy yield.

Also, rather than treating loss settings as default values, it is important to set them with a substantiated rationale for each project. Losses from wiring distance, soiling, mismatch, temperature, shading, and the like vary depending on roof conditions and facility usage. In the final report, you should be able to explain not only the annual energy production but also the assumptions that led to that result.

PVSyst is an effective tool for evaluating the power generation of rooftop installation projects. However, the tool’s results are influenced by the quality of the input conditions. By collecting accurate on-site information, organizing the conditions for each roof, and aligning the design details with the simulation settings, the power generation forecast will be closer to real-world practice. When using the PVSyst manual, it is important to read it not merely as an operational procedure but as a guide to assess the risks of rooftop installation projects.