7 Items to Check String Design in the PVSyst Manual

Table of Contents

• The significance of verifying string design in the PVSyst manual

• Assumptions to Check First in String Design

• Checklist item 1: Read the solar PV module specifications correctly

• Check Item 2: Verify the consistency between the number of series-connected cells and the voltage range

• Checklist item 3: Align PCS and MPPT input conditions

• Checklist item 4: Verify the number of parallel strings and the input current

• Checklist item 5 Do not mix different orientations, tilts, and shading conditions

• Checklist item 6: Bring loss settings and cable conditions closer to practical values

• Checklist item 7 Return warnings, errors, and report results to the design

• Approach to Applying the PVSyst Manual in Site Design

• Summary

The significance of verifying string design in the PVSyst manual

In the design of photovoltaic power generation systems, string design—how many modules to connect in series, how many strings to place in parallel, and which input circuit to assign them to—has a major impact on energy yield, equipment protection, constructability, and future maintainability. Many people who want to consult the PVSyst manual are not merely interested in learning screen operations; they want to know whether the entered conditions are reasonable as an actual design, what to review when warnings appear, and how to interpret the figures in the report.

String design may at first glance look like the task of matching the number of modules to the PCS capacity. However, in reality it is necessary to consider simultaneously the open-circuit voltage at low temperatures, the operating voltage at high temperatures, the MPPT tracking range, the input current limit, differences in orientation and tilt, the effects of shading, cable losses, the overloading ratio, equipment specifications, and the division of circuits for construction. If you judge based on only one of these factors, even if it works in simulation, rework can occur during the detailed design or construction stages.

When reading the PVSyst manual, it is important not only to memorize the meanings of the screen items but also to understand why those items are related to string design. For example, increasing the number of modules in series makes it easier to reduce the number of circuits, but it raises the maximum voltage at low temperatures. Reducing the number of modules in series provides more margin on the maximum voltage, but it may cause the MPPT lower voltage limit to be undershot at high temperatures. Increasing the number of parallel strings allows effective use of the PCS input, but attention must be paid to the input current and the conditions of the combiner box.

In this article, we outline seven items that are particularly easy to overlook in practice when checking string design while consulting the PVSyst manual. To be useful not only for those performing string design in PVSyst for the first time but also for those who already have operational experience yet find themselves unsure about the meaning of warnings or about design decisions, we explain how on‑screen checks connect to design judgments.

Prerequisites to Check First in String Design

Before verifying the string design, you must first review whether the project's overall assumptions are correctly set. Even if you carefully adjust only the string configuration, if the assumptions about meteorological data, installation site, orientation, tilt, modules, PCS, and system capacity are off, the design decisions themselves will become unstable.

Particularly important are the ambient temperature conditions at the installation site. In string design, the open-circuit voltage at low temperatures and the operating voltage at high temperatures are major considerations. In cold regions, module voltage increases at low temperatures, so if you put too many modules in series you may exceed the PCS's maximum input voltage. Conversely, in high-temperature areas or under conditions where module temperature tends to rise, such as on roofs, the operating voltage decreases, and whether it remains within the MPPT tracking range becomes a concern.

Also, it is important whether the modules are mounted on a single surface or are divided across multiple orientations and tilts. Even strings connected to the same PCS can have different peak output east versus west, different tilts on each roof surface, or different shading patterns; if such conditions exist, simply combining them into the same input can cause discrepancies between simulation results and actual behavior.

When reading the PVSyst manual, it is important not to treat the string configuration screen in isolation, but to view it as part of a continuous workflow that includes the project, system, orientation settings, nearby obstacles, loss settings, and report output. String design sits at an intermediate point connecting not only electrical design but also layout planning, construction planning, site conditions, and maintenance planning. Therefore, if you omit confirming the assumptions, inconsistencies are more likely to surface in later stages.

Checklist Item 1 Read the photovoltaic module specifications correctly

The starting point for string design is the specifications of the photovoltaic module. When you select a module in PVSyst, the rated output, open-circuit voltage, short-circuit current, maximum power operating voltage, maximum power operating current, temperature coefficients, and so on become the basis for the design calculations. If the module you select here differs from the model actually adopted, all subsequent judgments about the number of modules in series, current, losses, and energy production will be incorrect.

Particular attention should be paid to the open-circuit voltage and the temperature coefficient. Module voltage increases as temperature decreases. When determining the number of modules connected in series in a string, you need to consider not only the open-circuit voltage at Standard Test Conditions but also the open-circuit voltage at the expected minimum temperature. For example, even if the PCS input limit is met under standard conditions, it may be exceeded during cold winter conditions. Warnings in PVSyst serve as a prompt to check for such conditions.

The maximum operating voltage at peak output is also important. This relates to the voltage range that the MPPT tracks during normal operation. If the number of modules in series is too low, the string voltage can drop at high temperatures and approach the PCS’s MPPT lower voltage limit. If the MPPT falls outside its range, the system cannot operate at the ideal maximum power point, which may lead to power generation losses.

Short-circuit current and maximum output operating current are related to verifying the number of parallel strings and the PCS input current. In recent years, the adoption of high-output modules has increased, and module currents tend to be higher. Therefore, if strings are paralleled in the same way as before, the margin against the PCS input current limit and the allowable current of connected equipment may be reduced.

When reviewing the PVSyst manual, it's important not to treat module data merely as selectable items; make a habit of cross-checking them against the actual datasheet. Confirm conditions that affect the design — model number, power output, voltage, current, temperature coefficients, the presence of bifacial generation, low-irradiance characteristics, etc. — before proceeding to string configuration to reduce later revisions.

Check Item 2: Verify consistency between the number of cells in series and the voltage range

The most fundamental aspect of string design is the number of modules in series per string. The number of series-connected modules is the central factor that determines the string voltage. Increasing the count raises the voltage, and decreasing it lowers the voltage. Within this simple relationship there are several checkpoints to consider, such as maximum input voltage, MPPT range, start-up voltage, and variations at low and high temperatures.

The first thing to check is whether the maximum open-circuit voltage at low temperature exceeds the PCS's maximum input voltage. In PV systems, when module temperatures are low—such as in the early morning or during winter—irradiance can cause the string open-circuit voltage to increase. Designs that exceed the PCS maximum input voltage should be avoided from the perspective of equipment protection. If PVSyst issues a warning when you increase the number of modules in series, you need to verify this low-temperature voltage.

The next thing to check is whether the operating voltage at high temperatures remains within the MPPT range. In summer and for roof-mounted installations, module temperatures tend to be higher, which reduces the operating voltage. In designs with a small number of modules in series, the operating voltage may fall below the MPPT lower limit at high temperatures. If the PCS falls outside the range it can track, it will also affect the power generation simulation.

Furthermore, the PCS start-up voltage should also be checked. If periods during which the string voltage does not meet the PCS start-up conditions increase, it may affect the start and stop of power generation in the morning and evening. The impact on annual energy yield varies depending on the conditions, but it cannot be overlooked when assessing the validity of the design.

The number of modules in series affects installation and inventory management. If all strings can be made with the same number of modules, management becomes easier, but depending on the roof shape, obstacles, and layout constraints, some strings may have a different number of modules. In that case, it is necessary to carefully judge whether strings with different series counts may be connected to the same MPPT. Connecting circuits whose string voltages differ greatly to the same MPPT will shift the optimal operating point and cause losses.

When consulting the PVSyst manual, it's important not simply to check whether values fall within the recommended range but to be aware of how voltages change across multiple conditions—minimum temperature, maximum temperature, normal operation, and startup conditions. The number of modules in series is an item that, once decided, affects the layout drawing, single-line wiring diagram, cable planning, and installation procedures, so it should be carefully confirmed early in the process.

Verification Item 3: Align the input conditions of the PCS and MPPT

In string design, verifying the PCS specifications is essential. The PCS has parameters such as maximum input voltage, MPPT voltage range, maximum input current, number of MPPT circuits, number of input terminals, rated AC output, and allowable overloading range. Don’t be reassured just because you selected a PCS in PVSyst; you must confirm that the specifications of the actual model to be adopted match the design conditions.

What is particularly important is how strings are assigned on a per-MPPT basis. When a PCS has multiple MPPTs, the basic rule is to make the strings connected to each MPPT as similar in conditions as possible. Grouping strings with the same orientation, the same tilt, the same number of modules, and similar shading conditions makes it easier for the MPPTs to operate efficiently.

Conversely, if you mix on the same MPPT circuits that are east-facing and west-facing, south-facing and north-leaning, sides with heavy shading and sides with little shading, or circuits with different numbers of modules in series, their voltage–current characteristics will not align and losses are more likely to occur. PVSyst's simulation allows you to set the conditions, but in detailed design you must verify the allocation of input circuits as well; otherwise the on-site wiring and the analysis model will not match.

The input current of the PCS is also important. With recent high‑power modules, the current per string increases, so the number of strings that can be connected in parallel per MPPT may be limited. You need to compare the maximum input current listed in the datasheet with the actual short‑circuit current and the current calculated considering temperature and irradiance conditions. Even if PVSyst does not issue a warning, there may be other constraints in actual equipment specifications or installation standards.

Also, the PCS oversizing ratio needs to be checked. Designing the DC-side capacity larger than the AC-side capacity is commonly done, but if it is excessively large clipping losses increase, so economic considerations and assessments of equipment loading are necessary. In PVSyst you can review annual energy production and the breakdown of losses, allowing you to compare the benefits and losses of oversizing.

When using the PVSyst manual to check PCS and MPPT conditions, it is important not only to match the input values shown on the screen but also to verify that they correspond to the actual wiring plan. Even if the simulation model is neatly divided, if the site drawings show different connections the analysis results become difficult to use as a basis for design. Cross-checking PVSyst settings, electrical drawings, and equipment specifications from the same viewpoint improves the accuracy of string design.

Checklist Item 4: Verify the number of parallel strings and the input current

While the number of modules in series per string determines the voltage, the number of parallel strings mainly relates to the balance of current and capacity. How many strings are connected in parallel to a PCS or MPPT affects system capacity, input current, combiner boxes, cable sizing, protective devices, and constructability. When consulting the PVSyst manual, be aware that the number of parallel strings is not simply a numeric input field but represents the actual on-site circuit configuration.

By increasing the number of parallel strings, you can increase DC capacity even with the same number of series-connected modules. However, this also increases the MPPT input current. The PCS has maximum allowable input current and short-circuit current ratings, and designs that exceed those limits must be avoided. In particular, when using high-current modules, a parallel count that was acceptable under previous design assumptions may no longer provide sufficient input current margin.

Also, when using junction boxes or combiner boxes, you must verify the ratings of the inputs from each string, fuses, disconnect switches, cables, terminal blocks, and other components. Even if the system design is valid in PVSyst, if the actual component selection does not meet allowable current, temperature conditions, or installation regulations, a design change will be necessary. While simulations are strong for energy yield assessment, they do not automatically guarantee the detailed conditions of all installation components.

The number of parallel strings also affects maintainability. When measuring the current of each string or isolating a faulty string, inspections become easier the clearer the circuit configuration is. If the number of strings set in PVSyst matches the on-site labels, the combiner-box circuits, and the circuit numbers in the monitoring system, troubleshooting after commissioning will proceed smoothly.

Furthermore, when determining the number of parallel strings, consider how the modules are grouped in the layout. On building roofs, the number of modules per roof surface and the location of obstructions can make it difficult to secure strings with the same number of modules. For ground-mounted installations, it can be easier for construction to group strings by racking row. Rather than simply filling the PCS inputs, it is important to create a configuration that is easy to wire and inspect on site.

When checking the number of parallel strings against the PVSyst manual, confirm not only the on-screen capacity, current, and warnings, but also that these are consistent with the single-line wiring diagram and the layout plan. Practical verification should consider not only the impact the number of strings has on energy production, but also whether the electrical installation will be safe and compliant, whether the wiring can be carried out on-site without difficulty, and whether future maintenance will be straightforward.

Checklist Item 5: Do not mix different orientations, slopes, or shading conditions

An aspect that is easy to overlook in string design is the difference in conditions among strings connected to the same PCS or the same MPPT. In solar power generation, performance characteristics change depending on orientation, tilt, and whether shading is present. If strings with different conditions are casually connected to the same MPPT, their maximum power points will not align, potentially causing generation losses.

For example, east-facing and west-facing roofs have different times for their generation peaks. East-facing roofs have higher output in the morning, while west-facing roofs have higher output in the afternoon. If you put these two on the same MPPT, the optimal operating point differs by time of day, making it difficult for the MPPT to optimally track each string. Similarly, surfaces with significantly different tilt angles or surfaces that receive shadows from surrounding obstacles may also be better treated as separate systems.

Shading conditions require particular attention. Even brief shading can affect the current across an entire string, potentially causing bypass diode activation and mismatch losses. PVSyst can account for nearby obstacles and shading effects, but if the 3D model or obstacle conditions you input deviate from the actual site conditions, the results will differ. In string design, it is important to verify which modules are included in the same string and whether they experience similar shading patterns.

In addition, snow accumulation, soiling, surrounding trees, parapets, adjacent buildings, and equipment can all cause differences in power generation at the string level. If conditions such as certain rows receiving more shade, only part of a roof being more prone to soiling, or uneven effects from snow shedding exist, mixing those strings under the same MPPT can lead to actual operational losses that are larger than the simulation results.

When reading the PVSyst manual, it is important to review azimuth and tilt settings, the concept of subarrays, and the handling of nearby obstacles without separating them from string design. Rather than viewing string configuration as merely an electrical number‑matching exercise, thinking from the perspective of how to group modules with similar power‑generation behavior will improve the accuracy of the design.

In practice, when reviewing the layout plan it is also important to verify where the modules that will be included in the same string are physically located. Even if conditions appear identical in PVSyst, the actual roof surface may contain a mix of areas near obstructions and open spaces. You should review site photos, drone images, survey data, roof plan drawings, and so on, and reflect differences in generation conditions in the string design.

Checklist Item 6: Adjust loss settings and cable conditions to reflect practical values

To reflect string design in energy yield assessment, checking the loss settings is also essential. In PVSyst, you can set and review various loss factors such as wiring losses, mismatch losses, temperature losses, soiling losses, and conversion losses. Those most closely related to string design are DC-side wiring losses, inter-string mismatch, and non-uniformities caused by orientation and shading.

Cable losses depend on the distance from the string to the junction box and to the PCS, the cable size, and the current level. For ground-mounted installations, the distance from the array to the PCS can be long, while rooftop installations may have complex wiring routes. Entering only a standard loss rate in PVSyst may not match the actual wiring plan. If approximate cable lengths and routes are known at the design stage, it is desirable to make them as close to real-world values as possible.

When the number of modules in series or in parallel changes, the balance between voltage and current also changes, affecting wiring losses. In general, for the same transmitted power, higher voltage makes it easier to suppress current and thus reduce losses. However, if you increase the series count too much to raise the voltage, you will face maximum input voltage issues at low temperatures. In other words, you cannot determine the series count solely for the purpose of loss reduction.

Mismatch losses also need to be checked. If outputs between strings or modules are not aligned due to module-to-module variation, soiling, degradation, temperature differences, shading, orientation differences, etc., the generated energy will be lower than the ideal. In PVSyst you can set them as loss items, but in actual designs it is necessary to mitigate mismatch by grouping modules with similar conditions on the same string or MPPT.

Temperature conditions also affect string design. On roofs, ventilation can be poor, and module temperatures can rise easily. When temperatures are high, power output drops and operating voltage also falls. By checking the temperature model and installation conditions in PVSyst, it becomes easier to determine whether the system will remain within the MPPT range at high temperatures and how much the energy yield will decrease.

When checking loss settings in the PVSyst manual, it is important not to use the default values as-is but to consider whether they are appropriate for the project's conditions. For projects with long cable runs, multiple roof planes, significant shading effects, or hot climates, reviewing the loss settings directly affects decisions on string design. To use simulation results in practice, you must be able to explain how well the input values reflect the site conditions.

Checklist Item 7 Return warnings, errors, and report results to the design

When progressing with string design in PVSyst, warnings or errors regarding voltage ranges, input current, overloading, equipment conditions, sub-array configuration, and the like may appear. These are not merely operational nuisances but important signs to review the design conditions. While calculations may proceed even when warnings appear, in practice you need to understand what they mean and determine whether they are acceptable for the design or require correction.

If a warning about voltage at low temperatures appears, the number of modules in series may be too high. In this case, possible actions include reducing the number of modules per string, changing the PCS, or checking the design temperature conditions. However, reducing the number of modules in series may then bring the system closer to the MPPT lower limit at high temperatures, so it is important not to make a decision based on only one side.

If a warning about the MPPT range appears, the string voltage may not be within the PCS's tracking range. You should consider increasing or decreasing the number of modules in series, reviewing the module model, checking the PCS input specifications, or separating strings with different conditions. The key point is to determine a voltage range that is valid at both high and low temperatures.

If a warning about input current appears, the number of parallel strings or the module current may be exceeding the PCS's allowable rating. In this case, measures such as reducing the number of strings per MPPT, changing the allocation of input circuits, or reviewing the number or model of PCS units are necessary. Extra caution is required with high‑power modules.

In the report results, it is important to look not only at the annual energy production but also at the breakdown of losses. If clipping losses are large, the DC/AC ratio may be excessive. If mismatch losses or shading losses are large, there may be room to review string configuration, MPPT allocation, and layout planning. If cable losses are large, it provides an opportunity to reconsider cable sizes, wiring routes, and PCS placement.

When using the PVSyst manual, it is important not to make clearing warnings your only objective. You need to understand the design-related causes behind warnings and, after making corrections, verify how energy production, losses, equipment conditions, and constructability will change. Simulation is not a one-time run; it is a tool to review results, return to the design, and reconfirm.

How to Apply the PVSyst Manual to Site Design

To make practical use of the PVSyst manual in professional work, it is important not to treat operating procedures, design theory, and site conditions separately. Being able to enter the number of strings on the screen is not the same as being able to construct them that way on site. A high simulated energy yield is not the same as an excellent design when equipment protection, constructability, and maintainability are taken into account.

In string design, the first step is to keep voltage and current within their specified ranges. However, beyond that, considering groupings by roof surface and racking row, cable routes, the locations of junction boxes, the placement of the PCS, inspection flow during maintenance, and provisions for future replacements increases the completeness of the detailed design. PVSyst is effective for quantitatively checking energy yield and losses, but it is the designer’s responsibility to reflect site constraints in the inputs.

Also, when sharing PVSyst results internally or externally, it is important to be able to explain under which conditions the simulation was run. If the module model, PCS model, number of modules in series, number in parallel, MPPT assignment, azimuth, tilt, shading conditions, loss settings, and temperature conditions are organized, the discussion points in a design review will be clear. Conversely, if you present only a report while the conditions are ambiguous, it will be difficult later to explain "why this string configuration was chosen."

String design is not only intended to maximize power generation but also to reduce risk. By checking whether voltages will not rise excessively in winter, whether the system will not fall outside the MPPT range in summer, whether strings affected by shading can be isolated, whether the PCS input is not being overstrained, and whether cable losses are not excessive, you can reduce post-construction problems.

To grasp site conditions, actual measurements and on-site inspections are important in addition to drawings. In particular, terrain undulation, roof surface height, surrounding obstacles, existing equipment, orientation checks, and distance checks affect simulation conditions. By using high-precision positioning devices that can be linked with smartphones, it becomes easier to reflect on-site position information and survey results in design considerations. For example, using an iPhone-mounted GNSS high-precision positioning device such as LRTK makes it easier to identify discrepancies between desk-based design and actual site conditions when performing on-site checks and layout planning for solar power installations.

The purpose of reading the PVSyst manual is not simply to learn how to operate the software. The goal is to organize the design assumptions, understand the meaning of input values, review the results to identify improvements, and implement them into a design that can be used on site. Among these tasks, string design is a crucial process that connects energy yield, equipment specifications, constructability, and maintainability.

Summary

When checking string design in the PVSyst manual, it is important to first select the module and PCS specifications correctly, then verify in order the number of modules in series, the number of parallel strings, MPPT assignment, orientation and tilt, shading conditions, loss settings, and warning messages. String design is not simply the task of deciding how many modules to connect; it is a design process that simultaneously considers voltage, current, energy yield, losses, safety, constructability, and maintainability.

Particularly important are that the open-circuit voltage at low temperatures does not exceed the PCS's maximum input voltage, that the operating voltage at high temperatures falls within the MPPT range, that strings with different conditions are not mixed per MPPT, and that the number of parallel strings is reasonable relative to the input current limit. If the string configuration is decided without confirming these points, simulation results and the actual plant behavior are likely to diverge.

PVSyst warnings and reports are material for reviewing the quality of a design. Rather than merely clearing warnings, it is important to consider why a warning has been raised and what will improve or worsen if you change the number of modules in series or the number of strings in parallel. By looking not only at energy production but also at the breakdown of clipping loss, cable loss, mismatch loss, and shading loss, you can identify areas for improving string design.

In practice, it is essential to ensure that the settings in PVSyst match the layout drawings, single-line wiring diagrams, equipment specifications, and site conditions. Even if a design is valid in the software, if it does not correspond to the actual roof shape, racking layout, cable routes, locations of junction/connection boxes, or the conditions of the PCS input terminals, revisions will be required in later stages.

To master the PVSyst manual, the quickest way is to understand not only the on-screen operation procedures but also how each input value relates to design decisions. By carefully reviewing the seven items of string design, you can increase the reliability of energy-yield assessments and make explanations easier during design reviews and pre-construction checks. To improve the quality of photovoltaic installations, it is important to verify PVSyst settings in connection with site conditions.