PVSyst String Design Method | 8 Steps to Understand PCS Selection

Table of Contents

• Basics to grasp before performing string design in PVSyst

• Step 1 Check the specifications of the solar modules

• Step 2 Determine the upper and lower limits of the number of series-connected modules based on the design temperature

• Step 3 Check the PCS input conditions and narrow down candidates

• Step 4 Combine the number of series modules and the number of parallel strings

• Step 5 Check the balance between DC capacity and AC capacity

• Step 6 Enter the system configuration into PVSyst

• Step 7 Review warnings and loss results and revise the design

• Step 8 Compile the design for practical use, including PCS selection

• Accurately understand site conditions to improve the accuracy of string design

Basics to Understand Before Performing String Design in PVSyst

In designing photovoltaic power generation systems, it is essential to decide how many photovoltaic modules to connect in series and how many strings to connect to the PCS. Using PVSyst, you can proceed with the design while checking not only the energy yield simulation but also the relationships among the modules, the PCS, the number of strings, the voltage range, the current range, the capacity ratio, and the losses. For practitioners searching for "how to use PVSyst," string design is a part that is easy to get stuck on at first. This is because the interface has many input fields, making it hard to know which figures to look at to make a decision.

What's most important in string design is that the open-circuit voltage at low temperatures does not exceed the PCS's maximum input voltage, that the operating voltage at high temperatures does not fall below the PCS's operating range, and that the input current when connected in parallel does not exceed the PCS's allowable range. These are not just desk calculations; they are also reflected in warnings and simulation results in PVSyst. In other words, the basic workflow is to first confirm the electrical feasibility conditions, and then adjust while considering power generation, losses, PCS capacity, and site conditions.

Also, string design cannot be separated from PCS selection. Even with the same number of modules, the optimal configuration changes depending on the PCS’s input voltage range, number of input circuits, maximum input current, rated output, and approach to overloading. Rather than simply "connecting as many modules as possible," it is important to find a configuration in which the PCS operates efficiently throughout the year and is less likely to experience excessive clipping or operation outside its voltage range.

Designing in PVSyst involves the interrelationship of local weather conditions, minimum and maximum temperatures, module specifications, PCS specifications, installation orientation, tilt angle, shading effects, wiring distances, grid connection conditions, and so on. Therefore, string design should not be considered independently but treated as part of the overall power plant design conditions. Below, to enable even beginners to make decisions in line with practical workflow, the process from string design to PCS selection is organized into 8 steps.

Step 1 Check the specifications of the photovoltaic module

The first thing to check is the electrical characteristics of the photovoltaic modules to be used. In PVSyst you select module data and proceed with system design, but it is not sufficient to simply accept the values displayed on the screen. In practice, you verify the module's nominal maximum power, open-circuit voltage, maximum operating voltage, short-circuit current, maximum operating current, and temperature coefficients. Particularly important for string design are the open-circuit voltage and the maximum operating voltage, and how each of them changes with temperature.

The voltage of a photovoltaic module varies with ambient temperature and cell temperature. In general, lower temperatures increase voltage, while higher temperatures decrease voltage. Therefore, during cold winter periods the open-circuit voltage rises, posing a risk of exceeding the PCS’s maximum input voltage. Conversely, during hot summer periods the operating voltage falls, posing a risk of dropping below the PCS’s operating range. The number of modules connected in series per string must be determined within a range that satisfies both conditions.

What you need to be careful about here is not to rely solely on the nominal values. The numbers listed in a module's datasheet are values measured under specific standard conditions. In the actual field, cell temperature can fluctuate significantly above and below those standard conditions. Therefore, you should use the meteorological conditions and design temperature set in PVSyst and design taking into account voltage changes at low and high temperatures.

Also, even within the same output class of modules, there are types with relatively higher voltage and types with relatively higher current. Higher-voltage modules tend to reach the upper limit on the number of modules in series more quickly, while higher-current modules are more likely to be affected by PCS input current constraints. When selecting modules in PVSyst, it is important to check not only the magnitude of the output but also whether the voltage and current characteristics are suitable for pairing with the PCS.

Step 2: Determine the upper and lower limits of the number of cells in series from the design temperature

When determining the number of modules in series in a string, first calculate the upper and lower limits. The upper limit is determined by ensuring the open-circuit voltage at low temperature does not exceed the PCS's maximum input voltage. The lower limit is determined by ensuring the operating voltage at high temperature does not fall below the PCS's operating range, particularly the lower limit for maximum power point tracking (MPPT). The range bounded by these two conditions becomes the practical candidate range for the number of modules in series.

When assessing cold-temperature conditions, we evaluate how much the module's open-circuit voltage will increase based on the lowest expected local temperature. In cold regions or at high elevations, the minimum temperature can fall considerably, so even with the same module you may need to reduce the number of modules connected in series. Conversely, in warm regions the voltage rise at low temperatures is relatively small, which can provide more margin for the allowable maximum number of modules in series.

When evaluating high-temperature conditions, assume a rise in cell temperature during midsummer. Module temperature increases not only because the ambient air is hot but also further when installed on rooftops or close to the ground surface. As module temperature rises, the operating voltage decreases, and if the number of series-connected modules is too low, it becomes difficult for the PCS to enter its tracking range. Even if it appears to be generating power, be aware that there may be an increased amount of time during which the PCS cannot operate efficiently.

In PVSyst, you can check whether the number of modules in series is within the appropriate range for the selected module and PCS combination. If a warning appears, you should either increase or decrease the number of modules in series or reassess the PCS candidates. What is important here is not to aim solely at eliminating the warning. You need to verify together whether there is sufficient margin in the voltage range, whether stable operation can be maintained throughout the year, and whether the design is reasonable given the local conditions.

When deciding the number of modules in series, handling remainders is also a practical point. For example, if there is a remainder when dividing the total number of modules by the number of modules in series, the lengths of the strings connected to the same PCS may not match. Basically, strings connected to the same input are easier to manage—and tend to exhibit less variation in generation characteristics—if they consist of the same number of modules. If you must use different configurations, check the PCS input units and the specifications of each tracking circuit, and consider treating them as separate systems in PVSyst.

Step 3: Check the PCS input conditions and narrow down the candidates

When selecting a PCS, you should not decide based only on the rated output; you need to carefully check the input-side conditions. The main items related to string design are maximum input voltage, operating voltage range, tracking voltage range, maximum input current, allowable short-circuit current, number of input circuits, number of tracking circuits, and the number of strings that can be connected. If these conditions do not match the module configuration, warnings may appear in PVSyst or the design may not be feasible in practice.

The maximum input voltage of the PCS should be compared with the string open-circuit voltage at low temperature. Designs that leave no margin here should be avoided. Considering the site's minimum temperature, module temperature coefficient, installation variability, and data rounding, configurations that only barely meet theoretical limits become difficult to handle in practice. Even if PVSyst does not report an error, it is advisable in design reviews to verify that there is an adequate safety margin.

The tracking voltage range is the range in which the PCS can track the module's maximum power point. The longer the time the string's operating voltage stays within this range, the easier it is for the PCS to extract the generated power. If the number of modules in series is too small, the voltage tends to fall below the lower limit at high temperatures, and if the number in series is too large, it approaches the upper limit at low temperatures. Therefore, when selecting a PCS, check whether the number of series-connected modules you intend to use fits within the tracking voltage range without issue.

Input current is also important. In recent years, many high-power modules have large currents, and depending on the number of strings connected in parallel, it is easy to hit the PCS input current limit. Even if a PCS has multiple input circuits or tracking circuits, you need to check the maximum current per circuit, the overall maximum current, and how short-circuit current is handled. Just because the number of strings can be connected physically does not mean it will be electrically feasible.

When comparing PCS candidates, creating and evaluating multiple configurations in PVSyst makes decision-making easier. Even with the same rated output, the optimal string configuration changes if the input voltage range or the number of input circuits differs. If the number of modules varies by on-site plot, or if there are multiple orientations and tilts, how you divide the tracking circuits is also important. The PCS is the central equipment of a power plant and greatly influences the flexibility of string design, so it is also important not to narrow down candidates too early.

Step 4 Combine the Number of Cells in Series and Parallel

Once the candidate series-module counts and the PCS input conditions become clear, next combine the number of parallel strings. Decide the number of modules in series per string, calculate the required number of strings from the total module count, and assign those strings to each PCS input and to the tracking circuits. In this process, consider not only satisfying both the voltage and current conditions but also the site layout and constructability.

For example, increasing the number of modules in series reduces the number of strings required for the same capacity. Fewer strings make it easier to keep the number of junction boxes and wiring down, but they also make it more likely to approach the voltage upper limit at low temperatures. Decreasing the number of modules in series can provide voltage margin, but it increases the number of strings and affects PCS input current and wiring planning. Which is better depends on module specifications, PCS specifications, site conditions, installation orientation, and wiring distances.

In PVSyst, when configuring strings, the basic principle is to connect strings with the same conditions to each tracking circuit. If strings with largely different orientation, tilt, shading conditions, or numbers of modules in series are mixed on the same tracking circuit, it becomes difficult to fully extract the power output. If roof surfaces are divided into multiple sections, or for ground-mounted installations where shading conditions differ by zone, consider designing with separate tracking circuits.

Also, when allocating the number of strings, we check the capacity balance for each PCS. When using multiple PCS units, if capacity is overly concentrated in one PCS, the evaluation of power generation and losses will be biased. In PVSyst, we enter the configuration separated by PCS units and verify that the module capacity connected to each PCS, the number of strings, and the input conditions are appropriate. It is also an important check to ensure that the module layout on the design drawings matches the input configuration in PVSyst.

When determining the number of parallel strings, consider future maintenance. In facilities with a large number of strings, it is important to configure them so that, in the event of an anomaly, it is easy to trace which string has the problem. If string numbers, connection points, zones, and PCS numbers are organized during the design stage, post-construction inspections and investigations into the causes of reduced power generation will proceed smoothly. PVSyst designs should be compiled not only for simulation but also in a form that can be handed over to actual operation.

Step 5 Check the balance between DC capacity and AC capacity

Once the string configuration is completed, check the balance between the DC capacity and the AC capacity. The DC capacity is the total capacity on the photovoltaic module side, while the AC capacity is the output capacity on the PCS side. In photovoltaic power generation, a design that makes the module capacity slightly larger than the PCS capacity is sometimes adopted. This is because the modules do not always generate at their nominal output and are affected by solar irradiance, temperature, azimuth, tilt, shading, soiling, and other factors.

However, if the DC capacity is increased too much, the PCS may reach its output limit during periods of strong solar irradiance, increasing the time during which generated power cannot be fully utilized. This phenomenon occurs as the PCS output hitting a ceiling. In PVSyst it can be identified as a loss item in the simulation results. Within a certain range it may be acceptable from the viewpoints of economics and annual energy production, but if it is excessively large you will need to review the PCS capacity and string configuration.

The appropriate ratio of DC capacity to AC capacity varies depending on the region and design policy. In areas with good solar irradiance, low temperatures that tend to increase module efficiency, or installations facing south with tilt angles close to optimal, the PCS limit may be reached more easily. On the other hand, for east–west oriented installations, systems distributed across multiple orientations, or installations with a certain degree of shading, peak output is dispersed, and even with the same capacity ratio, curtailment losses may be smaller.

As a way to use PVSyst, first create a configuration that is electrically valid, and then compare simulations while monitoring the capacity ratio. By creating cases that change PCS capacity, cases that change the number of modules in series, and cases that change the number of PCS units, it becomes easier to see differences in annual energy production, losses, peak shaving, and equipment configuration. What is important is not simply choosing the case with the maximum annual energy production, but making a judgment that includes constructability, maintainability, equipment constraints, grid conditions, and on-site area constraints.

Also, the balance between DC capacity and AC capacity is an item that is often checked in plant presentation materials and internal approval documents. You should be able to explain why this PCS capacity was chosen, why this number of strings was selected, how much loss results from oversizing, and whether voltage and current constraints are being satisfied. PVSyst result screens and reports are also useful for organizing the rationale for those explanations.

Step 6 Enter the system configuration in PVSyst

When entering the string design in PVSyst, first set the project's location, meteorological conditions, azimuth, tilt, module, and PCS, and then enter the number of modules in series and the number of parallel strings on the system configuration screen. The screen may display recommended ranges and warnings for the selected module and PCS combination. Beginners tend to overlook the messages shown here, but they provide important clues for verifying the validity of the string design.

The input workflow is: first select the module to be used, then select the PCS. Next, enter the number of modules per string and the number of strings. Based on the entered configuration, PVSyst calculates the total module capacity, PCS capacity, voltage range, current range, capacity ratio, and so on. If errors or warnings appear here, check the causes and correct them.

Common warnings include voltage being too high at low temperatures, string voltage being too low relative to the tracking voltage range, input current being too large, and module capacity being too large relative to PCS capacity. These can sometimes be resolved simply by adjusting the numbers slightly, but fundamentally they may indicate that the combination of module and PCS is not a good match. Forcing ahead without understanding the meaning of the warnings will reduce the reliability of the power generation assessment.

In systems with multiple orientations or multiple tilts, it is important to separate sub-arrays in PVSyst. Even when connected to the same PCS, groups of modules with different orientations or tilts have different power generation characteristics. Therefore, treating them separately in the inputs produces a simulation that more closely reflects reality. Especially for rooftop installations, because the times of peak generation differ for south-, east-, and west-facing surfaces, it is important to align the assignment of tracking circuits with the configuration in PVSyst.

After input, always verify that the total capacity matches the number of modules shown on the drawings. Even if the model is valid in PVSyst, if the module count differs from the drawings, the simulation results do not represent the actual design. In string design, a difference of a single module can affect the overall handling of remainders and PCS allocation. Proceeding while cross-checking the design drawings, equipment specifications, and PVSyst input values reduces rework in later stages.

Step 7 Review warnings and loss results and revise the design

When you run a simulation in PVSyst, check the warnings and losses on the results screen. Losses related to string design include losses associated with the PCS operating range, losses from the PCS output limit, module losses due to temperature, mismatch losses, wiring losses, and shading losses. Review these to assess whether the string configuration and PCS selection are feasible.

First, what I want to check is whether there are any remaining warnings regarding the voltage range. If the open-circuit voltage at low temperature is too high, possible measures include reducing the number of modules in series, choosing a PCS with sufficient margin in maximum input voltage, or reviewing the module specifications. If the operating voltage at high temperature is too low, measures required include increasing the number of modules in series, choosing a PCS with a wider tracking voltage range, or re-evaluating the PCS input splitting.

Next, check the losses caused by the PCS output limit. In designs with an increased capacity ratio, this loss may occur to some extent. The important point is whether that loss is acceptable as part of the design intent. Increasing module capacity to raise annual power generation can result in a design where some generation cannot be captured at peak times. However, if the losses are too large, it may be better to increase the PCS capacity.

Wiring losses are another item that is easy to overlook. In configurations with low string voltage and high current, wiring losses tend to increase. For ground-mounted installations with long cable runs or sites where routing to the PCS is complex, it is necessary to check the electrical design together with the PVSyst inputs. If the wiring conditions in PVSyst remain at their default values, they may not match the actual losses. String design and wiring design may appear to be separate tasks, but they are closely related when evaluating energy yield.

When shading is present, the shading pattern at the string level is also important. If some modules within the same string are shaded, it can affect the power output of the entire string. When performing shading analysis in PVSyst, check not only the 3D scene and obstacle settings but also whether the string assignments are realistic. If shading is concentrated in specific rows or sections, how those parts are connected to PCS inputs will change how losses manifest.

When making design revisions, it is important not to change too many conditions at once. Create cases that change only the number of modules in series, only the PCS capacity, or only the assignment of the number of strings, and compare the results to make it easier to identify the cause. PVSyst is well suited for comparing multiple cases, so you can optimize step by step while retaining the rationale for the design.

Consolidate into a practical design including Step 8: PCS selection

Ultimately, we will compile it not only so that it is valid in PVSyst but also in a format that can be presented as a practical design. Items to be summarized are: the selected module, PCS capacity, number of PCS units, number of modules in series per string, number of strings, number of connections per PCS, DC capacity, AC capacity, capacity ratio, major losses, presence or absence of warnings, and design assumptions. By organizing these, they can be used for internal review, confirmation with the construction company, specification checks before ordering, and inspections after operations begin.

When selecting a PCS, not only the energy yield but also the clarity of the equipment configuration is important. Reducing the number of units can sometimes simplify the configuration, but it can increase the load on each unit and reduce flexibility for individual sections. Increasing the number of units can make zoning and orientation-based control easier, but it increases the number of devices, required installation space, and maintenance scope. Based on PVSyst results, assess the balance between energy yield, losses, constructability, and maintainability.

In practice, it is also important to ensure alignment between the number of PCS input circuits and the on-site layout. Even if things can be arranged neatly on the drawings, if the actual wiring routes become too long or the connection points too complex, construction errors and inspection burdens will increase. String design is not a numerical input for power generation simulation but the design of the electrical circuits that will actually be assembled on site. Always confirm that the configuration determined in PVSyst can be implemented in the site drawings.

Also, as the rationale for selecting the PCS, it is ideal to be able to explain why a PCS with a different capacity or different input conditions was not chosen. For example, organize reasons such as there being margin in the voltage range, the input current staying within limits, the capacity ratio being appropriate, peak losses being within acceptable range, and the ability to appropriately divide modules among multiple orientations. If it is difficult to convey this using only the PVSyst report, leave the reasons for your decisions in a design memo so they are easier to review later.

Furthermore, when considering future expansions or replacements, designing with some margin can be helpful. Because PCS and modules are used for long periods, it may become impossible to obtain equipment with identical specifications in the future. Excessively specialized string configurations or designs that operate at the limits of input conditions can narrow options during maintenance. Making the configuration standard and easy to explain at the initial design stage also contributes to long-term operational stability.

Accurately account for local site conditions to improve the accuracy of string design

String design in PVSyst can be very useful if the input values are correct. However, if site conditions are unclear, no matter how carefully you input data on the screens, discrepancies will arise between the simulation results and the actual power plant. In particular, the installation area, azimuth, tilt, ground elevation differences, surrounding obstructions, wiring routes, and PCS installation location influence string design and PCS selection.

If a design is carried out with inaccurate on-site dimensions or spatial relationships, problems can arise such as modules that appeared to fit on the drawings not actually fitting in place, more shading than anticipated, longer wiring distances, or having to change the placement of the PCS. As a result, it may become necessary to modify the string configuration created in PVSyst later. Accurately capturing site conditions early in the design stage is important not only to improve simulation accuracy but also to reduce rework.

Especially for ground-mounted installations, the topography before and after site preparation, surrounding structures, slopes, fences, adjacent buildings, and trees affect shading and layout. For rooftop installations, roof shape, level differences, equipment, handrails, lightning protection equipment, and maintenance walkways restrict module placement. Accurately identifying these factors and reflecting them in the layout and shading conditions in PVSyst increases the validity of the string design.

Therefore, as a means of linking desk-based design and on-site verification, it is effective to utilize tools that can acquire high-precision location information. LRTK is a GNSS high-precision positioning device that can be attached to an iPhone, allowing on-site location information to be easily obtained and used for design and verification tasks. In candidate site surveys for solar PV installations, it is important to confirm the installation area, grasp the current topography, record the positions of surrounding obstructions, and geotag photos. By accurately preserving this on-site information, it becomes easier to organize the input assumptions for PVSyst.

PVSyst string design is not just the task of matching the numbers of modules and PCS. It is a comprehensive design process: understanding the site conditions, satisfying voltage and current constraints, checking capacity ratios and losses, and selecting a PCS with construction and maintenance in mind. At first the many input items may feel difficult, but if you consider them in the order of module specifications, temperature conditions, PCS input conditions, number of modules in series, number of parallel strings, capacity ratio, loss checks, and site conditions, the decision flow can be organized. By combining analysis in PVSyst with acquiring site information via LRTK, you can reduce the discrepancy between desk design and actual field conditions and achieve more reliable photovoltaic design.