How to Set Mismatch Losses in PVSyst｜5 Basic Items

Table of Contents

• Understand what mismatch losses are

• Basic flow to check mismatch losses in PVSyst

• Basic Item 1: Configure mismatch due to module-to-module variability

• Basic Item 2: Check unevenness in string configuration

• Basic Item 3: Consider current differences caused by irradiance conditions and shading

• Basic Item 4: Account for variations in aging and soiling

• Basic Item 5: Review the loss results in the report

• Practical considerations when setting mismatch losses

• How to improve simulation accuracy to reflect site conditions

• Summary

Understanding What Mismatch Loss Is

Mismatch loss refers to the loss that occurs when the electrical characteristics of solar modules or strings are not perfectly matched. In solar power generation, many modules are connected in series or parallel to generate electricity. Ideally, modules of the same model would appear to produce the same current, the same voltage, and the same output. In reality, however, the power-generation characteristics of each module differ slightly due to manufacturing variations, differences in installation conditions, the way sunlight falls on them, temperature differences, soiling, degradation, and shading.

In series-connected modules, a module with lower current limits the current of the entire string. In other words, even if only one module has reduced output, its effect propagates to the whole string. In parallel connections, differences in voltage and current between strings can arise, causing the overall output to drop below the ideal value. This phenomenon, where the total output becomes smaller than the simple sum due to differences in the capabilities of individual modules or strings, is called mismatch loss.

In PVSyst simulations, annual energy yield is calculated by accumulating various losses in addition to the basic performance of modules and PCS. Mismatch loss is one of these, and even if its input value appears small, it affects annual energy yield, performance ratio, and electricity sales forecasts. Especially when generation forecasts are used by financial institutions, clients, internal approvals, or for design comparisons, it is important not to leave the mismatch loss as a mere default value but to verify that it matches the project conditions.

Mismatch loss does not simply mean "differences in module performance." In practice, it manifests as a combination of electrical individual variations, differences in string configuration, shading and uneven irradiation, variations in soiling and degradation, installation errors, terrain differences, and so on. Therefore, when deciding set values, you need to consider not only the numbers in the specifications but also the on-site layout conditions and operational conditions.

Basic workflow for checking mismatch losses in PVSyst

When checking mismatch losses in PVSyst, first open the target project and the simulation variant, and review the system configuration and the flow of loss settings. Generally, you set the location, meteorological data, azimuth, tilt, modules, PCS, and string configuration in the project, and then confirm the detailed loss conditions. Mismatch loss is often treated as a setting related to module quality and array losses, and is checked within the loss settings screen.

In practice, rather than immediately changing numerical values, it is important to first understand what assumptions the current settings are based on. Even if initial values are set, they are not necessarily appropriate for the project. Small-scale rooftop installations and large-scale ground-mounted installations differ in module count, number of strings, shading effects, construction conditions, and maintenance conditions. Initial values are merely standard assumptions and should be reviewed according to the design conditions.

As the order of checks, first confirm the module model and the allowable output difference. Next, check the number of modules in series per string, the number in parallel, and the connection configuration for each PCS input. Then verify whether arrays are grouped with the same orientation and tilt, whether there are any locations where shading occurs locally, and whether multiple planes are not combined into a single electrical system. Finally, review the loss diagram and reports to determine how much mismatch loss affects annual energy production.

What matters in using PVSyst is not the act of entering numbers into the on‑screen input fields themselves, but understanding what phenomena those numbers represent. Mismatch loss is a parameter that expresses site variability as a single loss rate, so to be able to justify the values you set later, link them to module specifications, drawings, string tables, shading analysis, site photos, installation conditions, and so on—this makes internal reviews and explanations to the client much easier.

Basic Item 1 Set mismatch caused by module-to-module variations

The first basic item to check is mismatch caused by individual module variations. Even modules of the same model do not all have exactly the same output characteristics. Due to manufacturing variations, there are ranges in maximum output, current, and voltage. Even if these are within the product specifications, those small differences can accumulate across the entire power plant and lead to differences in output at the string level.

When configuring this item in PVSyst, first check the specifications of the module you will use. The datasheet lists rated power, tolerance, short-circuit current, open-circuit voltage, maximum operating current, maximum operating voltage, temperature coefficients, and so on. Among these, the factors that are likely to relate to mismatch are variations in output and current. In particular, in series connections, modules with lower current become the limiting factor, so you need to be mindful of current differences between modules.

As a practical matter, it is not common to enter detailed measured values for each individual module. Therefore, in PVSyst a representative loss rate—the mismatch loss—is entered. In the early design stage, it is realistic to provisionally set a standard value and review it as the project's conditions become clearer. In the final simulation submitted to the client, the value is adjusted—taking into account the specifications of the selected modules, output tolerances, and quality control conditions—so that it is neither overly optimistic nor pessimistic.

If the mismatch caused by module-to-module variability is underestimated, the simulated energy yield tends to appear higher than the actual one. Conversely, if it is overestimated more than necessary, evaluations of design proposals can become overly conservative and may affect the profitability assessment of the power generation project. The important thing is not simply to make losses look small, but to use explainable values that reflect realistic variability.

Furthermore, in projects where module lot management and sorting are performed, it may be possible to reduce mismatch losses. For example, this applies when construction management involves grouping modules within the same power bin into the same string, or placing modules with similar specifications in the same area. Conversely, conditions such as mixing multiple lots, replacement units arriving later, or mixing different module types with similar specifications increase the likelihood of mismatch. The settings in PVSyst should be aligned with these actual construction and procurement conditions.

Basic Item 2 Check for inconsistencies in string configuration

The second basic item is uneven string configuration. Mismatch losses are affected not only by variations among individual modules but also by string design. If the number of modules in series per string, orientation, tilt, connection destination, or electrical conditions are not consistent, the operating points of strings connected to the same PCS input can become misaligned, which may lead to reduced output.

When configuring string layouts in PVSyst, check not only the number of modules and the number of PCS inputs, but also which surface or orientation each string is installed on. For example, if you mix a south-facing string and an east-facing string on the same PCS input, their power generation characteristics will differ by time of day. East-facing strings are stronger in the morning and south-facing strings are stronger at midday, so grouping them on the same input can shift the operating point away from the ideal. Similarly, be careful when combining surfaces with significantly different tilt angles on the same input.

Also pay attention to configurations where the number of modules in series per string differs. If, due to design constraints, some strings necessarily have fewer or more modules, the voltage conditions will change. Even if this is within the PCS's allowable input range, it can affect the operational balance when connected in parallel. After entering the system configuration in PVSyst, it is important not only to check that no errors or warnings are displayed, but also to verify that the string configuration matches the actual wiring plan.

In practice, even if drawings treat them as the same array, on site the way shadows fall and the terrain gradient can differ from row to row. In particular for ground-mounted installations, factors such as terracing or changes in grading, embankments, surrounding trees, adjacent equipment, fences, and differences in racking height can alter solar irradiance even across areas that appear to have the same azimuth and tilt. When such differences in conditions are significant, rather than simply treating everything as a single array, it is easier to explain losses—including those from mismatch—if you configure them separately by electrical system or by surface.

If the PVSyst results are better than expected, or if shading losses or mismatch losses are unnaturally small, review whether the string configuration has been overly idealized. Even if the simulation assumes strings are arranged uniformly, the actual site may not be uniform. When using PVSyst, do not rely solely on the input values shown on the screen; it is essential to cross-check them against the layout plan, single-line wiring diagram, and string table.

Basic Item 3: Consider differences in current due to solar irradiance conditions and shading

The third key factor is the difference in current caused by irradiance conditions and shading. Mismatch losses arise not only from differences in module specifications but also from variations in how sunlight falls on them. Because the output of solar modules is heavily influenced by irradiance, if only some modules in the same string are shaded, the current of those modules drops and the generation of the entire string is affected.

In PVSyst, you can reflect shading losses in the simulation by performing shading analysis and configuring nearby obstructions. However, shading loss and mismatch loss are not exactly the same. Shading loss represents the direct reduction in power generation caused by a decrease in solar irradiance due to obstructions. On the other hand, mismatch loss encompasses losses caused by an electrical imbalance between modules and strings due to non-uniform irradiance. In other words, for a site with shading you need to consider not only shading loss but also the mismatch effects induced by that shading.

For example, if thin shadows from utility poles or trees fall on the edge of an array, the shaded area may seem small, but if the shadow is concentrated on specific modules or cell strings, the electrical impact can be significant. Also, if shadows from adjacent rows fall on part of the modules in the morning or evening, they may occur only briefly but repeat throughout the year, resulting in losses that are difficult to ignore. When simulating with PVSyst, it is important to check not only the annual total shading losses but also the times of day when shadows occur and how the shadows are cast.

Non-uniform solar irradiance conditions are affected not only by shading but also by topography and layout. On sites that are long in the east–west direction, sloped terrain, roofs with level changes, or projects using multiple roof planes, the irradiance on each array can differ even at the same time of day. If strings with different irradiance conditions are combined into the same input, mismatches can increase. During the design phase, consider measures such as placing heavily shaded areas on separate circuits, separating faces with different orientations or tilts, and isolating rows that are prone to shading.

In PVSyst settings, it is important to review the mismatch loss values after performing shading analysis. In ideal projects without shading, it may be sufficient to focus on module-to-module variations. However, in projects with shading you must consider the impact of shading-induced nonuniform generation on the entire string. The idea that mismatch losses are unnecessary simply because shading analysis is set up as a separate item is dangerous. Distinguish the meanings of the two and adjust to realistic values while taking care not to double-count.

Basic Item 4 Assume variability in aging deterioration and soiling

The fourth basic item is variability in degradation and soiling. Mismatch losses can change not only in the plant’s first year but may vary as the years of operation progress. Solar modules degrade gradually over time, but not all modules degrade at the same rate. Installation location, temperature conditions, humidity, soiling, local shading, installation quality and other factors can cause differences in degradation rates among modules.

The same applies to soiling. The entire power plant does not become soiled uniformly. Rows near roads, rows near farmland, differences between windward and leeward sides, places with puddles or mud splash, locations prone to bird droppings, and spots where fallen leaves tend to collect can all experience localized soiling. When soiling concentrates on some modules, the current of those modules decreases, leading to mismatch within the string.

In PVSyst, settings for soiling losses and degradation over time may be handled separately, but it is also necessary to be aware of their relationship with mismatch losses. If soiling loss is set as a uniform percentage across the entire plant, it assumes all modules will become soiled in the same way. However, in reality there is variation in how modules become soiled, so aspects that cannot be represented by a uniform soiling loss may appear as mismatch.

In long-term power generation forecasts, it is important to consider not only the initial year but also the condition several years or a decade or more later. Even if initial module quality is high, insufficient maintenance can increase mismatch due to localized soiling or degradation. Conversely, in projects where regular inspections, cleaning, early detection of abnormal modules, and string monitoring are properly carried out, it becomes easier to curb the deterioration of mismatch.

When practitioners set mismatch loss in PVSyst, they should not determine the value solely by looking at initial performance but also take into account the operational management conditions. For example, whether there is a regular inspection plan, whether anomalies can be detected at the module or string level, how frequently cleaning is performed, and whether the surrounding environment has many sources of soiling. These conditions are not easily reflected directly in the simulation input values themselves, but they are important as a basis for explaining the mismatch loss.

Basic Item 5 Review the loss results in the report

The fifth basic point is to review how mismatch losses are reflected in the results of the PVSyst report. Simply entering values on the input screen does not mean you have correctly understood their impact on energy production. After running the simulation, check the loss diagram, annual production, performance ratio, system output, and monthly results to understand how mismatch losses are positioned within the overall picture.

In the loss diagram, the flow is organized from solar irradiance entering the module surface, conversion within the module, various losses, PCS output, to grid-side output. Among these, mismatch loss is displayed alongside other losses. Rather than concluding that a small value means it is unimportant, it should be viewed in relation to the other loss items. For example, in a project with large shading losses, if the mismatch loss is extremely small, there is room to verify whether nonuniform generation due to shading has been sufficiently taken into account.

When comparing multiple design proposals, confirm that the mismatch loss settings are based on the same assumptions. If only one proposal uses a relaxed loss setting while the other is conservative, the comparison will not be fair. In particular, when comparing differences in azimuth, tilt, number of strings, or shading conditions, you should consider whether it is appropriate to treat mismatch loss as the same fixed value.

When sharing reports within the company or with clients, it is reassuring to be able to concisely explain not only the mismatch loss values but also the assumptions behind them. For example, clarify whether the values consider module-to-module variation, whether they also include electrical nonuniformity caused by shading, or whether variations due to soiling and aging are treated as separate items. Because PVSyst reports present many figures, readers can become unsure about what each loss means. Practitioners should not only generate the report but also be prepared to explain the intent behind the loss settings.

When checking results, we look not only at annual values but also at monthly trends. If the effects of shading and irradiation conditions change significantly with the seasons, losses related to mismatch may also be concentrated in particular seasons or times of day. For projects where the solar altitude is low in winter and shadows lengthen, projects with strong morning and evening shading, or projects affected by the surrounding environment, it can be difficult to grasp the actual situation from annual averages alone. When using PVSyst, it is important not to pursue the annual generation number alone, but to interpret the underlying causes of losses.

Practical considerations when setting mismatch loss

When setting mismatch loss, the thing to avoid most is continuing to use the initial value without any justification. Initial values are convenient, but they do not automatically account for project-specific conditions. The appropriate approach varies depending on plant size, module specifications, string configuration, shading conditions, terrain, construction accuracy, and maintenance plans. While provisional values are acceptable in the early design stage, for simulations used for submission or decision-making this is an item that must be reviewed at least once.

What you should watch out for next is overlap with other loss items. In PVSyst, many losses are configured individually, such as shading losses, soiling losses, wiring losses, module quality losses, and temperature losses. If mismatch loss is set too high, you may end up double-counting the effects of shading or soiling that have already been considered under other items. Conversely, because some effects are treated uniformly under other items, you may underestimate the non-uniformities that would appear as mismatch. The important thing is to clarify what each loss item represents and to avoid both overlaps and omissions.

Also, when multiple people within a company use PVSyst, it is important to share rules for setting mismatch losses. If each person makes different judgments, forecasts of power generation can differ even for similar projects. Organizing internal rules on what values to use for standard projects, how to review settings for projects with shading, how to handle roofs with multiple orientations, and which conditions should be rechecked when design changes are made will stabilize quality.

It is also important to adjust the level of detail in your explanation depending on who you submit the simulation results to. For recipients familiar with electrical design, you can explain including string configurations and deviations in operating point. On the other hand, for clients or non-technical internal departments, it may be more effective to present it as an item that slightly reduces power generation due to variability in modules and strings. PVSyst settings are technical, but since they are ultimately used for business and construction decisions, the practical value lies in being able to explain them clearly.

How to Improve Simulation Accuracy by Incorporating On-site Conditions

To make mismatch losses more realistic, it is important to accurately capture the site conditions, not just the desk-based design assumptions. In simulations of solar power plants, meteorological data and equipment specifications tend to attract attention, but actual energy production is greatly affected by terrain, azimuth, tilt, surrounding obstructions, racking height, row spacing, and deviations in installation positions. If these conditions differ between the design drawings and the actual site, no matter how carefully you configure PVSyst, the results may deviate from reality.

Shading and the non-uniformity of solar irradiance, in particular, are closely related to mismatch losses. Accurately knowing the positions and heights of surrounding trees, buildings, fences, utility poles, slopes, and adjacent equipment improves the accuracy of shadow analysis and makes it easier to assess the validity of mismatch loss estimates. Conversely, if the locations of obstructions are only known approximately, the extent and timing of shading cannot be correctly evaluated, and loss settings tend to become subjective.

It is also important to measure the site's topography accurately. Even sites that appear flat can have slight slopes or undulations that affect the height of the mounting structures and the perceived spacing between rows. On sloped or graded land, solar irradiance conditions can vary from row to row. To make PVSyst inputs more realistic, site surveys before design, as-built verification after construction, and recording the positions of obstructions are useful.

For such on-site assessments, high-precision positioning using smartphones is also effective. LRTK is a GNSS high-precision positioning device that can be attached to an iPhone and can be used for acquiring on-site coordinates, recording the locations of features, verifying conditions before and after construction, obtaining point clouds, and geotagging photos. In planning and maintaining solar power plants, it is important to be able to quickly record on-site locations such as racking positions, obstructions, site boundaries, slopes, roads, and drainage facilities. To set realistic mismatch and shading losses in PVSyst, accurately capturing on-site location information — not just desk-based conditions — is a great help.

PVSyst is a powerful simulation environment for assessing energy yield, but its input conditions depend on the actual site conditions. By correctly understanding the site's geometry and obstructions and setting up simulation conditions while cross-checking them against the design drawings, you can improve the reliability of energy yield predictions, including mismatch losses.

Summary

To understand how to set mismatch losses in PVSyst, simply entering a loss rate in the input field is not sufficient. Mismatch losses arise from multiple overlapping factors—module-to-module variability, string configuration, shading and irradiance non-uniformity, soiling, and variability in aging and degradation—so when deciding on a value you need to review module specifications, the string table, layout drawings, shading analysis, site conditions, and the maintenance plan together.

As the five basic items, first check for mismatch caused by module-to-module variability, then review any unevenness in the string configuration. Next, take into account current differences due to shading and irradiance conditions, and also assume variations from soiling and age-related degradation. Finally, by rereading how mismatch losses are reflected in the PVSyst report, it becomes easier to explain the relationship between the input values and the power generation.

Mismatch loss often appears to be less than a few percent. However, it affects annual energy production and the performance ratio, and is relevant to design comparisons and feasibility assessments. In particular, projects with shading, roofs with multiple orientations, sloping terrain, large ground-mounted installations, or complex string configurations should be checked carefully. For practical use of PVSyst, it is important not to leave the default value as is, but to be able to justify it according to the project conditions.

Furthermore, improving simulation accuracy requires a solid understanding of on-site conditions. If rack locations, obstructions, terrain, site boundaries, and the positions of surrounding equipment can be recorded accurately, the rationale for shading analysis and loss settings becomes clear. By using iPhone-mounted, high-precision GNSS positioning devices such as LRTK, you can streamline on-site position recording, point cloud acquisition, and photo management, and it also helps confirm the assumptions to be entered into PVSyst. Not leaving power generation simulations to desk calculations alone but bringing them closer to the actual field conditions enables correct handling of mismatch losses and leads to more reliable photovoltaic system design.