5 Points for Reading Monthly Results in the PVSyst Manual

Table of Contents

• What does the monthly results screen in PVSyst show?

• The overall picture I want to check first in the monthly results

• Key points for interpreting the relationship between solar irradiance and power generation

• Key points to distinguish seasonal variation from temperature effects

• Key points for reviewing design conditions based on how losses increase

• Key points for using simulation results in presentation materials

• Misconceptions that commonly arise when reading monthly results

• The importance of linking on-site conditions with monthly results for decision-making

What does PVSyst's Monthly Results screen show?

When reviewing PV simulation results using the PVSyst manual, many people initially stumble over how to read the monthly results. If you only look at the annual energy yield, the project's overall outlook may seem apparent at first glance. However, in practice, relying solely on the annual figure can cause you to overlook the validity of the design conditions, the timing of losses, and seasonal generation trends.

Monthly results are not simply a table listing the power generated each month. They are important clues for interpreting, over time, factors such as solar irradiance, the effective light incident on the module surface, output reductions due to temperature, PCS-side constraints, losses like shading and soiling, and the final amount of energy that can be sent to the grid.

In particular, at the design and proposal stages, it is important to be able to explain why the annual power generation is what it is. Being able to grasp monthly trends—whether generation increases in spring and autumn, why it does not rise as much in summer despite higher solar irradiance, and how much it falls in winter—will give a deeper understanding of the simulation results.

Many people searching for the PVSyst manual want not only the operating procedures themselves, but also to know how to interpret the results, which numbers to prioritize, and where to look when energy production is lower than expected. Monthly results provide an entry point for answering those questions.

This article outlines five key points to keep in mind when reading monthly results in the PVSyst manual, organized from a practical, work-oriented perspective. Rather than simply checking the numbers on the screen, consciously reading them in a way that connects to design, explanation, verification, and review will allow you to make more effective use of the simulation results.

Overall picture to check first in the monthly results

When reviewing PVSyst's monthly results, it's important not to jump straight into the detailed loss items; first, check the overall trend. Get a rough understanding of how monthly energy production changes and which times of the year have relatively high or low production.

In solar power generation, solar irradiance conditions change with the seasons. Generally, the more solar irradiance there is, the more power output tends to increase, and the less solar irradiance there is, the more power output tends to decrease. However, power output is not determined by solar irradiance alone. Temperature, azimuth, tilt, shading, snow cover, soiling, PCS capacity, and wiring losses, among others, interact to affect it.

When reading the monthly results, the first point is to check whether increases or decreases in generation can be explained as natural seasonal variations. For example, if generation is low in winter, you need to distinguish whether that is due to reduced solar irradiance or shading effects from snow or low solar altitude. If generation does not rise in summer, it may not be caused by insufficient irradiance but could be related to reduced module output from high temperatures or output limits of the PCS.

When reviewing monthly results, do not look at energy generation by itself; check it together with solar irradiance and the flow of losses. If months with high solar irradiance also show high generation, the overall result is easy to understand. On the other hand, if a month has high solar irradiance but generation does not increase, there may be some kind of constraint or significant losses occurring in that month.

Also, the way monthly results are interpreted varies depending on the type of project. For large ground-mounted projects, surrounding topography, inter-row shading, and clipping caused by oversizing design are important. For roof-mounted projects, roof pitch, orientation, surrounding buildings, parapets, and shadows from equipment come into play. For self-consumption projects, you must consider not only the amount of generation but also its relationship with demand patterns.

At this stage, rather than judging the correctness of detailed numerical values, we check whether the monthly peaks and valleys align with the design conditions. If a south-facing project with an appropriate tilt shows a drastic drop in power generation in specific months, that becomes a cue to review shading conditions, meteorological data, and loss settings. Conversely, if seasonal changes are gradual and consistent with changes in solar irradiance, it becomes easier to proceed to the next checks.

When reading the PVSyst manual, it is important not only to memorize the meaning of each item individually but also to understand the monthly results as a single flow. The monthly figures are signals indicating how the design conditions are reflected in the results. If you grasp the overall picture first, your basis for judgment will be less likely to shift when you read the detailed loss items.

Key points for interpreting the relationship between solar irradiance and power output

What is particularly important in the monthly results is the relationship between solar irradiation and energy production. In PVSyst, based on meteorological data, horizontal-plane irradiation and tilted-plane irradiation are handled and ultimately converted into the energy received by the PV array. When reading the monthly results, first check which months have higher irradiation and which have lower, and then see how the energy production responds to that.

The important point here is not to confuse irradiance on the horizontal plane with irradiance on the module plane. Solar panels are not necessarily mounted horizontally. The actual irradiance received by the module plane changes with roof tilt, racking angle, and orientation. Therefore, even in months when horizontal-plane irradiance in meteorological data is high, the incidence conditions on the module plane are not necessarily optimal.

For example, in summer the solar altitude is high and the incident solar radiation on a horizontal surface tends to be larger. However, for installations with a large tilt angle, the angle of incidence on the module surface may not be optimal for the high summer sun. Conversely, in winter the hours of sunshine and the amount of solar radiation tend to be lower, but depending on the tilt angle this can work relatively advantageously for the low solar altitude.

PVSyst's monthly results allow you to verify these relationships numerically. Months with high energy production may not only have higher irradiance but also favorable incidence on the module surface and reduced losses. Conversely, months with high irradiance but stagnant generation should prompt suspicion of other factors such as temperature losses, PCS limitations, shading, or soiling.

What helps with this check is a sense of month-to-month generation efficiency. For example, if solar irradiance has increased substantially from the previous month but generated output has only risen slightly, the difference may indicate some kind of loss. Conversely, if generated output rises in step with the increase in solar irradiance, it is easier to regard the simulation results as reflecting a natural trend.

Also, when examining monthly results, consider the climatic characteristics of the site. The timing of the rainy season, typhoon season, snowfall season, dry season, and periods with frequent overcast skies varies by region. If you judge monthly power generation with a uniform, nationwide perspective, you may misinterpret regional characteristics. Even when checking results while reading the PVSyst manual, it is important to compare not only the figures in the software but also the local meteorological conditions.

The purpose of examining the relationship between solar irradiance and power generation is not merely to be satisfied with whether the output is high or low. It is to understand how design conditions affect power generation and, if necessary, to review orientation, tilt, layout, and loss settings. The monthly results contain hints for improving the design.

By addressing this point, you will improve your ability to explain the annual generation figures. Rather than merely stating how many kWh will be generated per year, you should be able to clarify which months see increases and which months see declines, and whether the reasons are solar irradiance, design conditions, or losses. To make proposals and internal reviews more persuasive, the relationship between solar irradiance and power generation should be carefully analyzed first.

Key Points for Distinguishing Seasonal Variation and Temperature Effects

What is easy to overlook when reading monthly results is the difference between seasonal variation and temperature effects. Solar power generation tends to increase when solar irradiance is high, while output decreases when module temperature rises. For this reason, in summer, even if solar irradiance is abundant, the generated power may not increase as much as expected.

When reviewing monthly results while consulting the PVSyst manual, it is risky to simply estimate higher generation for the summer. During periods of high ambient temperature when module temperatures rise, temperature-related losses increase. In particular, for roof-mounted projects the module temperature can rise more easily depending on the distance from the roof surface and ventilation conditions, so they may be more susceptible to temperature effects than ground-mounted installations.

On the other hand, in spring and autumn there is sufficient solar radiation and temperatures are not too high, so power generation efficiency can appear better. If monthly results show higher generation in spring or autumn, this may indicate a favorable balance between solar radiation and temperature conditions. Understanding that summer is not necessarily the month of maximum generation can help prevent misinterpretation of the results.

Although lower temperatures in winter tend to reduce thermal losses, solar irradiance and sunshine duration are insufficient and the sun's altitude is lower. As a result, even if temperature conditions alone appear favorable, total power generation is generally lower. Furthermore, in snowy regions it is necessary to consider snow-related shading and reflection, as well as the near-stop of power generation until snow is cleared.

In the monthly results, check which months show large losses due to temperature. If temperature losses are large in summer and the increase in power generation is being suppressed, that can be natural in the simulation. However, if losses are extremely large compared to expectations, you should check the inputs for module installation conditions, ventilation settings, the temperature model, and the mounting method.

When evaluating temperature effects, consistency with the design conditions is also important. For example, if the assumption is a ground-mounted installation with adequate ventilation, are the temperature conditions inadvertently set as if it were mounted flush to the roof? Conversely, if the installation is on a rooftop with limited rear ventilation, is the temperature rise being underestimated? These differences in inputs tend to show up in the summer values of the monthly results.

The relationship with the PCS output limit is also important. In somewhat oversized designs, during periods of high solar irradiance when module output is large, the PCS may cap the output. In the monthly results, it becomes easier to assess the appropriateness of the DC-to-AC ratio by seeing, during the high-irradiance periods of summer and spring, how much the AC-side output is being curtailed relative to the potentially available DC energy.

One thing to be careful about here is not to treat temperature loss and PCS limitation as the same thing. Temperature loss is the phenomenon in which a module’s power generation capability decreases due to a rise in temperature. By contrast, PCS limitation is the phenomenon where, at the stage of converting the generated DC power to AC, an upper limit is imposed by the PCS capacity or its control. Both reduce the amount of power generated, but their causes and mitigation measures are different.

If temperature losses are large, review of the installation method, ventilation, module selection, and racking conditions should be considered. If PCS limitations are large, revisit the DC/AC ratio, PCS capacity, the approach to oversizing design, and the relationship with feed-in restrictions and self-consumption conditions. In this way, when reading monthly results it is important to separate which losses are large in which seasons.

When you can distinguish seasonal variation from temperature effects, explanations of power generation become much more specific. If you can explain why power output does not peak in summer despite higher solar irradiance, and why generation efficiency appears higher in spring and autumn, confidence in the simulation results will also increase.

Key Points for Reassessing Design Conditions Based on How Losses Increase

When reading the monthly results from PVSyst, the way losses are assessed is extremely important. When energy production is low, many people only look at the final output. In practice, however, what is needed is to understand at which stage and in what way losses occur, and to use that understanding to revise the design conditions.

In solar power generation simulations, as solar irradiance reaches the modules, is converted into DC power, passes through the PCS and becomes AC power, and is ultimately consumed or connected to the grid, multiple losses occur. Typical losses include shading losses, reflection losses, temperature-related losses, mismatch losses, wiring losses, PCS conversion losses, clipping, soiling, and snow.

When examining monthly results, check whether these losses occur evenly throughout the year or are concentrated in specific months. If losses are significantly larger in certain months, conditions that commonly occur during those months may be influencing them. For example, if shading losses increase in winter, the lower solar altitude may cause shadows from nearby buildings, terrain, or front-row modules to extend further.

In ground-mounted projects where inter-row shading is a problem, winter shading losses tend to show up more clearly in the monthly results. Even if the annual figures make the impact look small, certain months can have a large effect on power generation. In such cases, this provides grounds for reviewing inter-row pitch, tilt angle, azimuth, installation height, and terrain slope.

In rooftop installation projects, shadows from surrounding buildings, rooftop penthouses, ventilation equipment, parapets, antennas, and adjacent equipment are reflected in the monthly results. In particular, because shadows tend to extend in winter and during morning and evening hours, if monthly shading losses are large, it is necessary to check the shading-condition inputs and the accuracy of the 3D model creation.

Losses from soiling are also important when examining monthly results. If soiling is set at a constant rate, its impact will be similar throughout the year, but if seasonal effects such as soiling, rainfall, snowfall, pollen, and dust or sand are taken into account, the pattern of monthly losses will change. In arid regions, near agricultural land, on construction sites, or in areas with heavy traffic, care should be taken not to underestimate the impact of soiling.

Wiring losses and PCS conversion losses are items that occur relatively year-round. However, during periods of higher power generation the current increases and the impact of losses tends to become larger, so when viewed on a monthly basis the amount of loss may appear to increase in high-generation months. This is not necessarily abnormal, but it is necessary to confirm the consistency of the loss rates with the design conditions.

Clipping is an aspect that requires particular attention in oversized designs. If the DC-side equipment capacity is increased while the PCS capacity is set relatively small, generated power can exceed the PCS output limit during periods of strong solar irradiance. That excess is not reflected in the final AC output. In monthly results, the impact of clipping can increase in months with favorable solar irradiance.

This loss is not necessarily bad. In an oversized design, the idea is to increase generation during low-irradiance periods — mornings, evenings, and winter — while accepting that some output will be clipped during high-irradiance periods. What matters is whether the losses from clipping are within acceptable limits from the perspectives of economics and plant utilization. Monthly results provide the basis for assessing that balance.

What matters when evaluating losses is not to focus solely on the fact that losses are large. If the losses are those anticipated in the design, they are an explainable result. The problem is when unintended losses are large or when the loss settings do not match the on-site conditions. It is important to detect discrepancies between input conditions and output results by reviewing the monthly results.

When reading the PVSyst manual, not only should you memorize the meaning of each loss term, but you should also adopt the perspective of checking in which months each loss increases and whether the reason can be explained by the design conditions; this makes it easier to review the results. Monthly trends in losses are a practical checkpoint for verifying design quality.

Key Points for Using Simulation Results in Presentation Materials

PVSyst's monthly results are not only for designers to check. They can also be used as materials to explain the projected power generation to the project owner, internal stakeholders, financial institutions, construction personnel, operations personnel, and others. Therefore, when reading the monthly results, it is also important to organize them into a form that is easy to explain.

First, when using monthly results in explanatory materials, it is important to convey not only the annual generation but also the seasonal generation trends. Make sure you can briefly explain the periods when generation is at its highest and when it tends to decline, and the reasons for those changes. For example, generation increases from spring to early summer because solar radiation conditions are favorable and temperature-related losses are relatively limited. Generation decreases in winter due to reductions in solar irradiance and hours of sunlight, the lower solar altitude, and, in some cases, the effects of snow or shading.

Next, it is also important not to explain monthly variations in excessive detail. PVSyst's monthly results are simulations and do not fully predict actual monthly energy generation. Weather conditions vary from year to year. If the rainy season in a particular year is prolonged, generation will decrease; if there are many sunny days, it may exceed expectations. In explanatory materials, you need to communicate, without exaggeration or omission, that the monthly results are forecasts based on standard conditions and that actual values may differ on a monthly basis.

However, it is important not to end with a mere caveat; you should be able to explain why you arrived at that estimate. Organizing the relationships among solar irradiance, temperature, shading, system capacity, PCS capacity, and loss settings will make it easier to answer stakeholders' questions.

In self-consumption projects, the relationship between monthly generation and demand is also important. Even in months with high generation, surpluses may increase if demand is low. Conversely, in months with low generation, the self-consumption rate can be high if the timing of demand aligns with generation. When using PVSyst’s monthly results, you need to explain not only the generation figures but also the demand patterns and how surplus power is handled.

For power-selling projects, projected annual generation and power sales revenue are important, but monthly generation fluctuations also affect cash flow and operations planning. When generation is concentrated in certain periods, this also affects decisions about when to schedule inspections and maintenance. This is because carrying out long-duration work that requires shutdowns during high-generation periods would cause significant losses.

When using monthly results in explanatory materials, rather than simply pasting screenshots from PVSyst, it is desirable to add supplementary explanations in language that is easy for the reader to understand. If there are too many technical terms, stakeholders will not be able to grasp the meaning of the numbers. For example, even when using the term "temperature loss," adding an explanation such as "solar cell output tends to decrease during periods of high ambient temperature" will make it easier to convey.

On the other hand, if you round numbers too much or cherry-pick only the favorable parts, you may have trouble explaining them later. Monthly results, including both good and bad months, constitute the annual estimate. Even if there are months with low power generation, if the reason is clear, it actually leads to a more reliable explanation.

When checking the monthly results items in the PVSyst manual, it is important not merely to read them as they are but to convert them into a form that can be used to explain the project. The figures intended for designers and the explanations for decision-makers should differ. To make monthly results useful in practice, both technical accuracy and clarity of explanation are required.

Common misunderstandings when reading monthly results

When reading monthly results, several misunderstandings can easily arise. The most common is assuming that the month with the highest generation will necessarily have the highest solar irradiance. In reality, because solar irradiance, angle of incidence, temperature losses, PCS limits, shading, and other factors combine, the month of peak generation may not coincide with the month of peak irradiance.

Another common case is judging that summer power generation is abnormal because it isn’t as high as expected. In summer, although solar irradiance is greater, module temperatures tend to rise and temperature-related losses can increase. Also, in over‑sized designs, output limits on the PCS side are more likely to occur. Therefore, even if summer generation does not reach its maximum, it is not necessarily due to input errors.

Conversely, it is premature to judge the design as poor based solely on lower winter power generation. Winter has shorter sunlight hours, a lower solar altitude, and is a season when shadows tend to lengthen. In snowy regions, there is also the impact of snow. It is necessary to separate whether the low winter generation can be explained by natural conditions or is due to design issues.

Furthermore, it is a misunderstanding to consider the monthly results as exactly matching the actual monthly power generation performance. Simulations are predictions based on the input meteorological data and design conditions. In actual operation, monthly differences can arise due to the weather in a given year, equipment failures, shutdowns, soiling, inspections, and changes in the surrounding environment. Monthly results are not a guarantee of monthly performance; they are intended to capture trends under standard conditions.

Furthermore, you should be cautious about judging good or bad solely by the loss rate. Even if a loss appears large, you must determine whether it was allowed by the design. For example, even if a certain amount of clipping due to overloading occurs, the design may be reasonable if annual energy production and economics improve. Losses are not always better just because they approach zero.

The same applies to shading losses. Trying to avoid shadows completely can greatly reduce the installed capacity or lower land-use efficiency. Allowing some shading loss may lead to higher overall energy production and profitability. When reading monthly results, it is necessary to consider not only the magnitude of the losses but also the overall objectives of the design.

When checking items against the PVSyst manual, people tend to focus on reading the numbers accurately. However, what matters in practice is interpreting the meaning of those numbers in context. Even for the same monthly generation, evaluations change depending on the region, installation method, contract terms, demand characteristics, and operational policies.

To avoid such misunderstandings, it is important not to view monthly results in isolation. Verify them alongside annual results, loss diagrams, design conditions, meteorological data, shading analysis, report outputs, and so on, and ensure consistency. Monthly results are part of understanding the overall simulation, yet they are an important element that directly informs practical decision-making.

The Importance of Linking On-site Conditions with Monthly Results for Decision-Making

To read PVSyst's monthly results correctly, it is essential to judge them in connection with site conditions, not just the figures shown in the software. The explanatory power of a solar power simulation increases the closer the input conditions are to the actual on-site situation. Conversely, if inputs are made without sufficiently grasping the site conditions, the monthly results may look neat yet still deviate from actual operation.

The most important site conditions are orientation and tilt. For rooftop installations, the orientation and pitch can vary from one roof surface to another. Even for ground-mounted systems, the overall site may not be perfectly flat, and grading slopes or terrain inclines can have an effect. If these conditions are oversimplified, the monthly solar radiation and the way shadows appear may not match reality.

Next, understanding surrounding obstacles is also important. Buildings, trees, mountains, slopes, utility poles, equipment, fences, and adjacent structures can change the impact of shadows depending on the season and time of day. In particular, in winter the sun's altitude is lower, so even distant obstacles can have an effect. If monthly results show a large drop in winter power generation, comparing them with the on-site shading conditions makes it easier to determine the reason.

Topography cannot be ignored either. In mountainous areas and on slopes, surrounding terrain can affect conditions around sunrise and sunset. If there are conditions that cannot be captured by meteorological data from flatlands alone, monthly results may differ from actual power generation trends. For development sites and ground-mounted projects, it is important to use survey data and terrain data to make the simulation assumptions as close to the actual site as possible.

The impact of soiling and snowfall also depends heavily on local conditions. In snowy regions, reductions in winter power output should not be treated merely as insufficient solar irradiance; factors such as the duration that snow covers the modules, how easily snow sheds due to tilt, and whether snow removal is carried out must be considered. Near agricultural land and at sites immediately after development, sand dust and mud splashing can have significant effects. Along coastlines, salt, and near factories, dust and other particulates should also be taken into account.

To reflect site conditions in the monthly results, a pre-design site survey is essential. By confirming elevation differences, nearby obstructions, rooftop equipment, existing structures, delivery routes, and the feasible installation area—items that cannot be determined from drawings alone—you can improve the accuracy of inputs to PVSyst. If the site survey is insufficient, explanations of the monthly results become abstract, making it difficult to account for differences from actual performance later.

What becomes effective here is a system for efficiently acquiring on-site location information and survey data. In solar power plant design, accurately grasping panel layout, surrounding obstacles, site boundaries, and terrain conditions contributes to improved simulation accuracy. By combining high-precision positioning using smartphones, point cloud acquisition, and on-site coordinate verification, you can reduce the gap between desk-based design and actual site conditions.

The monthly results from PVSyst are like a mirror of the input conditions. When site conditions are properly reflected, the month-by-month changes in energy production and losses become easier to explain. Conversely, if the input conditions are misaligned with the actual site, the monthly results will also deviate from reality. Therefore, the ability to accurately grasp site conditions is just as important as the ability to interpret the monthly results.

Especially when looking ahead from the proposal stage through construction and operation, it is necessary not to treat simulation results as a one-off deliverable, but to improve accuracy through a process of site verification, design revision, re-simulation, and preparation of explanatory materials. Monthly results become a common language for confirming the validity of the design at each of those stages.

Summary

When reading the monthly results in the PVSyst manual, it is important not to focus only on the generation figures, but to systematically review each relationship with irradiance, temperature, shading, losses, PCS limits, and local site conditions. Monthly results are not only a breakdown of annual energy production but also an important reference for confirming how design conditions affect each season.

The first thing to check is whether the monthly peaks and troughs in power generation can be explained by normal seasonal variation. Then verify the relationship between solar irradiance and power output; if there are months with high irradiance but weak output, suspect temperature losses, PCS limitations, shading, soiling, or similar issues.

Next, separate seasonal variation from temperature effects. In summer, even with higher solar irradiance, temperature-related losses increase, so spring and autumn can appear more efficient. In winter, even if temperature conditions are favorable, power generation falls due to lower irradiance, shorter sunshine hours, shading, and snowfall. Understanding these seasonal characteristics will allow you to correctly explain monthly results.

Furthermore, examining how losses increase can prompt a reassessment of design conditions. Monthly loss trends—such as winter shading losses, summer temperature losses, clipping during overloading, and soiling or snowfall due to regional characteristics—offer clues for design improvements. Losses are not simply better when smaller; it is important to judge them in balance with the design objectives and economic considerations.

Monthly results can also be used for internal reviews and explanations to clients. Rather than showing only the annual power generation, being able to explain when generation increases and when it decreases—and why—will increase confidence in the simulation results. However, it is important to understand that monthly results do not guarantee actual monthly power generation and are standard estimates based on the input conditions.

And ultimately, the important thing is to judge by linking the results in PVSyst with the actual site conditions. If local information such as orientation, tilt, topography, surrounding obstructions, roof-mounted equipment, snow accumulation, and soiling is not accurately reflected, the accuracy of the monthly results will decrease. The reliability of the simulation depends not only on operating the software but also on how accurately the site can be assessed.

To efficiently carry out everything from on-site surveys to design, simulation, and preparation of explanatory materials, a system that incorporates location and terrain information collected on-site into the design process is useful. By employing an iPhone-mounted GNSS high-precision positioning device like LRTK, you can more smoothly carry out tasks such as confirming site coordinates, determining installation boundaries, and recording surrounding conditions. To translate PVSyst's monthly results into judgments that more closely reflect actual conditions, it is important to enhance both the ability to interpret simulation results and the ability to accurately capture site conditions.