5 Ways to Read PVSyst Monthly Results｜Understand Seasonal Differences

PVSyst's monthly results are not supplemental material for simply viewing the breakdown of annual energy production. They are important material for interpreting a project's seasonal differences and design tendencies from the monthly changes in energy production, solar irradiation, losses, and PR. PVSyst's Loss Diagram can be viewed not only on an annual basis but also by month, allowing evaluation of seasonal differences in losses. Also, PVSyst's normalized indicators—Yr, Ya, Yf, Lc, Ls—are organized on a per-day basis, and viewing them monthly makes it easier to grasp seasonal variations.

Especially for practitioners looking for information on "how to read PVSyst", the monthly results serve as an entry point for detecting anomalies that annual values cannot reveal. Even if the annual generation looks sufficient, a large drop only in winter or a shorter-than-expected increase in summer may indicate a design issue hidden in shading, temperature, irradiance conditions, or system losses. PVSyst states that the effects of losses can be used as hourly, daily, and monthly values, and its reading approach assumes returning to monthly results as well as annual results.

Also, monthly results are not simply tables for checking "months with higher values" and "months with lower values." They are materials that allow comparison, by month, of seasonal differences in site conditions, alignment of irradiance conditions, the way shadows form, the magnitude of temperature losses, and how downstream losses take effect. Once you can read monthly results as a progression, the reasons behind the annual values become easier to understand, and the accuracy of comparative assessments, internal explanations, and customer explanations improves.

This article explains how to interpret PVSyst's monthly results in practical work, organized from five perspectives. First, it outlines a flow: grasp the peaks and troughs of monthly generation, then examine the relationship with solar irradiance, from there check PR and losses, and finally connect to output figures closer to amounts sold to the grid and self-consumption. In the latter half, it also clarifies common points of misunderstanding and how the accuracy of site conditions affects how convincing the monthly results are.

Table of Contents

• Key Assumptions to Note Before Reading Monthly Results

• How to Read 1｜First, Grasp the Peaks and Troughs of Monthly Power Generation

• How to Read 2｜Look at Monthly Variations in Solar Irradiation and Deviations in Generation

• How to Read 3｜View PR by Month to Confirm Seasonal Patterns

• How to Read 4｜Narrow Down Causes Using the Monthly Loss Breakdown

• How to Read 5｜Connect Monthly Results to the Output Figures for Assessment

• Common Misunderstandings

• The Accuracy of On-site Conditions Affects the Credibility of Monthly Results

• Summary

Assumptions to Keep in Mind Before Reading the Monthly Results

Before reading PVSyst's monthly results, it's important to understand that monthly figures, like annual values, are just a snapshot within the overall flow. In the normalized indicators, Yr is defined as the ideal yield based on incident energy, Ya as the array output, and Yf as the final useful output, while Lc is Collection Losses and Ls is System Losses. Viewing these on a monthly basis makes it easier to see in which months losses are pronounced at each stage.

Furthermore, the Loss Diagram is explicitly stated to be not only always displayed in the annual report but also available on a monthly basis, and it can be used to evaluate seasonal impacts on losses. In other words, monthly results are not merely a supplement to the annual value, but an integral part of the formal interpretation used to verify the reasons behind the annual value. It becomes easier to understand why PVSyst provides a monthly display when you consider that ignoring seasonal differences makes it easy to overlook weaknesses in the design.

Furthermore, when reading monthly results, it is important to view the numbers as a flow through the year rather than tracking them as isolated monthly points. By following how generation, solar irradiation, PR, and losses move from spring to summer, summer to autumn, and autumn to winter, you can discern the characteristic behavior of the project. Once you can read it this way, you won't simply end with “this month is low,” but will find it easier to consider “why it fell in that month.” This is consistent with the fact that PVSyst’s Yr, Ya, Yf, Lc, and Ls are fundamentally organized as “relationships.”

How to Read 1｜First, grasp the peaks and valleys of monthly power generation

When reading monthly results, the first thing you should do is grasp the peaks and troughs of monthly power generation. Before tracking the numbers month by month in detail, look at where generation rises and falls over the year. This is less a check of the breakdown of generation and more an exercise to confirm the seasonal shape of the entire project. The reason monthly Loss Diagrams are considered suitable for evaluating seasonal differences is precisely that understanding this overall shape is important.

In practice, you can learn a lot just from these peaks and troughs. If there is a large drop in winter, shading from low sun angles or orientation conditions may be having a strong effect. If the increase in summer is not as large as expected, temperature-related losses or downstream losses might be influencing performance. Conversely, if the variation is relatively gentle throughout the year, the irradiance conditions and system configuration are straightforward, and it’s easier to consider the case explainable. This way of thinking is consistent with PVSyst’s idea that seasonal effects can be evaluated in the monthly display.

What’s important here is not to stop at a simple comparison between months with high and low monthly power generation. By checking whether the positions of peaks and troughs are natural or unnatural and whether the drops are steep or gradual, you heighten sensitivity to installation conditions and the way losses appear. The first step in interpreting monthly results is to grasp the annual profile rather than focus on the absolute values. From there, when you move on to the next level of analysis, it becomes easier to isolate the causes.

How to Read 2 | Examining Monthly Variations in Solar Irradiance and Deviations in Power Generation

Once you have identified the peaks and troughs in the monthly energy production, the next thing to check is how they relate to the monthly variation in solar irradiance. In PVSyst’s normalization metrics, Yr is the ideal yield based on the energy incident on the collector plane, and it is the quantity that forms the basis of power generation. Therefore, when interpreting monthly generation, you should first look at how much incident energy there was in that month.

This way of interpreting the data is important because it makes it easier to determine whether changes in power generation can be explained solely by changes in solar irradiance. If power generation also increases straightforwardly in months with high irradiance, you can assume that the incident-light conditions and loss structure are relatively normal. Conversely, if there are months when irradiance increases substantially but power generation does not grow as much as expected, there may be loss factors specific to those months. PVSyst's Lc and Ls are precisely the indicators for seeing "how much the yield has decreased from the ideal yield."

For example, if solar irradiance increases from spring to early summer but power generation levels off in midsummer, you should first suspect temperature-related losses. Conversely, if generation falls in winter by more than the decrease in irradiance, there may be overlapping shading or orientation issues. When reading monthly results, always look at variations in irradiance and power generation together, and treat any discrepancy between them as a clue to losses.

Also, this way of interpreting is useful for separating the influence of site conditions from that of design conditions. Solar irradiance itself is a natural condition, but how much of that month's irradiance is converted into electricity is a matter of design and losses. Therefore, when looking at monthly power generation, checking the irradiance together is fundamental to understanding the boundary between natural conditions and design conditions.

How to Read 3 | Look at PR by Month to Verify Seasonal Groupings

After looking at monthly generation and irradiation, checking the monthly PR next makes it even easier to see how the project is coming together. In PVSyst, PR is defined as the ratio of the effectively produced energy to the energy idealized using the nominal STC efficiency. For grid-connected systems, E_Grid is basically used as the numerator, so looking at the monthly PR shows how effectively the entire system was able to perform in that month.

What makes monthly PR useful is that it lets you see not simply whether generation was high or low, but how well performance held together under that month's conditions. For example, if PR falls in a month with high solar irradiance, that may indicate losses that are particularly strong in that month. Conversely, if PR remains stable in a month with low solar irradiance, it becomes easier to interpret that the way losses manifest is relatively mild. PR is often used as an annual summary metric, but looking at it monthly makes it easier to grasp seasonal patterns.

In practice, a drop in PR in midsummer is more likely to suggest temperature losses or overload conditions, while a drop in PR in winter is more likely to indicate problems with shading or irradiance conditions. Of course, PR alone does not reveal the cause, but using monthly PR makes it easier to see in which months performance issues are concentrated. PVSyst positions PR as an indicator suited to system quality comparison because it can aggregate these groups of losses into a single value.

However, it is important not to use monthly PR in isolation. Only by viewing it together with monthly energy generation and solar irradiance does the reason become clear. In practical terms, a useful reading order is to look at the peaks and troughs in monthly generation, check for deviations from irradiance, and then use monthly PR to confirm overall consistency—following this sequence makes judgment considerably easier.

Reading 4｜Narrow down causes using the monthly breakdown of losses

After reviewing the monthly PR, finally go back to the monthly loss breakdown and narrow down which losses are the weak points for that month. PVSyst explains that each item in the array and system losses is available as hourly, daily, and monthly values. In other words, losses such as shading, temperature, wiring, and conversion can be checked not only as annual totals but also for how they vary month by month.

This way of reading is important because it lets you map monthly anomalies to specific causes. For example, if PR is falling in summer, checking the monthly values for TempLoss or System Loss makes it easier to see the contributions of temperature and downstream losses. If the decline is large in winter, you are more likely to suspect contributions from ShdElec or losses related to irradiance conditions. Because PVSyst's grid system results define TempLoss, ShdElec, OhmLoss, EArrMPP, etc., tracking them month by month makes it easier to identify at which stage the reduction is occurring.

Also, when looking at losses it is useful to consider the front-end and back-end separately. In the normalized indicators, Lc is defined as Collection Losses and Ls as System Losses. If the front-end Lc is increased only in specific months, array-side issues such as shading, temperature, mismatch, or wiring are suspected. If the back-end Ls is large, you should prioritize investigating inverter or system-configuration problems. The monthly loss breakdown provides the material needed to carry out this separation in practice.

The key to reading monthly results is not to jump into the detailed loss items from the start. First look at the peaks and troughs, check for deviations from solar irradiance, confirm the PR, and only for months that still require an explanation delve deeply into the loss breakdown — that way you can efficiently read only the parts that matter. This sequence is effective for both beginners and operations staff.

How to Read 5 | Assess monthly results by linking them to exit figures

At the end of the monthly results, it is important to link them to output figures such as the month's electricity sold to the grid and the amount effectively utilized. In PVSyst's PR definition, for grid-connected systems E_Grid is treated as effectively produced energy, and when considering self-consumption E_Grid + E_Solar becomes available energy. In other words, the peaks and troughs and differences in losses in the monthly results ultimately appear as variations in the amount that can be sent to the grid and the amount available for use on the demand side.

In practice, looking at these output figures makes it easier to link the monthly results to business and operations. For power-selling projects, by seeing which months have strong grid injection volumes and which months dip, you can understand the makeup of the annual sales forecast. For self-consumption projects, it becomes easier to consider, month by month, how much E_Solar is effective and how much reliance on the grid increases. Monthly results only translate directly into practical decisions when they are connected to the final ways the power is used or dispatched.

Also, looking at the monthly output figures makes it easier to convey the significance of design changes. For example, as a result of implementing winter shading countermeasures, the improvement in annual power generation may appear small, but if the winter drop in E_Grid is mitigated, this can have significant practical implications. Alternatively, if PR improvements from summer temperature countermeasures are translating into increased electricity sales, that will be easier to explain as a monthly result. The output numbers are the final stage that links the interpretation of monthly results to business decision-making.

At this stage, the important thing is not to look at the output figures first. If you use them as a final check after reviewing monthly power generation, solar irradiation, PR, and losses, you will have greater confidence in those figures. Making monthly results useful in practice means ultimately being able to read which month and what aspects will be effective for the business and operations.

Common Misunderstandings

The most easily misunderstood aspect of PVSyst's monthly results is believing that a project is better the more months it has with high monthly generation. In reality, what matters is whether the monthly peaks and troughs are natural or unnatural, and which losses are causing them. The Loss Diagram is considered effective for assessing seasonal differences because examining month-to-month differences in loss structure is more meaningful than comparing absolute monthly values.

Another common mistake is to attribute all winter declines to seasonal factors. It's natural that solar irradiance is lower, but if the drop is larger than that, there may be issues with shading or orientation conditions. Conversely, mistaking sluggish summer performance for insufficient irradiance is also an error; looking at monthly PR or TempLoss can reveal that temperature effects are the reason. Since PVSyst variables explicitly show TempLoss and ShdElec, it's important not to dismiss monthly result anomalies based solely on intuition, but to go back and verify the variables.

Furthermore, using monthly results merely as a supplement to annual values is a missed opportunity. PVSyst is designed to show monthly Loss Diagrams and monthly loss values so they can be used to investigate the causes of the annual results. Even if the annual figures don’t look suspicious, viewing them by month can reveal biases in specific months. If you overlook those biases, you may miss design weaknesses.

The accuracy of on-site conditions affects confidence in monthly results

Accurate knowledge of site conditions is indispensable to make monthly results truly convincing in practical work. Because PVSyst can handle fine effects such as near shadings, wiring, module quality, and mismatch in detail, the accuracy of the entered site-condition data directly affects the results. When monthly shading losses or System Loss look unnatural, the cause is often not the calculation itself but the accuracy of the inputs for the site's relative positions and route conditions.

For example, in projects where there is a large drop only on winter mornings, the monthly results can look different with only slight differences in clearance from obstacles, height, or how orientation is set. Conditions such as wiring losses and those up to the injection point also make the monthly System Loss less convincing if on-site information—such as panel location or routing—is unclear. Because the timing of "when it takes effect" is more important for monthly results than for annual values, the precision of on-site conditions tends to have a larger perceived impact on the reliability of the results.

In that sense, having a means to capture on-site spatial relationships with high accuracy helps bring PVSyst’s monthly results closer to practical reality. If obstacle locations, equipment layout, orientation, and distances can be determined more precisely, it becomes easier to tighten the assumptions about Shading Loss, irradiance conditions, and wiring conditions. Rather than simply accepting the monthly figures, whether you can be satisfied with those numbers is largely determined by the accuracy of such on-site assessments.

This naturally leads to LRTK, an iPhone-mounted GNSS high-precision positioning device, as a means of accurately grasping on-site positional relationships. By making it easier to confirm positions on-site, measure distances to obstacles, and improve the reproducibility of equipment placement, it becomes easier to more accurately organize the assumptions that go into PVSyst. In practical work where you want to turn seasonal differences in monthly results into field-rooted decisions rather than leaving them as desk-based numbers, this kind of high-precision on-site understanding is a great help.

Summary

When reading PVSyst monthly results, first identify the peaks and troughs in monthly generation, then check for deviations from irradiance, confirm seasonal groupings with the monthly PR, narrow down causes using the monthly loss breakdown, and finally link these to output figures such as exported energy and self-consumption. Simply following these five steps turns the monthly results from a mere table into a powerful resource for interpreting seasonal differences and design quirks.

What's important is not to treat monthly results as mere supplements to the annual values. PVSyst provides monthly displays and monthly loss values so you can read seasonal differences and anomalies from them. When you can move back and forth month by month among the figures for generation, irradiation, PR, losses, and output, the meaning of the annual results becomes much clearer. This way of reading is useful for comparisons, internal explanations, and customer explanations.

And to make that interpretation even more reliable, it is essential to grasp the spatial relationships on site with high precision. If you want to organize shadows and layout conditions more accurately, considering the use of LRTK on an iPhone-mounted GNSS high-precision positioning device can also be effective. By combining the ability to correctly interpret PVSyst’s monthly results with the ability to accurately capture site conditions, it becomes easier to obtain a more convincing understanding of seasonal variations and to make design decisions.