【How to Read PVSyst Output Results｜8 Points for Interpreting Energy Production and Losses】

Table of Contents

• Interpret the outputs not only by the energy production but also from the underlying assumptions

• Verify the plausibility of the annual energy production as the final outcome

• Use monthly energy production to assess seasonal variation and anomalies

• Understand the difference between solar irradiation and effective solar irradiation

• Use the performance ratio as an indicator to assess the overall health of the system

• Use the loss diagram to trace where energy production is being reduced

• Verify the major loss items by separating site conditions and design conditions

• After reviewing the report, link it to on-site surveying and construction management

Interpret the output not only by power generation but also by the assumptions

When viewing PVSyst's output results, the first thing you want to check is the annual energy production. Because the ultimate amount of generation directly affects project feasibility assessments and design comparisons, it is natural to focus on the production figures. However, in practice you should not look at the production number in isolation; you must always verify the assumptions used to calculate that figure.

PVSyst simulation results are calculated from a combination of many input conditions, such as location, meteorological data, photovoltaic modules, PCS, azimuth, tilt, array configuration, shading, and loss settings. Even if the output appears large, if the meteorological conditions are set more favorably than they are in reality or if shading conditions are not adequately reflected, the predicted energy yield will be overestimated for a real-world generation plan. Conversely, if loss conditions are overestimated, the results may be more conservative than actual.

Therefore, the important initial attitude when reading the output results is to see them not as the definitive answer for energy generation, but as results for verifying the consistency of the input conditions. At the beginning of the report and in the summary section, basic conditions such as project name, site, meteorological data, system capacity, number of modules, PCS configuration, tilt angle, azimuth, and so on are displayed. If any of the values here differ from the on‑site planned conditions, the way you interpret the subsequent energy generation and loss figures will also change.

Particularly important to note is that even with the same installed capacity, the annual power generation varies depending on the installation site, orientation, tilt, shading, and temperature conditions. For example, a layout that is close to south-facing and a layout that includes east–west orientations will have different temporal distributions of power generation. If the tilt angle changes, the seasonal amount of solar irradiance captured also changes. Whether or not you include the effects of snow and shading from nearby buildings, trees, and terrain can also greatly change the breakdown of generation and losses.

Also, PVSyst's output does not merely show a single figure of "how many kWh are generated"; it presents the process leading to that number step by step. It starts with the solar irradiance on the horizontal plane, then the irradiance on the tilted plane, the effective irradiance accounting for shading and reflections, the module output, the array output, the AC output after conversion, and finally the grid output. Understanding this flow makes it easier, when generation is lower than expected, to distinguish whether the cause is the irradiance conditions, shading, temperature, or equipment configuration.

When you are not yet familiar with using PVSyst, it is important to make a habit of first cross-checking the input conditions and the report summary before reading the output results. Verify whether the system capacity is correct, whether the module model and PCS capacity match the design conditions, whether the azimuth and tilt align with the drawings and on-site conditions, and whether the meteorological data location is not substantially different from the target site. If you skip this basic check, even if you later read the loss items in detail, you may find that the fundamental assumptions were wrong.

The output is not a finished answer but documentation for verifying the design conditions. First, review the entire report to understand the conditions under which the results were calculated, and then interpret the energy production and losses—this is the first step to using PVSyst in practice.

Confirm the validity of the annual power generation as the final result.

One of the most closely watched items in PVSyst output is the annual energy production. Annual energy production is an indicator of how much electrical energy the target facility is expected to generate over the course of a year, and it is frequently used for business planning, revenue estimates, design comparisons, internal explanations, and customer briefings. In practice, because this figure is often transcribed into final submission documents, it is important to correctly understand what the number represents.

When reviewing annual generation, first check the units. Output results are commonly shown in energy units such as kWh or MWh. The larger the facility, the easier it is to understand the figures when viewed in MWh, but care is needed because unit mix-ups are likely to occur in comparisons and reports. For example, when comparing annual generation with monthly generation, or when converting to generation per unit of installed capacity, make sure the units are consistent.

It's important to confirm that the annual power generation is within a reasonable range relative to the installed capacity. Rather than judging simply by whether it is large or small, evaluate whether the value is plausible for a solar power system of the same scale by taking into account the area's solar irradiance, mounting tilt, orientation, shading, temperature conditions, and loss settings. For example, in an area with good solar irradiance, little shading, and an appropriate tilt angle, the annual generation per unit of installed capacity tends to be relatively high. Conversely, in locations with significant shading, severe high-temperature conditions, or a non-ideal orientation, annual generation can be lower.

A useful way to check annual generation is to look at generation per unit of installed capacity. If you only look at total generation, the figures tend to be larger for bigger installations, making it difficult to compare projects. Therefore, divide the annual generation by the installed capacity and view it as generation per unit capacity; this makes it easier to compare projects of different scales. If this value is unusually high compared with expectations, suspect possibilities such as insufficient loss settings, shading not being accounted for, or overly favorable meteorological conditions. Conversely, if it is unusually low, check for errors in azimuth or tilt settings, inconsistencies in PCS configuration, excessive loss assumptions, or over-accounting for shading conditions.

Annual energy yield is regarded in PVSyst as the final output result, but in practice it should not be used alone to draw conclusions; it should be checked together with other indicators. In particular, by examining its relationship with the performance ratio, loss diagram, monthly energy yield, plane-of-array irradiance, and effective irradiance, you can make a deeper assessment of the plausibility of the energy yield. For example, even if the annual energy yield is low, if the performance ratio is within a natural range and the irradiance itself is low, the main cause can be attributed to the location or meteorological conditions. On the other hand, if irradiance is sufficient but the performance ratio is low, there may be significant system-side losses.

Also, when interpreting annual power generation, be aware of the purpose of the simulation. During the preliminary assessment stage, the objective is to grasp the broad trends in power generation. In the basic design and detailed design stages, it is necessary to more accurately reflect equipment configurations and loss conditions and to be in a state where the basis for the power generation estimates can be explained. For pre-construction checks, it is important to ensure that differences between the drawings and on-site conditions are reflected. For post-operation performance comparisons, a perspective that analyzes the discrepancies between simulated values and measured values is required.

Annual energy production is the most straightforward figure in PVSyst’s output, but it is also the most easily misunderstood. Rather than simply assuming that a large number is good and a small number is bad, it is important to check the conditions under which that number was produced, whether it is reasonable relative to the system capacity, and whether it is consistent with the breakdown of losses.

Monthly Power Generation: Interpreting Seasonal Variations and Anomalies

After checking the annual power generation, next look at the monthly power generation. Monthly generation is important for identifying seasonal variations and anomalies that cannot be seen from the annual total alone. Solar power generation varies by season depending on solar irradiance, temperature, solar altitude, installation angle, and how shadows fall, so examining monthly changes makes it easier to judge whether the simulation results are realistic.

Generally, months with favorable solar irradiance and seasons with longer daylight hours tend to see increased power generation, while seasons with low irradiance or periods of unstable weather tend to experience reduced generation. However, it is not simply the case that summer always produces the most and winter the least. Summer has longer daylight hours but module temperatures tend to be higher, which can increase temperature-related losses. In winter, even with shorter daylight hours, the lower ambient temperatures can sometimes improve module efficiency. Depending on the installation angle, panels may better match the lower solar altitude in winter, allowing a certain level of generation to be maintained.

When reviewing monthly power generation, first confirm that the seasonal peaks and troughs align with the local climate patterns of the target area. If generation is unusually low in a particular month, check whether that month’s solar irradiance is actually low, whether shading effects are having a significant impact, or whether there are any anomalous settings. In particular, if shading from nearby buildings or terrain has been modeled, shading losses can increase during seasons with low solar altitude. In that case, monthly generation in winter may show a noticeable decline.

On the other hand, be cautious if the monthly power output is unnaturally flat throughout the year. In actual solar power generation, solar irradiance conditions change with the seasons, so it is natural for monthly power output to show some variation. If the variation is too small, check whether the meteorological data, tilt conditions, and azimuth conditions are being properly reflected. Of course, the magnitude of variation differs depending on the region and equipment conditions, but if the results differ significantly from local meteorological experience, it is worth reviewing the input conditions.

Monthly power generation is also useful for maintenance and operational planning. In months when generation is high, opportunity losses from outages or work tend to be greater, so this affects the scheduling of inspections. In months with lower generation, the impact of maintenance work may be relatively small. Additionally, knowing the expected generation for each month provides a benchmark for comparing actual values after operations begin. If a month’s actual generation falls significantly below the simulated value, it prompts checks for weather factors, equipment outages, soiling, shading, grid-side constraints, and the like.

In PVSyst’s monthly output, you can check not only energy production but also the monthly variations in solar irradiance and performance ratio. Even in months when energy production is low, if solar irradiance is low that is a natural result. If irradiance is high but energy production is not increasing, you need to check system-side factors such as temperature losses, conversion losses, shading losses, and clipping. Thus, monthly energy production should not be viewed on its own; it is important to read it in combination with monthly irradiance and losses.

In practice, there are cases where only the total annual energy production is included in the documentation, but checking by month is indispensable for explaining the validity of the design. By looking at monthly generation you can tell whether the simulation reflects local seasonal variations, whether there are any abnormal drops in particular months, and whether the pattern of losses looks natural. When reading PVSyst output, after confirming the annual generation you should always make a habit of following the monthly trends.

Understanding the Difference Between Solar Radiation and Effective Solar Radiation

To correctly interpret PVSyst's output results, it is necessary to understand the differences among the items related to solar irradiance. Because electricity generation originates from sunlight, irradiance is the starting point for calculations. However, reports may display multiple irradiance items, and confusing the irradiance on the horizontal plane, the irradiance on the tilted plane, and the effective irradiance makes it difficult to understand the losses.

The first thing to check is the solar irradiance on a horizontal plane. This indicates the basic solar radiation conditions based on meteorological data. It is the starting point for assessing how favorable the site’s meteorological conditions are for power generation. However, photovoltaic modules are often installed at a fixed tilt and azimuth rather than horizontally. Therefore, solar irradiance on the horizontal plane alone cannot determine the actual irradiance incident on the module surface.

Another important factor is the irradiance on the tilted surface. This describes the amount of solar radiation incident on the surface where a module is installed. Depending on the installation azimuth and tilt angle, the solar irradiance reaching the module surface can vary even at the same location. For example, if the azimuth is appropriate and the tilt angle suits local conditions, the irradiance on the module surface is likely to be favorable. Conversely, if the azimuth is significantly misaligned or the tilt angle is inappropriate, the amount of solar radiation collected can decrease.

Another perspective is effective solar irradiation. This can be understood as the amount of solar irradiation that actually contributes to power generation, derived from the solar irradiation incident on the tilted surface after accounting for effects such as shading, reflection, and the angle of incidence. If power generation is lower than expected, it is important to distinguish whether the solar irradiation on the horizontal plane is low, whether the conversion to the tilted surface is unfavorable, or whether losses from shading or reflection occur before reaching the effective solar irradiation.

When reviewing the solar irradiance item, one thing to keep in mind is that higher irradiance does not necessarily lead to higher final power generation. Even if irradiance is sufficient, final generation can decrease if there are conditions such as large temperature losses, a poor balance with PCS capacity, a lot of shading, large wiring losses, or output limitations. Conversely, even if irradiance is somewhat lower, the performance ratio can be favorable if losses are small and the design is appropriate.

In PVSyst output, tracing the flow from solar irradiance to energy production makes it easier to identify design weaknesses. First check the horizontal plane irradiance to understand the site's meteorological conditions. Next look at the tilted plane irradiance to see how the installation azimuth and tilt angle affect solar capture. Then examine the effective irradiance to determine how much impact shading, reflections, and similar factors have. Keeping this sequence in mind allows you to explain not just that energy production is low, but at which stage the reduction occurs.

In practical work, whether you understand how to read the solar irradiance items greatly affects your ability to explain a report. When customers or internal stakeholders ask, "Why does this amount of energy generation occur?", being able to explain the flow from irradiance, through losses, to the final generated energy can increase confidence in the simulation results. When learning how to use PVSyst, understanding the difference between irradiance and effective irradiance is extremely important.

Use the performance ratio as an indicator to assess the overall health of the system

Along with energy production, the performance ratio is an important output in PVSyst. The performance ratio is an indicator for evaluating how efficiently the system is generating power relative to solar irradiance conditions. Annual energy production is greatly influenced by system size and irradiance conditions, but the performance ratio is useful for verifying overall system losses and the soundness of the design.

The performance ratio is, simply put, a way of assessing how much of the theoretically obtainable generation remains in the actually expected output. In PVSyst, many factors are reflected in the calculations, including meteorological conditions, module characteristics, temperature, shading, wiring, conversion, and mismatch. By examining the performance ratio, you can grasp the overall efficiency of the system including these losses.

When checking the performance ratio, it's important not to assume that higher numbers are unconditionally better or that lower numbers are unconditionally worse. If the performance ratio is excessively high, loss settings may not have been sufficiently accounted for. For example, shadow losses may not have been included, soiling losses may have been underestimated, temperature conditions may be set more favorably than the actual situation, or wiring and conversion loss settings may be too optimistic.

Conversely, if the performance ratio is low, possible causes include harsh site conditions, excessive shading, large temperature losses, an impractical equipment configuration, or losses being overestimated.

What is important is not to use the performance ratio as a standalone pass/fail decision, but to view it together with the loss diagram and monthly generation. Even if the annual performance ratio looks normal, examining it by month can reveal a large drop in specific seasons. For example, if the performance ratio falls in winter, shadowing caused by the reduced solar altitude may be a factor. If it falls in summer, temperature losses due to high ambient temperatures may be affecting it. Looking at the monthly performance ratio makes it easier to find problems that are hidden in the annual average.

The performance ratio is also useful when comparing multiple design options. For example, when comparing several options at the same location that change tilt angle, azimuth, number of modules, or PCS capacity ratio, looking at the performance ratio as well as the annual energy generation allows you to determine which option converts solar radiation into electricity most efficiently. However, a higher performance ratio does not always mean the option is optimal. You also need to consider equipment capacity, site constraints, constructability, cost, and operating conditions. It is important to treat PVSyst’s output results as one of the inputs for design decision-making.

Performance ratio can also be used for post-operation performance comparisons. Even if actual generation is lower than the simulation, differences in weather conditions can affect the amount of energy produced. Therefore, rather than simply comparing generation quantities, looking at how much is being generated relative to the actual irradiance conditions makes it easier to assess the health of the installation. The performance ratio calculated by PVSyst can be used as a reference baseline at the design stage and applied to post-operation analysis.

To acquire practical-level skill in using PVSyst, you need the ability to interpret not only annual energy production but also the performance ratio. The performance ratio is an indicator of the system's efficiency relative to irradiance conditions and serves as an important clue for verifying the appropriateness of loss settings and the overall soundness of the design. When assessing whether the energy production is reasonable, always check it together with the performance ratio.

Track where power generation is being reduced in the loss diagram

When reading PVSyst outputs, the loss diagram is extremely important. By looking at the loss diagram, you can understand at which stages and by how much energy is lost from the solar irradiance to the final AC output. The total energy production alone does not reveal the causes, but by tracing the loss diagram it becomes easier to identify design issues and configuration errors.

The loss diagram shows, step by step, the flow of solar irradiance energy from the sun as it reaches the module surface, is converted into electricity, becomes DC output, is converted to AC by the PCS, and is finally delivered to the grid. In this flow, energy is reduced by factors such as shading, reflection, angle of incidence, temperature, module characteristics, mismatch, wiring, conversion, shutdown, and curtailment. By seeing which losses are largest, you can concretely explain why power generation has decreased.

When reading a loss diagram, first follow the overall flow from top to bottom. In the initial stage, solar irradiance on the horizontal plane is converted to solar irradiance on the tilted surface. Here, the tilt angle and azimuth are relevant. Next, proceed to the effective solar irradiance that accounts for shading and reflection. If there is a large loss at this stage, check for surrounding obstacles, terrain, inter-row shading, and the effects of the angle of incidence.

Once the modules enter the power-generation stage, temperature losses and losses due to module characteristics will occur. Solar photovoltaic modules tend to lose output as temperature rises, so in hot regions or under installation conditions with poor ventilation, temperature losses can be significant. Care is also required where rear-side ventilation is poor, such as for roof-mounted installations or low mounting racks.

Furthermore, on the DC side, mismatch losses and wiring losses occur. Mismatch losses result from differences in characteristics between modules and variations in solar irradiance conditions. Wiring losses are affected by cable length, current, conductor cross-sectional area, and wiring layout. If these losses are large, it is necessary to review the string configuration, wiring design, and voltage conditions.

At the stage of passing through the PCS, losses occur due to conversion inefficiencies and capacity constraints. The PCS is a device that converts direct current (DC) to alternating current (AC), and its conversion efficiency results in a certain amount of loss. Also, if there are periods when the DC-side input exceeds the PCS's processing capability, the output may be limited. If this is significant, check the balance between PCS capacity and module capacity. However, because some degree of limitation may be acceptable by design, do not simply judge the presence of losses as bad; evaluate them against the design intent.

What you should pay particular attention to in a loss diagram is whether any single loss item is disproportionately large. If shading loss is large, check the on-site obstruction conditions and 3D settings. If temperature loss is large, check the installation method and ventilation conditions. If wiring loss is large, check cable lengths and voltage design. If conversion loss or limitation loss is large, check the PCS configuration. The loss diagram serves as a map for locating the areas that need correction.

As you become familiar with using PVSyst, you will be able to infer to some extent which input conditions are affecting the results just by looking at the loss diagram. Rather than only looking at the generation figures, following the flow of energy in the loss diagram and being able to explain where reductions occur leads to simulation verification that is trusted in practice.

Check the main loss items separately for site conditions and design conditions

PVSyst’s loss items span a wide range, but in practical work it is easier to organize them by broadly dividing them into losses caused by site conditions and losses caused by design conditions. Rather than viewing all losses in the same way, it is important to distinguish which losses are due to site constraints and which losses can potentially be improved through design or equipment selection.

Typical losses caused by site conditions include shading losses, incidence-angle losses, soiling losses, and some temperature-related losses. Shading losses arise from surrounding buildings, trees, terrain, utility poles, fences, and shadows between rows of mounting structures. Especially for ground-mounted and rooftop installations, how accurately on-site obstacles are reflected has a large impact on the results. If shading conditions are simplified while on-site surveys are insufficient, discrepancies with actual power generation are likely to occur.

Incidence angle losses are the effect whereby sunlight striking the module surface at an oblique angle reduces the usable irradiance due to reflection and similar effects. This is related to the installation azimuth and tilt angle, and the movement of the sun. Soiling losses are affected by sand and dust, pollen, bird fouling, the surrounding environment, and cleaning frequency. Temperature losses are related not only to ambient temperature but also to the installation method and ventilation conditions. Installations close to the roof surface, poorly ventilated layouts, and regions prone to high temperatures tend to experience larger temperature losses.

On the other hand, losses resulting from design conditions include wiring losses, mismatch losses, PCS conversion losses, losses due to capacity constraints, and losses related to the voltage range. These can sometimes be improved through design measures. If wiring losses are large, review the wiring routes, cable lengths, cable sizes, and string configuration. If mismatch losses are large, check module layout, the way strings are formed, and variations in shading. If PCS-related losses are large, check PCS capacity, the input circuitry, and the balance with DC capacity.

Dividing losses into site conditions and design conditions makes it easier to explain them to stakeholders. For example, if shading losses are large, you can consider whether to accept them as a site constraint or improve them by changing the layout, removing trees, or adjusting the installation area. If wiring losses are large, they may be improvable through design changes. If temperature losses are large, consider whether you can review the rack height or ventilation conditions. By organizing loss components from the perspective of their potential for improvement in this way, you can use the simulation results to inform the next design decisions.

Also, when reviewing loss items, it is important to verify not only the magnitude of the numbers but also whether there is a basis for the settings. Confirm how much soiling loss was assumed, whether wiring losses are based on the actual wiring plan, whether the shading model is based on on-site surveys or drawings, and whether the temperature conditions match the installation method. Loss settings without supporting justification will have weak explanatory power, even if the numbers appear reasonable.

When using PVSyst output in practical work, loss items are not merely results but clues for design improvement. By sorting out which losses are large, which losses are difficult to avoid because of local conditions, and which losses can be improved by design changes, you increase the value of the simulation. Don’t stop at merely reading the energy production; it’s important to identify the next actions from the loss items.

After reviewing the report, link it to on-site positioning and construction management

After checking the PVSyst output results, it is important to connect the figures in the report to on-site design, construction, and operations and maintenance. Simulation is not a task that ends on paper; it is a means to correctly reflect site conditions and to manage the project so that the installed equipment is arranged as designed and approaches the expected power generation. In particular, when interpreting energy yield and losses, the accuracy of on-site information such as shading, orientation, tilt, layout, topography, and obstacles determines the reliability of the results.

If PVSyst output shows that shading losses or the effects of orientation and tilt are significant, on-site position and layout checks become important. Even if drawings appear to show no problems, on the actual site the way shadows fall and the installation conditions can change due to surrounding structures, ground elevation after site preparation, racking locations, access paths, fences, equipment positions, and so on. If the layout shifts during construction, the actual equipment conditions may no longer match the assumed energy yield and loss conditions used in the simulation.

In such situations, leveraging high-precision on-site positioning makes it easier to connect the design conditions examined in PVSyst with the actual conditions in the field. LRTK, as an iPhone-mounted high-precision GNSS positioning device, can be used on site for position verification, point cloud acquisition, coordinate recording, photo management, and more. In the planning and construction management of solar power plants, it is important to accurately understand racking positions, pile locations, equipment boundaries, surrounding obstacles, and ground topography. If on-site coordinate information can be acquired with high precision, it becomes easier to compare the layout and shading conditions assumed in simulations with the actual construction status.

For example, if PVSyst output indicates large winter shading losses, you can verify on site the locations of structures or terrain causing the shading and cross-check them against the information on the design drawings. In projects where ground elevation changes before and after site formation, changes in topography can affect shading and mounting angles. If the positions of piles and racking can be checked during construction, you can determine early whether they have been placed according to the design. This reduces discrepancies between the simulation and on-site construction and makes it easier to mitigate the risk of reduced power generation.

Even during the operation phase of a power plant, recording on-site information is important. If power output is lower than the simulation, you should not only review the PVSyst settings but also check on-site for newly occurring shading, changes in the surrounding environment, soiling of equipment, ground or drainage problems, deviations in installation positions, and other issues. If you retain photos with high-precision location information and point cloud data, it will be easier to analyze the causes later.

To deepen practical use of PVSyst, both the ability to interpret output results and the ability to accurately assess on-site conditions are necessary. Not only checking annual energy production, monthly energy production, performance ratio, and the loss diagram, but also thinking about how to verify those results on site makes simulations more practical. By reading energy production and losses in PVSyst and recording the site's location and shape with high precision using LRTK, it becomes easier to perform consistent checks from design and construction through operation and maintenance.

In planning solar power generation, it's important not to separate desk-based simulations from actual on-site conditions. Correctly read PVSyst's outputs, understand which losses are related to which site conditions, and then measure the site with high precision. Establishing this workflow enhances the ability to explain power generation and makes it easier to implement design changes, verify construction, and improve operation. LRTK is an effective option for confirming issues revealed by PVSyst on site and supporting quality control of solar power generation equipment.