6 steps to investigate causes of low power generation in the PVSyst manual

Table of Contents

• What to consider first when PVSyst shows low energy production

• Step 1 Verify meteorological data and site conditions

• Step 2 Check for input errors in the azimuth and inclination angles

• Step 3: Verify the combination of modules and inverters

• Step 4 Check shadow analysis and the effects of nearby obstructions

• Step 5 Check that the loss setting is not excessive

• Step 6 Isolate the cause from the results screen and loss plot

• Practical considerations when investigating the causes of low power output

• Summary

What to Consider First When PVSyst Shows Low Energy Production

When you run a generation simulation following the PVSyst manual, the annual energy production may sometimes appear lower than expected. In solar power system design, the final energy yield is determined by multiple overlapping factors such as module capacity, installation angle, local solar irradiance, the presence or absence of shading, wiring losses, and inverter conditions. Therefore, if the results are low, it's premature to immediately blame only the module capacity or system configuration.

When you feel the power generation is low, it's important not to look only at the results but to check the input conditions step by step. PVSyst can handle many configuration items, but a single input mistake can greatly affect the overall results. For example, if the site conditions differ from the actual project, if the azimuth is misinterpreted, or if shading settings are entered excessively, the power generation will drop significantly.

Also, on PVSyst's results screen, it is important not just to look at the annual energy production but to examine the breakdown of losses. If you can identify at which stage energy is being lost, you can narrow down the possible causes considerably. Depending on whether the issue is low at the solar irradiance stage, large array losses, prominent inverter losses, or excessive losses due to wiring or temperature, the settings screens you should examine will differ.

The purpose of using the PVSyst manual in practice is not just to learn screen operations. It is to understand the relationship between the input settings and the results, and to be able to explain why the energy yield turned out as it did. To provide justification to the client, in-house design staff, and construction personnel, you need the ability to systematically identify the causes of low energy yield.

This article organizes six steps to investigate the causes of low power generation, referring to the PVSyst manual. From initial setup to interpreting the results screen, we review, in order, the points where practitioners are most likely to get stuck.

Step 1 Check meteorological data and site conditions

The first step in investigating the cause of low power output is to verify the meteorological data and site conditions. In solar power simulations, which region’s solar irradiation data is used has a major impact on the results. Even if module and inverter settings are correct, if the site conditions differ from those assumed for the project, the generated power will not match expectations.

The first thing to check is whether the project location is set correctly. In PVSyst you select a site and run simulations based on latitude, longitude, elevation, and meteorological data. Even if you think you selected a nearby site, solar radiation and temperature conditions can differ in mountainous, coastal, and urban areas. In regions with large terrain variations, trends in power generation can differ even within the same prefecture, so you need to confirm that you are using data close to the project site.

Next, check the type of meteorological data. If the calculated annual energy production is low, the meteorological data being used may be conservative. If you can compare multiple meteorological datasets, comparing each dataset's solar irradiance and temperature trends will reveal the reasons for differences in energy production. If you proceed without checking the data's reliability, the years covered, and its applicability to the region, it will be difficult to explain the design values later.

Also check that the values of horizontal-plane irradiance and tilted-surface irradiance are not extremely low. Even if the design conditions appear acceptable, the simulated irradiance can become low depending on the choice of meteorological data and conversion settings. While consulting the PVSyst manual, it is important to review the meteorological data import method, units, site information, time zone, and so on to ensure there are no inconsistencies.

Ambient temperature conditions should not be overlooked. Solar modules produce less output as temperatures rise. For projects in high-temperature regions, even with adequate solar irradiance, temperature-related losses can be large and the expected energy yield may fall short. Conversely, if the temperature data are set higher than actual conditions, temperature losses can be overestimated, making the estimated generation appear lower.

In snowy regions, whether snow-related losses occur and the reflective conditions are also subjects to be checked. If shading by snow is taken into account, winter power output can drop significantly. Whether snow will be cleared in actual operation, whether natural snow-shedding is expected, or whether the tilt angle is sufficient will change how losses are treated. By checking meteorological data and local conditions, it becomes easier to distinguish whether a reduction in power output is due to a design error or to regional characteristics.

Step 2 Check for Input Errors in Azimuth and Tilt Angles

The next things to check are the azimuth and tilt angles. Even if you set them while consulting the PVSyst manual, if you mistake the azimuth reference or input direction, the predicted energy production can be greatly reduced. Especially for projects in Japan, because south-facing is often used as the reference, if you do not correctly understand the azimuth definition in the software, you may end up simulating in an unintended orientation.

In solar power generation, the amount of solar radiation modules receive changes depending on which direction they face. The closer they are to facing south, the more stable the annual power output tends to be, while east- or west-facing orientations alter the generation pattern by time of day. As the orientation approaches north, the annual amount of solar radiation received can drop significantly. If the power output is low, first check whether the set azimuth matches the actual drawings and on-site conditions.

Tilt angle is also important. For ground-mounted installations, the rack angle, and for rooftop installations, the roof pitch directly affect energy production. If the tilt angle is too shallow, it can be advantageous for summer generation, but from the perspective of optimizing annual output, it can limit total energy production. Conversely, if the tilt angle is too steep, it may be beneficial in winter but reduce solar gain in summer. The optimal angle depends on whether the project's objective is to maximize annual energy production or to prioritize winter generation.

A common input mistake is errors when converting roof pitch into an angle. On drawings the pitch may be shown as a rise-to-run ratio or other slope notation, but PVSyst sometimes requires the tilt to be entered as an angle. If you mistake the slope notation for an angle and enter it as-is, the resulting tilt will differ from the actual condition. When energy production is extremely low, checking these basic inputs should take priority over module performance.

Also, when dealing with multiple roof surfaces or arrays with multiple orientations, confirm that each azimuth and tilt angle is correctly separated. If you combine them into a single array, the results may not match the actual power generation trends. For east-west roofs, single-sloped roofs, and complex ground-mounted projects, the way arrays are divided can itself affect the results.

When reading the PVSyst manual, it is important not to view azimuth and tilt angles merely as input fields, but to understand them as prerequisites for solar energy capture. If the energy production is low, before proceeding to the results screen, first check that the orientation and angle of the installation surface match the drawings, site conditions, and design intent.

Step 3 Confirm the combination of modules and inverters

If there are no issues with weather conditions and installation angle, check the combination of modules and inverters. The cause of low power generation is not simply that the capacity is small; if the string configuration, voltage range, inverter capacity, oversizing ratio, temperature conditions, and so on are not appropriate, the energy that should be generated may not be fully converted.

First, check that the module model and capacity have been selected correctly. PVSyst uses a module database for simulations, but if you choose a similar part number by mistake, the output, temperature coefficients, voltage, and current conditions will change. Even if the manufacturer name or model number is similar, results will differ if the rated output or cell configuration is different. Cross-check the selected data against the specification sheet of the module you actually plan to use and review whether the chosen data match.

Next, verify the inverter selection. If the inverter capacity is too small, output limiting can occur during generation peaks, increasing clipping losses. Conversely, if the inverter capacity is too large, efficiency may be poor during periods of low-load operation. When generation is low, check not only the inverter’s rated capacity but also its efficiency curve and input voltage range.

String design is also a major point. If the number of modules in series is too small, it may be difficult to reach the inverter’s operating voltage range, which can reduce efficiency. If the number of modules in series is too large, the open-circuit voltage at low temperatures may exceed the upper limit. If PVSyst is showing warnings, it is important not to downplay their meaning and to review the number of strings and the series count.

Also, when using an inverter with multiple MPPTs, check whether arrays with different azimuths or tilts are being combined into the same input. Connecting strings with different irradiance conditions to the same MPPT can cause mismatch and reduce power generation. For east- and west-facing arrays or arrays with differing tilts, the way they are electrically combined has a large impact on simulation results.

When referring to the PVSyst manual, it's important not to simply select equipment and stop; you should take care to verify that the module–inverter combination is electrically sound. If annual energy production is low, don't just look at equipment capacities—examine the system as a whole to see during which times of day and in what ways losses are occurring.

Step 4: Verify shadow analysis and the effects of nearby obstructions

The impact of shading is a very important cause of low power generation. PVSyst can account for shadows from nearby obstacles, terrain, buildings, trees, and rows of mounting structures. Shadow settings are useful for bringing the model closer to reality, but if the input conditions are overestimated, the power generation will be shown as lower than necessary.

First, check whether the objects casting shadows are positioned correctly relative to the real-world layout. Even small errors in building height, distance, or orientation will change how shadows fall. This is especially true when the sun's altitude is low—during mornings, evenings, or winter—because shadows lengthen and the placement of obstructions has a large effect on the results. If power output is low, review the 3D scene and shadow analysis settings and verify they match the on-site conditions.

Next, check that the range used for shadow calculations is not excessive. If distant obstacles that do not actually have an impact are entered as large objects, or unnecessary obstacles are left in the calculation, losses may be significantly overestimated. You should also confirm that shadow conditions that were conservatively applied in the early design phase are not being used unchanged in the final simulation.

In ground-mounted projects, inter-row shading between arrays is also important. If the row spacing is narrow, shadows from the front rows can fall on the rear rows during winter or in the morning and evening, reducing power generation. When accounting for inter-row shading in PVSyst, check that the racking pitch, module height, tilt angle, and ground slope are entered correctly. Using values that differ from the actual racking design will cause shading losses to be overestimated or underestimated.

On rooftop installation projects, parapets, rooftop penthouses, HVAC equipment, adjacent buildings, antennas, and the like can cause shading. Even a small obstacle can cast a shadow on part of a string, which may greatly affect power generation. Because this cannot be judged by simple area ratios alone, it is important to check at what times of day and over what extent the shadows will fall.

Shading losses include geometrical shading losses and electrical mismatch losses. When shading affects some modules, it can impact the output of the entire string. While consulting the PVSyst manual, make sure you understand which shading losses are being included in the calculations. If low generation is caused by shading, it is important not simply to remove obstacles but to distinguish between shading that actually exists on site and shading that has been over‑entered into the model.

Step 5 Check that the loss setting is not excessive

When energy output is low, check whether the loss settings are excessive. In PVSyst you can set various losses such as temperature losses, wiring losses, mismatch losses, soiling losses, degradation, auxiliary consumption, and availability losses. These are necessary to run simulations close to real conditions, but applying multiple conservative loss assumptions can greatly reduce the energy output.

First, what I want to check is soiling loss. When dust, pollen, bird droppings, yellow sand, etc. adhere to the surface of a solar module, it cannot receive sufficient solar irradiance and its power output decreases. However, how much soiling loss to expect varies depending on the region, tilt angle, rainfall, and cleaning frequency. If a large value is entered without clear justification, it can cause the predicted power generation to be low.

Next, check the wiring losses. Losses vary depending on the cable lengths and cross-sectional areas on the DC side and the AC side, as well as the voltage conditions. If the cable length is entered longer than it actually is, or if the resistance value is set too high, the generated power will decrease. Cross-check with the design drawings and the single-line diagram to ensure there are no inconsistencies in the input values.

Temperature losses are also important. Module temperature is affected not only by ambient air temperature but also by the mounting method and ventilation conditions. Roof-flush installations tend to trap heat, while ground-mounted or rack-mounted installations tend to have better ventilation. Selecting thermal conditions that do not match the installation type can lead to excessive temperature losses. If generation is low in high-temperature regions, it is necessary to check both the meteorological data and the temperature-loss settings.

Mismatch losses should also be reviewed. Differences between individual modules, variations in string conditions, and shading variability can reduce the output of the entire array. However, if values are set significantly higher than standard, you should verify their justification. It is common practice to conservatively assume multiple losses, but if you cannot explain the basis for each, the reliability of the simulation results will be reduced.

Also, if system availability or downtime rates have been set, check those values as well. When considering maintenance outages, equipment failures, grid constraints, etc., the annual power generation will be lower. It is important to confirm whether the outages are actually to be expected or whether conservative values from past projects are merely being reused.

When using the PVSyst manual, you should understand each loss item individually and clarify which losses have been entered and on what basis. If the energy production is low, individual losses may seem small, but their combined effect can make a large difference. Reviewing the loss settings is an essential step in investigating the causes of reduced generation.

Step 6: Isolate causes from the results screen and loss plot

Finally, check the results screen and the loss diagram to isolate the causes of low energy production. The PVSyst results are not sufficient if you only look at the annual energy production. By tracing at which stage the energy is being lost, you can identify the specific causes.

The first things to look at are the annual final generation and the specific yield. By checking how much energy is being produced relative to the installed capacity, you can assess the reasonableness of the project. However, even if this figure is low, do not immediately conclude that the overall design is poor; instead, check the breakdown of losses.

In the loss diagram, you can check the flow starting from solar irradiance, the irradiance received on the array plane, module output, inverter output, and finally the output to the grid. By seeing where this flow drops significantly, you can determine which settings screens to check. For example, if the irradiance on the array plane is low, check the azimuth angle, tilt angle, weather data, and shading conditions. If losses are large at the module output stage, check temperature losses, low-irradiance characteristics, mismatch, soiling losses, and so on. If losses are large at the inverter stage, review inverter capacity, input voltage range, clipping, and efficiency conditions.

Monthly power generation is also important. With only the annual values, you cannot see in which season generation is low. If it is low only in winter, solar irradiance conditions, snow, shading, and tilt angle may be involved. If it does not increase as much as expected in summer, temperature-related losses or inverter output limits might be affecting it. If generation is weak in the mornings and evenings, you can suspect the influence of orientation or surrounding shading.

Also, review the warnings and caution messages in the simulation report. In PVSyst, warnings may appear about string voltage mismatches, compatibility with the inverter, inconsistencies in input conditions, and so on. If power generation is low and you ignore the warnings, you may miss the underlying cause. Warnings are not merely formal notices; they are important clues for verifying the design conditions.

When reading the results screen, it is important not to focus on a single number but to consider the input conditions, the loss diagram, the monthly results, and any warning messages together. By referring to the PVSyst manual and understanding which screen and which settings are reflected in the results, you can logically explain the causes of low energy yield.

Practical considerations when investigating the causes of low power output

When PVSyst indicates a low energy yield, practitioners often try to find the cause quickly and immediately change the settings. However, changing values without justification makes it impossible to tell later which change affected the results. When investigating the cause of low energy generation, the sequence of checks and careful record-keeping are important.

First, save the conditions before making any changes. Before modifying weather data, the installation angle, equipment configuration, loss settings, shading conditions, and so on, duplicate the original project to make comparisons easier. If you overwrite a single file repeatedly, you will not be able to return to the initial conditions, making verification work difficult.

Next, make changes one at a time. For example, if you change the meteorological data, the tilt angle, and the loss settings all at once, even if the power output increases you won't be able to determine which change was effective. When investigating the cause, the basic workflow is to change one setting, check the results, and record the differences.

It's also important to think of site conditions and design intent separately. Constraints that actually exist on site, such as shading and orientation, cannot be easily removed. On the other hand, input errors and overly conservative settings can be corrected. By distinguishing and organizing whether the cause of low power generation lies in site conditions or in setting errors, the direction for design improvements becomes clear.

When explaining to colleagues or clients, you should not simply say "the power generation is low"; you need to be able to explain "which losses are large," "whether those losses are due to site conditions," and "whether there is room for improvement through design changes." PVSyst reports are useful as explanatory materials, but simply showing the output figures is not enough. In practice, you are expected to understand the background of the numbers and provide the rationale for your judgments.

Moreover, PVSyst results are only simulations and may differ from actual power generation. Operational-phase factors that can affect output include construction quality, operation and maintenance, soiling, equipment failures, grid constraints, and inter-annual weather variability. Therefore, during the simulation stage it is important to ensure the validity of the input conditions and the explainability of the results.

Investigating the causes of low power generation is not merely troubleshooting. It is a crucial step to improve the accuracy of design conditions, identify project risks early, and lead to more convincing proposals. Carefully verifying the relationship between settings and results while utilizing the PVSyst manual enhances the reliability of power generation simulations.

Summary

To investigate the causes of low energy output in the PVSyst manual, it is important not to look only at the results screen but to check the input conditions in sequence. The reasons for a low displayed energy output are divided into multiple elements, such as meteorological data, site conditions, azimuth angle, tilt angle, equipment configuration, shading conditions, loss settings, and inverter parameters. Rather than suspecting only one factor, you need to isolate the cause while considering the overall flow.

The first thing to check is the meteorological data and the site conditions. If you use data from a region different from the project site, or if the solar irradiance and temperature conditions differ from actual conditions, the power output can change significantly. Next, check the azimuth and tilt angles. Mistakes in the orientation or angle inputs are a common cause of reduced power generation.

On top of that, we check the combination of modules and inverters. If there are problems with the model, capacity, string configuration, input voltage range, or MPPT allocation, the energy that could be generated cannot be fully utilized. Furthermore, in the shading analysis, we review the effects of buildings, trees, inter-row shading, rooftop equipment, and so on, and confirm whether the settings match the actual conditions.

Loss settings are also important. If parameters such as soiling, wiring losses, temperature, mismatch, or downtime rate are overestimated, the energy yield can be reported lower than it should be. Finally, review the results screen and the loss diagram to see at which stage significant losses occur. Combining annual generation, monthly generation, the loss diagram, and warning messages makes it easier to explain the causes logically.

Because PVSyst is a multifunctional simulation tool, the reliability of the results depends on understanding the input conditions. Rather than reading the PVSyst manual merely as an operational guide, it is important to use it as a handbook to understand how parameter settings affect energy production. When generated energy is low, instead of hastily changing numbers, check in the following order: weather data, installation angle, equipment configuration, shading, losses, and results. By doing so, you can clarify the causes of reduced generation and bring the simulation results closer to ones that are easy to explain in practice.