How to Compare Design Proposals in PVSyst | 7 Ways to Interpret Power Generation Differences

Table of Contents

• Things to organize before comparing design proposals in PVSyst

• When looking at differences in generation, do not judge by annual values alone

• Comparison Point 1: Look at the difference in annual energy generation

• Comparison Point 2: Look at the differences in monthly energy generation

• Comparison Point 3: Look at differences in PR and specific yield

• Comparison Point 4: Identify the factors that caused differences in the loss diagram

• Comparison Point 5: Look at differences in shading losses and layout conditions

• Comparison Point 6: Consider overloading (DC/AC oversizing) and PCS-side constraints

• Comparison Point 7: Look at the intent of design changes and whether they can be adopted in practice

• Common mistakes when comparing design proposals

• Practical methods for conducting comparisons

• Summary

Things to sort out before comparing design proposals in PVSyst

The purpose of comparing design proposals in PVSyst is not simply to find the one with the highest electricity generation. In practice, a comprehensive judgment is required that includes generation, system capacity, land use, constructability, maintainability, the impact of shading, whether there are any infeasibilities in the electrical design, and how easy the proposal is to explain in future documentation. Even a proposal with a slightly higher generation is not necessarily the optimal choice if it is difficult to construct, makes wiring more complex, worsens inspection routes, carries a greater risk of shading, or is disadvantageous for future expansion or maintenance.

Before comparing design proposals, the first thing to do is align the assumptions of the proposals being compared. For example, if one proposal uses different meteorological data, another has different panel specifications, and a third even changes the loss settings, you cannot correctly determine the cause of differences in energy output. The basic rule for comparison is to isolate changes one at a time as much as possible. If you are comparing tilt angles, keep the number of panels, orientation, PCS capacity, loss conditions, and weather conditions the same. If you are comparing layout proposals, keep equipment conditions the same as much as possible and check how much difference is caused by shading, spacing, and orientation.

Also, PVSyst simulation results are calculations based on the input conditions. The reliability of the results can vary greatly depending on the accuracy of on-site surveys, the precision of terrain information, the fidelity of obstacle representation, the validity of meteorological data, and the content of the equipment specifications entered. When comparing design proposals, you should not judge solely by the numbers in PVSyst; it is necessary to verify how accurately the input conditions reflect the actual planned site.

What practitioners should pay particular attention to is whether the "content you want to compare" and the "conditions being changed" are aligned. For example, if you simultaneously compare a proposal that increases the number of panels to boost power generation with a proposal that changes the tilt angle, it becomes difficult to tell whether the difference in power output is due to the increase in installed capacity, the change in tilt angle, or changes in shading losses. If you create multiple proposals while the purpose of the comparison is unclear, you will have difficulty explaining the results later.

Therefore, when comparing design proposals in PVSyst, first clarify what you want to evaluate. For example, the metrics you should examine will differ depending on whether you want to choose the layout with the highest power generation efficiency for the same capacity, aim to maximize energy yield on limited land, select a stable option that minimizes shading impacts, or check the balance with PCS capacity. By deciding the objective in advance, it becomes easier to organize how to interpret the simulation results.

When assessing differences in power generation, do not judge solely by annual values

When comparing design proposals in PVSyst, the first thing that tends to catch the eye is the annual energy production. Annual energy production is an easy-to-understand metric and is important for checking the overall merits of design proposals. However, it is dangerous to draw conclusions based solely on annual energy production. This is because even with the same annual energy production, seasonal generation patterns, the breakdown of losses, output limits at peak times, the timing of shadow occurrence, and efficiency per unit of installed capacity can differ.

For example, suppose Option A has a higher annual power generation but suffers significant temperature losses in summer and is more susceptible to shading in winter. On the other hand, Option B has slightly lower annual generation but smaller month-to-month variability, limited shading losses, and is easier to manage in terms of maintenance and construction. In this case, Option A may look better based on simple annual generation alone, but considering practical operational stability and ease of explanation, Option B may be more likely to be adopted.

Also, differences in generation can easily change depending on differences in installed capacity. Increasing the number of panels tends to increase annual generation, but whether that increase is efficient is a separate matter. You need to check whether generation has increased in proportion to the increase in installed capacity, or whether it has not increased as much as expected due to shading, PCS constraints, wiring losses, etc. Therefore, when looking at annual generation, it is important to also check efficiency indicators such as specific yield and PR.

Furthermore, when comparing design proposals, it is important to interpret not only the "magnitude of the difference" but also the "meaning of the difference." If annual power generation changes by a few percent, there may be clear differences in the design conditions. Conversely, when the difference is very small, taking into account uncertainties in input conditions and year-to-year variations in meteorological data, it may not be significant in practice. Rather than treating numerical differences as absolute, it is necessary to consider whether the difference can be used for design decisions or whether it falls within the range of error and underlying assumptions.

In PVSyst comparisons, taking annual energy production as the starting point, we sequentially check monthly energy production, specific yield, PR, loss diagrams, shading losses, PCS-side constraints, and differences in input conditions. By following this sequence, you can use differences in energy production as inputs for design decisions rather than viewing them superficially.

Comparison Point 1: Examine the Difference in Annual Power Generation

The first step in comparing design proposals is to check the differences in annual energy production. Annual energy production is a basic metric that indicates how much electrical energy a design proposal is expected to generate over the course of a year. In PVSyst’s result screens and reports, this can be viewed as the energy sent to the grid or as the final usable energy. When comparing multiple proposals, first lay out each proposal’s annual energy production side by side to see which proposals produce more and which produce less.

However, at this stage what matters is not ranking the options but grasping the magnitude of the differences. For example, if the difference in annual energy output is very small, it may indicate that there is no significant difference in generation performance among the design proposals. In that case, there is greater room to base the decision on factors other than energy output—such as constructability, maintainability, site utilization, material quantities, and ease of future modifications. Conversely, if there is a clear difference in annual energy output, you should examine in more detail why that difference occurred.

When examining annual power generation, always check the installed capacity as well. Even if an option shows high generation, if it's simply because it has more panels, you can't claim its generation efficiency is high. It's natural for options with larger installed capacity to produce more, but you must assess whether that increase is reasonable. For example, if you significantly increase installed capacity but see only a small increase in generation, shading losses, output limitations, or reduced layout efficiency may be affecting the result.

It is also important to evaluate annual energy production against the design intent. If the plan aims to maximize generation, the magnitude of annual production becomes an important decision factor. On the other hand, if the priority is to construct without undue difficulty within a limited site and to achieve stable long-term operation, having the largest possible annual production alone is not necessarily the correct answer. Even if there are differences in output, it is necessary to consider what additional design burdens are being introduced to obtain those differences.

When comparing annual power generation, it is also important in practice to manage the names of each option in a clear and understandable way. For example, rather than simply naming them "Option 1" and "Option 2", giving names that indicate the changes—such as "south-facing standard option", "low-tilt option", "high-density layout option", or "shadow-avoidance option"—makes it easier to judge when reviewing reports later. By recording not only the numerical results of simulations but also which conditions were changed for each option, the accuracy of comparisons is improved.

Comparison Point 2: Looking at Differences in Monthly Power Generation

The next thing to check after annual energy production is the difference in monthly production. Even if annual values show little variation, month-by-month analysis can reveal differences in specific seasons. Checking monthly production is especially important in regions where tilt angle, azimuth, shading, temperature conditions, and snow effects are factors. PVSyst reports allow you to review monthly energy production, solar irradiation, and loss trends, enabling you to understand the seasonal characteristics of each design option.

For example, design proposals with different tilt angles can produce different generation patterns in summer and winter. A lower tilt angle may be advantageous in summer, while a higher tilt angle may receive more solar radiation in winter. Even if the differences look small when looking only at annual generation, clear trends can appear on a monthly basis. Which design should be adopted depends on which season’s generation you prioritize.

Also, the effects of shading change with the seasons. During periods when the solar altitude is low, the influence of surrounding obstacles and inter-row shading tends to increase. Therefore, even if the annual value appears acceptable, caution is required if power generation drops significantly in specific winter months. When planning power generation projects and equipment operations, not only the annual total but also monthly stability are important decision-making factors.

When looking at monthly generation, it is not sufficient to simply check which months have higher or lower output. Focus on months where there are large differences between design proposals and verify what is happening in those months. If the month showing a difference is in summer, temperature-related losses or output limits on the PCS side may be involved. If it is in winter, shading, solar incidence angle, or terrain conditions may be involved. In regions with a rainy season or frequent cloudiness, it is also necessary to check trends in the solar irradiance data.

Monthly comparisons are also useful when explaining things to stakeholders. Showing only the annual energy production can make it difficult to convey why a given proposal is preferable. However, if you can explain monthly trends, you can communicate concrete reasons for design decisions—for example, "this proposal is slightly worse on an annual basis but has smaller declines in winter," or "this proposal achieves high summer generation but shows noticeable winter shading losses." In practice, it is important not only to consider the magnitude of the figures but also to present comparisons that can be explained.

Comparison Point 3: Look at the difference between PR and specific power generation

When comparing design proposals in PVSyst, in addition to annual energy production you should always check PR and specific yield. PR is used as a performance indicator that shows how efficiently the system converted the incident solar energy into electrical energy. Specific yield is an indicator for assessing the energy produced per unit of system capacity. Both are important when comparing proposals with different system sizes.

An option with a large annual generation may at first glance appear superior. However, if that option’s installed capacity is larger than the others, the increased generation may simply be due to the increased capacity. In such cases, checking the specific generation allows you to compare efficiency per unit of capacity. If the specific generation declines even after increasing installed capacity, factors such as shading from high-density layouts, constraints on the PCS side, temperature rise, or deterioration of wiring conditions may be affecting performance.

Looking at PR makes it easier to understand the magnitude of losses for each design option. A design with a high PR can be considered to generate power relatively efficiently under the input conditions. Conversely, a design with a low PR may have some larger losses. However, judging solely by PR is risky. Because the way PR appears varies with design parameters, meteorological conditions, and equipment configuration, it is necessary to check it together with loss diagrams and monthly trends.

What you should pay particular attention to are proposals that show high annual generation but low PR or specific yield. Such proposals may be boosting generation by increasing installed capacity, but could be disadvantaged in terms of efficiency. While this kind of design can be an option in plans that make maximum use of the site, it is necessary to understand to what extent the design-related burdens and losses have increased relative to the increase in generation.

Conversely, there are proposals whose annual energy production is not maximal but that have high PR and specific yield. Such proposals may be generating efficiently while keeping installed capacity down. If you want to leave more room on the site, reduce shading risk, or prioritize constructability and maintainability, they become strong design options. When comparing in PVSyst, treating absolute energy production and efficiency metrics separately allows for more practical decision-making.

Comparison Point 4: Look at the factors that caused the differences in the loss plot

Checking the loss diagram is crucial for understanding differences in power generation among design options. By examining the loss diagram, you can identify at which stages and what types of losses occur as solar irradiance reaches the panel surface, is converted into electrical power, and ultimately becomes usable electrical energy. If there is a difference in energy production, confirming which loss items are causing that difference will reveal potential points for design improvement.

For example, if design proposals with the same installed capacity show differences in energy production, first check the loss diagram for shading loss, temperature loss, mismatch loss, wiring loss, PCS conversion loss, and output limitation. By seeing which items are larger, you can infer the cause of the difference in energy production. If shading loss is large, review the layout and the conditions of any obstructions. If temperature loss is large, consider the installation method, ventilation conditions, and the temperature rise on the module surface. If losses on the PCS side are large, check the capacity ratio and the operating range.

When looking at loss charts, it is easier to understand if you compare each option’s loss components in the same order. Rather than looking at a single option and concluding that “the losses are large,” check which components have increased compared with the other options. In comparisons of design options, differences are often more important than absolute values. For example, if thermal losses are similar across all options, temperature-related loss may not be a primary factor in that comparison. On the other hand, if only one option shows large shading losses, there may be a problem with that option’s layout conditions.

Also, loss diagrams are effective as explanatory materials for stakeholders. Rather than simply explaining low energy production as "poor conditions," being able to say "in this proposal losses from inter-row shading have increased, and as a result annual energy production has decreased" makes it easier to convey the need for design changes. To make practical use of PVSyst output, it is important not only to read the numbers but to translate them into language that leads to design decisions.

After confirming the differences in the loss diagram, consider whether those losses can be improved. Shading losses can sometimes be improved by changing the arrangement or spacing. Wiring losses can sometimes be improved by reviewing cable length, cross-sectional area, and equipment layout. Constraints on the PCS side can sometimes be improved by reviewing capacity and configuration. On the other hand, losses caused by weather conditions and regional characteristics can be difficult to change significantly through design. By separating losses that can be improved from those that should be accepted, you can link the comparison results to the next design review.

Comparison Point 5: Examine differences in shading loss and layout conditions

When comparing design proposals in PVSyst, shading losses are an item that requires particular attention. In photovoltaic installations, shadows from surrounding buildings, trees, terrain, rows of racking, and ancillary equipment affect energy production. The impact of shading not only reduces overall energy yield but also influences monthly generation trends and time-of-day output. Therefore, when comparing layout proposals, it is necessary to carefully interpret the differences in shading losses.

When panels are placed densely, it becomes easier to increase the installed capacity. However, if inter-row spacing is insufficient or the clearance from obstacles is inadequate, shading losses may increase. In that case, even if annual energy production increases, the efficiency per unit of installed capacity may decline. When comparing in PVSyst, separate the effect of increasing the number of panels from the impact of increased shading losses.

When assessing shading losses, it is important not only to look at the annual total but also to check which seasons experience the most shading. In regions where the sun’s altitude is lower in winter, shading patterns can change seasonally even with the same layout. For projects where winter generation is important, even a small amount of shading may not be negligible. Conversely, although there may be a certain level of shading loss over the course of a year, it can be acceptable when considering the design objectives and site conditions.

Also, the impact of shading relates not only to the layout but also to string configuration and electrical connection conditions. Even with the same shade, its effect on power generation changes depending on which area is shaded and at what times. It is important to consider not simply whether shading exists, but the shading’s location, timing, extent, and frequency. When interpreting PVSyst results, also check how closely the shading modeling conditions match the actual on‑site conditions.

A common mistake in comparing shading losses is oversimplifying on-site obstacles and terrain. While it may be acceptable to use simplified models for comparisons in the early design stages, when narrowing down candidate designs it is necessary to reflect site conditions more accurately. In particular, in locations with undulating terrain, in areas surrounded by buildings or trees, or where low solar elevation has a strong influence, the accuracy of site information directly affects the comparison results.

Comparison Point 6: Examine Overloading and PCS-side Constraints

When comparing design proposals, the balance between panel capacity and PCS capacity is also important. A design that makes the panel capacity larger than the PCS capacity makes it easier to use the PCS effectively during times or seasons with low solar irradiance, while during times of strong irradiance the output may be curtailed by the PCS's maximum. When comparing multiple proposals in PVSyst, verify how different degrees of oversizing affect energy yield, output limiting, PR, and the loss diagram.

When the oversizing ratio is increased, annual energy production tends to rise. However, beyond a certain level, the increase in energy production obtained from the added panel capacity may become small. This is because the periods when the output on the PCS side is capped become more frequent. If PVSyst’s loss diagram or result items show an increase in losses related to output limitation, carefully assess to what extent the added panel capacity is being utilized.

What's important when comparing oversizing is not simply whether there is output limitation. You need to confirm whether the losses from output limitation are acceptable relative to the increase in energy production from the added capacity. Even with some output limitation, generation during the morning, evening, and low-irradiance periods can increase, potentially making it advantageous over the course of a year. On the other hand, if output limitation becomes too great, generation will not rise in proportion to the added capacity, and the design efficiency will decline.

Also, constraints on the PCS are not limited to capacity. Input voltage range, string configuration, temperature conditions, number of modules in series, number of parallel strings, and other factors are also relevant. When comparing design proposals, you need to check not only the energy yield but also whether the electrical design is feasible. Even if the simulation runs successfully in PVSyst, you still need to separately verify that there are no issues in light of actual equipment specifications and installation conditions.

In comparisons on the PCS side, we consider not only maximizing power generation but also operational stability. Even if a proposal produces slightly more power, careful judgment is required at adoption if it leaves no margin in PCS operating conditions, makes the configuration more complex, or would make future maintenance difficult. The PVSyst comparison results are material for confirming the validity of the electrical design, and verifying the specifications of the actual equipment and the site conditions is indispensable for the final decision.

Comparison Point 7: Examine the intent behind design changes and whether they can be adopted in practice

Finally, it is important to confirm the intent of design changes and whether they can be adopted in practice. In PVSyst you can create various design proposals and compare power generation and losses. However, a proposal that performs well in simulation cannot necessarily be adopted on-site as is. In practice, you also need to consider constructability, maintainability, safety, site utilization, laws and regulations, delivery and access routes, racking layout, drainage planning, inspection access routes, and so on.

For example, suppose there is a proposal to arrange panels at high density to increase power generation. On PVSyst the annual energy yield might increase, but on site there could be insufficient inspection walkways, mowing and cleaning may become difficult, and the flexibility of racking installation may be reduced. In such cases, it is necessary to compare the increase in generation with the on-site operational burden and make a decision.

Also, do not judge proposals that alter the tilt angle or azimuth solely by energy production. Even if changing the tilt angle improves energy yield, it can affect wind loads, racking specifications, construction costs, and maintainability. Even if adjusting the azimuth increases energy production, it can lead to poor compatibility with the site shape and reduce layout efficiency. PVSyst results are a valuable basis for decision-making, but adopting a design proposal requires alignment with on-site conditions.

It is also important to clarify the intent behind design changes. Explain why the proposal was created, what you aimed to improve, and summarize which metrics improved and which deteriorated as a result. It is uncommon for all metrics to improve simultaneously. For example: power generation increased but shading losses also increased, PR fell but annual generation rose, constructability improved but generation dropped slightly—design proposals have both advantages and disadvantages.

A practically useful comparison in business is not about simply choosing the option with the highest numerical value, but about choosing an option whose adoption can be justified. When explaining to stakeholders, saying only "this option has the highest annual energy production" may be insufficient. Aim to be able to explain, "this option offers a good balance of annual energy production, shading losses, PCS constraints, and constructability, and is also well suited to site conditions." It is important to organize the PVSyst comparison results as the evidence supporting that explanation.

Common Mistakes When Comparing Design Proposals

A common mistake when comparing design proposals in PVSyst is drawing conclusions when the comparison conditions are not aligned. For example, if one proposal changes the panel specifications, another changes the tilt angle, and the loss settings also differ, you cannot correctly identify the cause of the difference in energy production. In comparisons, the basic rule is to fix as much as possible everything except the item you want to change. Even when changing multiple factors at the same time, you must make sure you can later explain which factors had what degree of impact.

Another common mistake is judging solely by the annual energy production. Annual energy production is important, but unless you examine monthly generation, PR, specific yield, loss diagrams, shading losses, and output curtailment, you cannot grasp the characteristics of a design proposal. In particular, when comparing proposals with different installed capacities, you cannot judge efficiency by annual energy production alone. If you select a proposal without checking generation per unit capacity, you may later discover the problem that “despite increasing capacity, the effect is small.”

Also, taking shading input conditions lightly is another mistake. Although shading has a significant impact on power generation, comparisons are sometimes proceeded with using the simplified conditions from the early design stage. In particular, in locations with surrounding obstructions or uneven terrain, the accuracy of shadow modeling will affect the results. At the stage of deciding which proposal to adopt, it is important to verify shading in a way that is as close as possible to the actual site conditions.

Furthermore, isolating simulation results from practical, on-site conditions is also a problem. Even if a proposal shows high energy yield in PVSyst, it is meaningless if it cannot be constructed. Proposals that are difficult to maintain, that do not provide adequate inspection access, that have overly complex wiring, or that require unrealistic installation conditions tend to be difficult to adopt in practice. When comparing design options, it is necessary to consider generation performance and site conditions simultaneously.

Lastly, a common mistake is inadequate documentation of comparison results. If you don’t record which option changed what, why that option was created, and what changed as a result, it becomes difficult to explain later. Rather than relying solely on PVSyst file names or variant names, organize the changes and the reasoning behind your decisions in writing; this will be helpful for internal reviews and when explaining things to clients.

Practical, Easy-to-Use Approach to Making Comparisons

When comparing design proposals using PVSyst in practical work, it is effective to first create a baseline proposal and then change one condition at a time from that baseline. The baseline should reflect site conditions, equipment conditions, a standard layout, and reasonable loss settings. From there, create proposals with different tilt angles, different azimuths, different layout densities, different PCS capacities, and proposals that prioritize shade avoidance.

When creating each proposal, clearly specify the changes. For example, when comparing tilt angles, keep other conditions as similar as possible. When comparing layout density, check the change in the number of panels together with the change in shading loss. When comparing PCS capacity, check the relationship between annual power generation and output limitation. In this way, deciding in advance which indicators to look at for each comparison purpose makes interpretation of the results more consistent.

When organizing comparison results, check annual generation, monthly generation, specific yield, PR, main losses, shading losses, output limitations, and practical points to note. Even if not in table form, arranging the characteristics of each option in prose makes judgment easier. For example, summarizing like "The high-density layout option increases annual generation but has larger shading losses in winter and a lower specific yield" and "The shade-avoidance option yields slightly less annual generation but has a more stable PR and makes it easier to secure maintenance access" will help lead to design decisions.

Comparisons are not completed in a single step. In practice, you narrow down promising options in an initial comparison, then run simulations again that reflect site conditions and construction conditions. In the early stages of design you grasp the overall trends, and as you move into detailed design you refine the input conditions. In particular, because results change when terrain, obstacles, equipment specifications, or loss conditions change, it is important to reconfirm with the latest conditions.

When you're still getting used to PVSyst, it's easy to simply follow the numbers on the screen. However, what matters in practice is reading the context behind those numbers. By considering why a given design option produces more energy, why there's a difference in a particular month, why the PR decreases, or why losses increase, PVSyst becomes not merely calculation software but an analysis tool that supports design decisions.

Summary

The basic method for comparing design proposals in PVSyst is to check, in order, the annual energy yield, monthly energy yield, PR, specific yield, loss diagram, shading losses, constraints on the PCS side, and practical feasibility for adoption. When examining differences in energy yield, it is important not to simply choose the proposal with the larger number, but to interpret which conditions caused that difference. By confirming whether the system capacity differs, the shading differs, the tilt angle or azimuth differs, or whether output limits have increased, you can practically judge the quality of a design proposal.

It is especially important not to draw conclusions based solely on annual energy production. Annual figures are a convenient entry point for comparison, but you cannot see the real differences unless you examine monthly variations and the breakdown of losses. Even options with higher energy production may have lower efficiency per capacity, larger shading losses, stricter PCS constraints, or be disadvantageous for construction and maintenance. Conversely, an option with slightly lower annual energy production may be easier to adopt in practice if it offers superior stability, constructability, and maintainability.

To make effective use of PVSyst, it is important to establish a reference case, clearly identify the changes, and organize the results so that you can explain the comparisons. By recording, for each design option, what was changed and which metrics improved or worsened, you improve the quality of internal reviews and client explanations. PVSyst results serve as material to support design decisions, and reading them together with site and construction conditions allows you to make judgments that are closer to practical, real-world conditions.

Moreover, to improve PVSyst’s comparative accuracy, the accuracy of on-site information entered into the simulation is also important. If you can accurately determine the planned site’s location, elevation, topography, obstacles, existing structures, and surrounding environment, it becomes easier to evaluate shading and layout conditions. Using position information and point cloud data acquired on site in design studies can reduce the gap between desk-based assumptions and the actual site.

LRTK is a high-precision GNSS positioning device that can be attached to an iPhone. For site surveys of solar power generation facilities and for assessing existing conditions before design, accurately acquiring on-site location information and recording structures, terrain, and reference points is essential. As a preliminary step before comparing design proposals in PVSyst, securing high-precision on-site coordinates and surrounding conditions makes it easier to improve the accuracy of shading analysis, layout planning, and pre-construction checks. Enhancing the quality of positional data collected on site therefore plays a significant role in bringing simulation-based comparisons closer to real-world practice.