7 Points to Watch When Setting Solar Irradiation Data in PVSyst

Table of Contents

• Why solar irradiation data settings matter in PVSyst

• Point 1: Match the installation site with the location of the solar irradiation data

• Point 2: Check not only annual figures but also monthly biases

• Point 3: Understand and handle differences in time resolution and data format

• Point 4: Don’t decide based on solar irradiation alone without checking temperature and surrounding conditions

• Point 5: Don’t separate the settings for azimuth and tilt from the irradiation data

• Point 6: Don’t casually mix multiple data sources

• Point 7: Reverse-check the irradiation settings while reviewing calculation results

• Perspectives to link PVSyst irradiation settings to practical accuracy

Why solar irradiation data settings matter in PVSyst

When running generation simulations in PVSyst, the first thing practitioners should be aware of is that the way solar irradiation data is set up becomes the foundation for the entire calculation. No matter how carefully you arrange equipment configuration, installation orientation, or loss assumptions, if the underlying concept of the solar irradiation data is off, the final generation forecast will be unstable. In other words, solar irradiation data should not be treated as a mere initial input but as a prerequisite that supports all subsequent decisions.

In practice, when people think about generation simulations they tend to focus on system capacity, layout, and loss allocation. However, if the assumption about how much solar irradiation the system will receive is inappropriate, any comparison becomes less meaningful. For example, even a small discrepancy in site conditions can change not only the annual generation figure but also the monthly trends and apparent temperature effects. When handling solar irradiation data in PVSyst, it is important to be aware of the ripple effects on the entire simulation, not just the most conspicuous numbers.

Additionally, irradiation data settings can look reasonable on the surface yet be weak in practical use. Assuming a nearby representative station or reusing assumptions from a previous nearby project because the sites are close or the annual averages are similar can blur the differences between alternatives or weaken the rationale during explanations. Because PVSyst tends to reflect the input assumptions directly in the results, how you handle irradiation data directly affects the reliability of those results.

Furthermore, solar irradiation data is not something you set once and forget. As a project progresses, you may need to increase site resolution, change comparison targets, or revise layout assumptions. Each time, you must check whether the irradiation data settings still match the current purpose. Below are seven commonly overlooked points to consider when setting solar irradiation data in PVSyst, organized from a practical perspective.

Point 1: Match the installation site with the location of the solar irradiation data

When setting solar irradiation data in PVSyst, the first thing to watch is whether the installation site and the location represented by the irradiation data truly match. This seems basic, but in practice it’s a surprisingly easy point to leave ambiguous. Using data from a representative station near the candidate site or reusing assumptions from a nearby location in a previous project will produce plausible-looking results. However, if you use them without considering whether the site difference is truly negligible, the validity of your results can become shaky.

This discrepancy becomes especially apparent when dealing with comparative projects or multiple candidate sites. If the installation sites differ but the approach to irradiation data is not unified, it becomes hard to tell whether differences among alternatives stem from design conditions or from differing site assumptions. Because PVSyst results are often used to compare scenario differences, unclear site consistency weakens the persuasive power of those comparisons. Even if you present a list of annual generation numbers, mismatched base locations make the materials difficult to use in practice.

To prevent this, make the recognition of the installation site as clear as possible when selecting irradiation data. Be able to explain which location you used as the reference and why you adopted that location’s data. Doing so makes judgments easier to revisit later. Even when using a representative station for preliminary studies, keep in mind that it is a provisional assumption. If you clarify your approach to site selection early, it will be easier to revise when on-site conditions become clearer.

Also note that site matching is not only about coordinates. Check whether the surrounding terrain and installation environment align with the impression given by the irradiation data you’re using. When handling irradiation data in PVSyst, do not simply pick available data; think about whether that location is a reasonable representative for this project. Taking care here will significantly reduce explanation burden and re-calculation work downstream.

Point 2: Check not only annual figures but also monthly biases

A common oversight when setting irradiation data is to be reassured by annual totals or averages alone. Indeed, annual representative values are easy to understand and compare. However, in PVSyst simulations the monthly distribution of irradiation greatly affects how results appear. Even if annual values are similar, differing monthly biases can change seasonal generation variability, the appearance of losses, and the impression of comparison alternatives.

In practice, annual generation often becomes the main decision factor, which naturally draws attention to annual totals. However, depending on system orientation, installation conditions, or operational considerations, the weakness or strength in a particular season can affect project decisions. If you set irradiation data without checking monthly tendencies, it will be hard to determine whether any unnatural biases in the calculation results stem from the irradiation settings or from other assumptions. When using PVSyst, you need to pay attention to the distribution shape as well as the annual value.

To address this point, after setting the irradiation data, at minimum check whether the monthly tendencies present any extreme mismatches. If one month looks unnaturally high or low, investigate why. Of course regional characteristics and seasonality may naturally produce differences; the important thing is whether the bias aligns with the project assumptions and whether you can explain it later. Being aware of distribution at the irradiation-setting stage makes later checks easier.

This perspective is also useful when creating comparison scenarios. Even if two alternatives have similar annual generation, differing monthly behaviors can change how they are evaluated on site or presented internally. When setting irradiation data in PVSyst, do not rely solely on the annual totals. Grasping monthly biases brings consistency to result interpretation and reduces hesitation during report preparation and explanation.

Point 3: Understand and handle differences in time resolution and data format

When setting irradiation data in PVSyst, you need to understand not only the numbers themselves but also the time resolution and format of the data. Even if data look the same at first glance, the simulation detail and interpretation differ depending on whether the data assume hourly/time-dependent variation or are closer to monthly representative values. Using such data ambiguously can produce numbers but blur what those numbers actually mean and their comparative significance.

In practice, the required resolution depends on the simulation purpose. For preliminary comparisons, grasping the overall trend is a priority, while more detailed design studies may require awareness of fluctuations. If you use data without considering format differences, assumptions adequate for one project may be reused in another where they are insufficient, leaving you short of the needed precision. Although PVSyst inputs may be entered in a similar way, the trustworthiness of results depends on understanding the nature of the source data.

To address this, be aware in advance of the level of granularity at which the data will be reflected in results before setting irradiation data. It is not necessary to always use the highest resolution. What matters is judging whether the data’s granularity matches the purpose of the current study. There is no need to force an overly complex premise for a simple comparative document, nor to keep coarse premises when a detailed comparison is required.

Understanding data format differences also speeds up reviews when results seem odd. If generation trends deviate from expectations, you can suspect not only the system conditions but also the data’s resolution or handling. PVSyst presents numerical results up front, but those numbers inherit the nature of the source data. That’s why it’s important to use irradiation data with an understanding of its format and time resolution.

Point 4: Don’t decide based on solar irradiation alone without checking temperature and surrounding conditions

Solar irradiation data are important, but it is risky to complete PVSyst settings based only on irradiation. In practice, people often look first at irradiation conditions and roughly judge a site’s generation potential. However, temperature and surrounding environment conditions also affect results. If irradiation looks good but other assumptions are not consistent, the simulation can easily become overly optimistic.

For example, temperature effects can vary depending on the installation environment, and surface conditions or layout assumptions can make equipment behavior and losses look different even under the same irradiation premise. Although irradiation data are the starting point in PVSyst, final generation is determined by the overlap of multiple conditions. Therefore, when selecting irradiation data, it is important to consider how it ties to local surrounding conditions and other meteorological inputs.

As a mitigation measure, when you choose irradiation data consider whether the site’s environmental conditions and the project’s design assumptions are compatible. Don’t conclude that high irradiation alone makes the site advantageous; also check whether equipment and loss conditions feel consistent with that premise. PVSyst simulation inputs may look like independent pieces stacked together, but in reality they interact. Treat irradiation settings as the entry point to overall design, not an isolated decision, which makes later consistency checks easier.

This viewpoint also stabilizes result interpretation. If annual generation falls short of expectations, you won’t immediately blame irradiation alone. When setting irradiation data in PVSyst, include the relationship with other meteorological conditions and installation environment to improve practical accuracy.

Point 5: Don’t separate the settings for azimuth and tilt from the irradiation data

One often overlooked point when setting irradiation data in PVSyst is considering irradiation separately from azimuth and tilt settings. Irradiation data are a fundamental site-bound condition, but the energy actually received by the equipment depends on orientation and slope. If you treat irradiation selection and azimuth/tilt as separate issues, the linkage between assumptions weakens and it becomes harder to justify the results.

In practice the flow is often to decide site conditions first and then consider azimuth and tilt. That order is natural, but be aware that the meaning of solar irradiation data can change depending on the installation conditions decided later. For example, in a project where optimal orientation cannot be achieved, evaluations change even if the irradiation premise is the same. In PVSyst, do not evaluate irradiation as a standalone good-or-bad item; be mindful of how you will use it within the installation conditions for that project.

To avoid this oversight, when setting irradiation data simultaneously consider the assumed azimuth and tilt for that site. Even if details are undetermined, having a rough direction brings consistency in result interpretation. Conversely, if you initially use favorable irradiation assumptions and later add harsh installation conditions, the gap between initial expectations and reality tends to widen and internal explanations will require more rework.

This viewpoint is also essential when preparing comparison alternatives. Even if irradiation data are the same across cases, differences in azimuth and tilt require separating which differences stem from site factors and which from installation choices. When handling irradiation data in PVSyst, do not put site conditions and installation conditions into separate boxes. Linking both improves the explanatory power of simulation results and simplifies design comparison.

Point 6: Don’t casually mix multiple data sources

A common practical issue in irradiation data settings is unintentionally mixing multiple data concepts. You might reference irradiation assumptions from a past project, update other conditions for the current project, and also consult trends from a different nearby site for comparison—before you know it, assumptions become a patchwork. In that case, the numbers may look reasonable but it becomes hard to trace which condition came from where, making explanation and reproduction difficult.

As a practitioner, it’s natural to want to utilize past project data and to refer to nearby cases, and doing so is not inherently bad. However, what matters is explicitly separating the reference scope from the assumptions you actually commit to for the current project. PVSyst simulation reliability depends on aligned assumptions. If you unconsciously mix multiple data sources or concepts, it becomes difficult to judge whether observed differences are due to design factors or to differing data premises.

To address this, clearly state the reason for adopting the irradiation data and consolidate the assumptions into a single consistent thread for the current project. If you reference past projects, document what parts you referenced and what you overwrote with current conditions. For comparison scenarios, check whether the treatment of irradiation data differs between alternatives and ensure comparability. Reusing data for efficiency should not increase verification and explanation burden later.

Also, mixing data sources creates a heavy load during report creation. You may have results but be unable to coherently explain the assumptions in words. When setting irradiation data in PVSyst, do not merely gather available information; organize assumptions into a coherent set for the project. When assumptions are unified, result verification and explanation become much easier.

Point 7: Reverse-check the irradiation settings while reviewing calculation results

Finally, an important point is not to stop after setting irradiation data. In PVSyst, you must review calculation results and use them to retrospectively check whether the chosen irradiation data were truly appropriate. Even if the inputs looked plausible at entry, if the resulting figures, monthly trends, or loss behavior feel off, you need to reconsider the irradiation settings. If you neglect this, initial assumptions will become fixed in the results.

In practice, people can feel reassured once the simulation runs and then stop at checking annual generation only. But what you should really check is whether those numbers align with project intuition. If monthly variations look unnatural, the difference between alternatives is unexpectedly large or small, do not just suspect equipment conditions—also question whether the irradiation data treatment is at fault. PVSyst makes it easy to validate premises against results, and iterating in this way improves simulation quality.

As a countermeasure, after setting irradiation data fix a set of primary result review points. By checking annual projections, monthly tendencies, differences between alternatives, and how losses appear in a consistent order, you are more likely to notice weak assumptions or inconsistencies. In practical use, it is more realistic to refine premises while observing results than to expect a single perfect input. Treat irradiation settings as something to be inspected after input as well.

Furthermore, this reverse-checking mindset facilitates team review. A different team member can point out inconsistencies in the results relative to the irradiation assumptions. Correct handling of irradiation data in PVSyst requires not only careful input but also a willingness to question premises through the results. Setting is not the end—consider confirmation of consistency based on results to be part of practical irradiation data work.

Perspectives to link PVSyst irradiation settings to practical accuracy

What ties together the seven points above is the necessity to treat solar irradiation data not as a mere initial input, but as a premise that determines the quality of the entire simulation. Matching the installation site, checking monthly distributions, understanding data format, ensuring consistency with meteorological and installation conditions, avoiding mixed sources, and reverse-checking from results—these are not steps to make numbers look good but perspectives to approach results that are usable in practice. Generation simulation in PVSyst is not determined solely by equipment conditions; interpretation can vary greatly depending on how irradiation data are set.

For practitioners, the goal is not to choose the irradiation setting that gives the highest numbers. What matters is creating assumptions that are explainable for the project, robust in comparisons, and easy to keep consistent even after later condition changes. Vague irradiation handling destabilizes design comparisons, internal explanations, and loss interpretation. Conversely, careful irradiation settings increase confidence in the results and make report preparation and stakeholder communication easier.

If you truly want to improve irradiation-setting accuracy, do not limit yourself to desk-based data selection. A shallow understanding of site location, site orientation, surrounding terrain, potential shading, and layout constraints will weaken decisions about how to use irradiation data. While irradiation itself can be a broad-area premise, what matters in practice is how to translate it into site-specific conditions. In other words, irradiation-setting accuracy is tied to the accuracy of on-site understanding.

In that sense, when you want to secure position information and coordinates on site more reliably, using an iPhone-mounted high-precision GNSS device such as LRTK can be effective. If on-site position information and site conditions are more easily organized, it becomes easier to judge locations when selecting irradiation data in PVSyst and to check consistency with azimuth, tilt, and layout conditions. By combining desk-based PVSyst simulation with LRTK-supported on-site surveying, irradiation settings become not just data choices but design decisions grounded in the field. Correct handling of irradiation data not only improves generation forecast accuracy but also strengthens the practical linkage between desk work and on-site work.