【How to Use MET Files in PVSyst｜4 Checks to Prevent Configuration Errors】

Table of Contents

• First, understand what a MET file is

• Basic workflow for using MET files in PVSyst

• Check 1: Verify site information and coordinates are correct

• Check 2: Verify the meteorological data period and time resolution/unit are correct

• Check 3: Verify that items such as solar irradiance, temperature, and wind speed are appropriate

• Check 4: Verify units, time zone, and missing values

• Common configuration mistakes when using MET files

• Practical checking procedures and data management approach

• Collect and organize site information to improve PVSyst analysis accuracy

• Summary

First, understand what a MET file is

When conducting energy production simulations with PVSyst, the handling of meteorological data is the first important consideration. The power output of a solar power system is not determined solely by the capacity of the photovoltaic modules or the specifications of the power conditioner. In reality, many conditions are intricately related, including the solar irradiance at the installation site, ambient temperature, wind speed, rainfall patterns, surrounding topography, and shading conditions. Among these, the meteorological data that are used directly in PVSyst's calculations are extremely important information that form the foundation of the simulation results.

MET files are a type of meteorological data file used by PVSyst. Generally, it is easiest to think of them as files that format monthly or hourly meteorological information for a specific location so it can be handled within PVSyst. In PVSyst, the project's site settings are associated with the meteorological data, and that data is used to calculate annual energy production, losses, performance ratio, and so on. In other words, if the MET file is not appropriate, no matter how carefully the system conditions are entered, the reliability of the simulation results will be reduced.

One common stumbling block for practitioners who are just starting to use PVSyst in a professional setting is treating a MET file as merely an import file. If they assume that importing the file means the setup is complete, or that no errors means there are no problems, they can overlook issues such as the site location being offset, units being different from those expected, or time handling being misaligned. PVSyst is specialized analysis software and performs calculations strictly according to the input values, so verifying the assumptions on the input data side is essential.

Especially in the design and feasibility evaluation of photovoltaic power generation, a difference of a few percent in annual energy production can affect decisions on financial plans and equipment specifications. Errors in MET file settings may look like small differences on the simulation screen, but they can significantly impact the final energy production, loss assessment, equipment selection, and explanatory documentation. Therefore, when using MET files in PVSyst, it is important to always check not only the import operation but also the four elements: location, period, items, and units.

This article organizes, in a way that is easy for beginners to understand, the basic concepts of using MET files in PVSyst and the checkpoints to prevent configuration mistakes. It explains from a practical, ready-to-use perspective for those who already use PVSyst but feel uncertain about handling meteorological data, or who have received MET files and don’t know what to check.

Basic Workflow for Using MET Files in PVSyst

The workflow for using MET files in PVSyst, viewed broadly, follows the sequence of choosing the site, preparing the meteorological data, associating it with the project, and using it in the simulation. The exact screen names and button labels may differ depending on the environment, but the practical approach is common. First, clarify “which site’s power plant you will analyze” and prepare meteorological data appropriate for that site. Next, import that meteorological data in a format PVSyst can handle and link it to the project you are creating.

What is important here is not to consider the MET file in isolation. A MET file only becomes meaningful when combined with site information, system conditions, azimuth, tilt, shading settings, loss settings, and so on. For example, even if system capacity and panel layout are entered correctly, if the meteorological data are from a different region, the estimated power generation will not match the actual conditions on site. Conversely, even if you use meteorological data close to the site, if the project's coordinates or elevation are incorrect, the consistency of the documentation will be weakened.

When loading a MET file into PVSyst, first confirm what kind of meteorological conditions the file represents. Whether it is monthly average data, hourly data, typical year data, or data close to actual measurements will affect how you use it and how you interpret the results. Beginners tend to focus on the file format itself, but in practice it is important to verify "what this data was produced based on," "whether it is sufficiently representative of the site in question," and "whether it is appropriate for the simulation purpose."

After associating a MET file with the project, don’t immediately look at the final results; first review an overview of the meteorological data. Check whether the annual solar irradiation is unreasonably large, whether the monthly trends look natural, whether the seasonal temperature variations align with local expectations, and whether there are any missing or anomalous values. While PVSyst can perform detailed analyses, the final judgment on the validity of the input values rests with the user. The fact that the software was able to import the data and the decision that the data are suitable for practical use are not the same thing.

Also, before using MET files, decide within your company or project which meteorological data will be the standard to reduce later rework. When multiple people analyze the same power plant, if each person uses different meteorological data it becomes difficult to compare results. The required accuracy and level of explanation differ depending on the purpose—initial study, conceptual design, detailed design, materials for financial institutions, post-construction verification, etc. As a way of using PVSyst, not only should you simply import MET files, but you should also organize which data to use at each stage, as this leads to higher operational quality.

Check 1: Are the location information and coordinates correct?

The first check when using a MET file is whether the site information and coordinates match. When calculating energy production with PVSyst, the meteorological data is always tied to a specific location. Whether the site is mountainous, coastal, urban, or on a plain will affect solar irradiance, temperature, and wind conditions. Therefore, you need to confirm that the location of the meteorological data contained in the MET file matches the location set in the PVSyst project, or is within a practically acceptable range.

When checking coordinates, be careful about input errors in latitude and longitude. Mistakes such as entering latitude and longitude in reverse, using the wrong sign, misplacing the decimal point, or confusing degrees-minutes-seconds with decimal degrees can occur in practice. In particular, when entering location information received from external sources as-is, the notation format may not be consistent. Even if points look close on a map, they can be off by several kilometers to tens of kilometers, and in regions where weather conditions vary, this can affect simulation results.

Elevation is also an easily overlooked factor. In solar power generation analysis, elevation is related to temperature and atmospheric conditions. In areas with large elevation differences, temperature conditions can change even at the same latitude and longitude. Especially in mountainous and plateau regions, the elevation can differ greatly between the nearest meteorological station and the actual proposed plant site, so you should check not only the location information in the MET file but also the actual on-site conditions.

Also, even if the site name in PVSyst’s site settings and the MET file are similar, they are not necessarily the same location. Relying only on a broad-area name or municipality can overlook differences such as coastal versus inland, lowland versus upland, or flat versus sloped terrain. When using meteorological data in PVSyst, it is safer to verify by latitude, longitude, and elevation rather than by site name. When including this in project documentation, be prepared to describe the site conditions of the meteorological data used, so you can more easily support your rationale if questioned later.

A practical tip for checking coordinates is not to complete the process using only the PVSyst screen. Cross-check against design drawings, the candidate site coordinates, field survey data, terrain information, and layout plans to confirm that the site location is consistent. This is especially important when comparing multiple candidate sites, because it’s easy to accidentally reuse a MET file from a previous project and leave the point coordinates from that earlier project unchanged. Even if you change the file name or project name, the analysis will be incorrect if the internal location information has not been updated.

When you become familiar with PVSyst, you may create new projects by duplicating existing ones. This method is efficient, but it is dangerous if you skip checking the MET file settings. If the original project's site data and meteorological data remain and you only enter the new site's system conditions, the simulation may look correct while using the meteorological conditions from a different site. For new projects, make it a habit to always verify the correspondence between the site and the MET file first.

Check 2: Are the period and time units of the meteorological data consistent?

The second point to check is the period and time resolution of the meteorological data. MET files may be based on monthly representative values or on hourly data, among others. The type of data you use affects the granularity of the simulation results and what can be evaluated. If you only want to understand an approximate annual energy yield, monthly data may be acceptable in some cases, but if you need to consider shading effects, generation patterns by time of day, system constraints, or assessments close to output curtailment, data with finer time resolution becomes important.

What you need to watch for in practice is whether the period of the data fits the purpose of the analysis. If you use data from a single year, the results will vary depending on whether that year was average, had unusually high solar irradiation, or experienced a lot of unsettled weather. Even when using representative-year data, you need to understand the methodology by which that representative year was created. The annual energy production displayed by PVSyst is a calculation based on the meteorological data entered and does not fully guarantee future performance.

You also need to check the time units. When using time-resolved data, verify whether the values are hourly, aggregated from finer-resolution measurements, or converted from daily or monthly values. With coarse time units, it becomes difficult to represent short-term cloud effects or rapid weather changes. Conversely, in preliminary assessments, using excessively fine-grained data may still yield only limited overall accuracy if other assumptions are coarse. The important point is to judge whether the temporal resolution of the meteorological data is appropriate for the analysis objective.

When using MET files in PVSyst, pay attention to the year the data cover. In system design studies, it is common to estimate long-term generation trends based on historical weather data, but when using measured values from a specific year as-is, you need to check whether that year was atypical. For example, a year with prolonged rainfall, an unusually hot year, or a year with heavy snowfall may show different trends from a typical year. Relying solely on single-year data to assess project viability can lead to overestimation or underestimation.

Also, pay attention to gaps in the time coverage. Even if the data are treated as one year’s worth, some months or time periods may actually be missing. Even if PVSyst performs interpolation or conversion, large gaps in the original data can affect the reliability of the results. Missing irradiance data in particular tends to directly impact energy generation, so when you check the overview after importing, verify that the monthly values are not unnaturally low and that there are no consecutively missing periods.

In practice, it is important to be able to explain in submission materials and internal reviews which period of meteorological data was used. Rather than showing only the PVSyst results, recording the period covered by the MET file used, the time resolution, and the nature of the data will also be helpful when revisiting assumptions later. Power generation simulations are not created once and finished; they may be updated repeatedly due to design changes, equipment changes, layout changes, or re-evaluation of shading conditions. If the meteorological data assumptions change each time, comparisons become impossible.

Check 3: Are items such as solar radiation, air temperature, and wind speed appropriate?

The third point to check is whether the meteorological parameters included in the MET file are appropriate. In PVSyst’s energy yield calculations, solar irradiance plays a central role, but ambient temperature and wind speed also affect generation performance. Because photovoltaic modules tend to lose output as temperature rises, temperature conditions influence annual energy production and loss assessment. In addition, wind speed can affect estimates of module temperature, so it must be considered together with the mounting configuration and ventilation conditions.

There are several types of solar irradiance. Global horizontal irradiance, direct irradiance, diffuse irradiance, irradiance on a tilted surface, etc.—the calculation approach changes depending on which component is used. In PVSyst, the irradiance incident on the installation surface is calculated based on the input meteorological data. Therefore, it is important to check that the types of irradiance included in the MET file are appropriate and that the necessary components are present. When converting and using external data, take care not to misalign the original data field names with PVSyst’s corresponding fields.

For temperature, check not only the annual average but also seasonal variations. If winter temperatures are too high, summer temperatures are too low, or the month-to-month changes look extremely unnatural, the site or unit settings may be incorrect. Errors in the temperature unit or the handling of decimal points can affect the estimated module temperature and consequently cause the predicted energy yield to be off. After importing the MET file into PVSyst, always check temperature trends as well as irradiance.

Wind speed tends to be underestimated compared to solar irradiance and air temperature, but it is an item that should be checked in practice. In particular, when ventilation conditions differ—such as roof-mounted, ground-mounted, over-water installations, or installations close to walls—it can affect how module temperature is considered. Even when wind speed data are missing or represented by a single representative value, you should understand, when interpreting results, that there are limits to estimating cooling effects due to wind. When explaining PVSyst analysis results, knowing the extent to which meteorological data fields are available increases the credibility of your explanation.

Even items such as humidity and precipitation, which have only a limited direct impact on power generation calculations, can be useful for checking data consistency. For example, if something seems off — very high solar irradiance in a region with heavy rainfall, unusually warm winter temperatures in a cold area, or coastal locations showing inland-like temperature variability — it can prompt a review of the data site or the conversion methods. Not just loading the MET file, but checking whether the meteorological data follows plausible trends helps prevent configuration errors.

When using MET files received from external sources, also confirm who created the data and for what purpose. Data produced for preliminary estimates, for past projects at different sites, by simple conversion of monthly values, or prepared for detailed analysis will have different ranges of reliability. Even if the file extension is the same, the data contents and the creator’s intent are not necessarily identical. You should treat the fact that a file is in a format usable by PVSyst separately from whether the data are appropriate for the current project.

Check 4: Verify units, time zones, and missing values

The fourth checkpoint is units, time zones, and missing values. When handling MET files in PVSyst, even if the file loads successfully, discrepancies in the handling of units or time can affect the results. This is especially important when external data has been processed into MET files or when multiple people have exchanged the data; do not skip checking the units and the time reference.

When checking units, look at which unit the solar radiation is expressed in. Whether it is a value per unit time, a daily or monthly accumulation, or an energy amount per unit area will change how it should be handled. If you get the units wrong, the annual insolation can become extremely large or small. When viewing results in PVSyst, if the annual energy production falls outside a reasonable range, you should first suspect the units of the meteorological data, not just the system parameters.

Be careful with temperature units. They are generally handled in degrees Celsius (℃), but external data may contain different notations or values that have already been processed. If the temperature units are incorrect, it will affect estimates of module temperature and temperature-related losses. If a numeric value alone looks suspicious, check the unit-conversion history. In particular, data processed in spreadsheet software can lose column names and descriptions, making the units impossible to determine later.

Checking the time zone is important when using time-series data. Solar irradiance is closely related to the sun’s position, so if the timestamps are shifted it will affect the morning/afternoon distribution and shadow assessment. For example, whether the data were recorded in local time, based on standard time, or include time adjustments such as daylight saving time will change how the hourly data are interpreted. While projects within Japan may encounter less confusion, it is essential to confirm the time zone for overseas projects or projects covering multiple regions.

Pay attention to missing values such as blanks, zeros, abnormally negative values, and long runs of the same value. It is natural for solar radiation to be zero at night, but if it remains zero for extended periods during daytime, that may indicate missing data or conversion errors. For temperature and wind speed as well, if a constant value persists for a long time, it may be an imputed or placeholder value rather than a measured one. Even if PVSyst does not report an import error, you must separately verify whether the data are valid.

How to handle data with missing values depends on the purpose of the analysis. For preliminary assessments, you may allow certain imputations, but if the data are to be used for detailed feasibility evaluations or presentation materials, you should document your approach to missing-data handling. Indiscriminately replacing missing values with zeros can lead to underestimation of solar irradiance and power generation. Conversely, unnatural imputations can produce results that are better than reality. Because the quality of MET files is directly linked to the reliability of simulation results, treat missing-data handling with care.

As a practice when using PVSyst, it is important to make a habit of checking the annual and monthly graphs, the summary display, and the warning messages immediately after loading. Even if no major errors appear on the screen, verify that the monthly solar radiation values are not unnatural, that the seasonal variation in temperature looks reasonable, and that wind speeds are not excessively high. Since configuration errors often show up as anomalous numerical values, simply reviewing the overview of the meteorological data before running the simulation can prevent many mistakes.

Common configuration mistakes when using MET files

Common mistakes when using MET files in PVSyst are less about the file-loading operation itself and more about misconceptions regarding the underlying assumptions. A typical example is reusing a MET file from a past project as-is. If you assume a project in a similar region will be fine, in reality differences in elevation, distance from the coast, and terrain conditions can cause the meteorological conditions to be offset. Especially when using company-standardized template projects, make sure to check that the MET file association has not been left pointing to a previous project.

The next most common issue is a mismatch between the site settings and the meteorological data. In PVSyst projects, there are cases where a new site has been configured but the weather data remains for a different site. Because the project name and location are changed on the screen, the operator is likely to assume it is the correct project. However, if the meteorological data used for simulation calculations remains old, the results will not reflect the new site. When creating a new project, it is important to verify the site, the MET file, the layout, and the equipment conditions as a single set.

Also, mistakes occur from using data without understanding the difference between monthly and hourly values. Monthly data are useful for viewing annual trends, but they have limitations when it comes to checking time-of-day impacts or fine output fluctuations. Conversely, even if you are using hourly data, if that data was generated from coarse monthly values, it does not necessarily represent detailed hourly variations. Don’t judge by the file format alone; check the granularity at which the data was created.

Confusing units can also lead to serious mistakes. If the units for solar irradiance, temperature, wind speed, or the time reference are misaligned, PVSyst’s calculation results can change significantly. Especially when you process external data in a spreadsheet before importing it, unit information can be lost in the course of renaming columns or deleting unnecessary rows. If you have processed data, it is safer to keep a record of the relationship between the original raw data, the processed file, and the MET file imported into PVSyst.

Furthermore, it is also problematic to look only at the results and not review the meteorological data. If the annual energy production is higher or lower than expected, one tends to look for causes in the system capacity or loss settings, but in reality the cause can be the site specified in the MET file or the solar irradiance. In PVSyst analyses, many configuration items are interrelated, so when the results feel off, you need to go back and verify the meteorological data.

Practical Checking Procedures and Approaches to Data Management

When using PVSyst in practice, it is important not to rely solely on an individual operator's attention when checking MET files. Define the items to be checked for each project and ensure they can be verified against the same criteria by anyone, to reduce mistakes. Especially when multiple people are working, if the rationale for selecting meteorological data and the file update history are not shared, it can become impossible to determine the cause when comparing results later.

The first thing to confirm is whether the MET file to be used has been officially selected for this project. If provisional study files, internal review files, and client-submission files are mixed together, the likelihood of using the wrong file increases. Including the project name, site, creation date, data type, and version number in the file name makes management easier. However, do not rely solely on the file name; always check the site information and meteorological summary displayed in PVSyst.

Next, record the combination of the PVSyst project and the MET file. Keeping a record of which project file used which MET file and the equipment conditions in effect at the time of the simulation makes it easier to trace the change history later. Energy production simulations change depending on the panel model, PCS capacity, tilt angle, azimuth, shading conditions, and loss settings. If the meteorological data is changed along the way, it becomes difficult to determine which change affected the results.

For internal reviews, we recommend making not only the final results but also an overview of the input meteorological data a subject of verification. By checking annual solar irradiation, monthly solar irradiation, average temperature, the analysis period, site coordinates, and so on, obvious configuration errors can be detected at an early stage. Even if reviewers are not familiar with the detailed operations of PVSyst, they can still assess the plausibility of the site and seasonal trends. It is important that these assumptions are shared not only by technical specialists but also by design staff, business feasibility evaluators, and field surveyors.

Also, when updating a MET file, ensure it is not confused with past results. It is not uncommon to replace meteorological data for the same project. In initial assessments simple data are used, while more suitable data may be applied in detailed assessments. In such cases, when comparing old and new results within the same document, unless the differences in meteorological data are explicitly stated, it will be unclear whether the differences are due to equipment changes or to changes in the meteorological data.

The key in data management is reproducibility. Managing by criteria such as whether the same simulation can be reproduced months later, whether another person can run the calculation under the same conditions, and whether you can explain the usage conditions when asked by customers or stakeholders will improve practical quality. Learning how to use PVSyst is of course important, but in practice the ability to manage the relationship between input data and results is just as important.

Prepare local site information to improve PVSyst analysis accuracy

The MET file is an important input for PVSyst simulations, but analysis accuracy is not determined by meteorological data alone. To correctly reflect the plant’s installation environment, information on local terrain, surrounding obstacles, orientation, tilt, elevation, and the planned installation area must also be prepared. Even if the meteorological data are appropriate, if site conditions remain unclear it becomes difficult to accurately assess power generation and shading effects.

Especially for shading assessments, surrounding buildings, trees, slope faces, mountains, utility poles, and equipment can affect power generation. When setting shading effects in PVSyst, if the positions and heights of objects differ from actual conditions, the loss assessment will also be off. Alongside checking the MET file, it is important to accurately obtain on-site spatial information. If you cannot judge from drawings alone, confirm coordinates and heights on site and collect information that can be reflected in the analysis conditions.

Also, the coordinate accuracy of the planned installation site is important. If the candidate site's boundaries, racking layout, locations of junction boxes and PCS, access roads, or extent of site development remain ambiguous, uncertainties will persist in the layout produced in PVSyst. If the terrain has undulations, slope and orientation can affect power generation. Simulating while taking into account the site's topographical conditions, rather than relying solely on a simple planar layout, leads to an assessment that is closer to real-world practice.

High-precision positioning data is useful for organizing this kind of on-site information. LRTK is a GNSS high-precision positioning device that attaches to an iPhone, allowing easy acquisition of on-site location information and enabling its use in organizing the groundwork for design and surveys. In candidate site surveys for solar power generation, gathering information before simulation is important—confirming the installation area, recording the locations of obstacles, adding location information to photos, and organizing field notes. In addition to correctly using MET files in PVSyst, accurately aligning on-site coordinate information makes it easier to reduce discrepancies between desktop studies and actual field conditions.

Because PVSyst's analyses produce results based on the input conditions, it is important to carefully handle both meteorological data and on-site data. Check the location, period, variables, and units in the MET file, and if you can also capture the site position and obstacle information with high accuracy, the explanatory power of the power generation simulation will improve. For consistently managing everything from initial studies to detailed design, pre-construction verification, and post-completion records, user-friendly high-precision positioning tools such as LRTK are an effective option for practitioners using PVSyst.

Summary

Using a MET file in PVSyst is not simply a matter of loading the file and assigning it to the project. To improve the reliability of the energy yield simulation, you need to verify which location the MET file’s meteorological data correspond to, what period the data cover, which meteorological variables are included, and whether there are any issues with units, time zone, or missing values. Especially for beginners, it’s easy to feel reassured once the import operation completes, but in practice the checks performed after importing are the important ones.

To prevent configuration errors, first check the site information and coordinates. Verify that the target site and the MET file location match and that there are no major discrepancies in elevation or regional characteristics. Next, check the period and time resolution of the meteorological data to determine whether they are appropriate for the analysis purpose. Furthermore, confirm that items such as solar irradiance, air temperature, and wind speed are appropriately included, and finally check the units, time zone, and missing values. Simply making these four checks a habit will prevent many mistakes in PVSyst meteorological data settings.

Also, PVSyst results depend on the input conditions. Even if the MET file is appropriate, if the site coordinates, topography, shading, or layout conditions are ambiguous, the explanatory power of the simulation results decreases. Practitioners using PVSyst in real-world projects should pay attention not only to quality-checking meteorological data but also to acquiring and managing on-site information. By utilizing LRTK as a GNSS high-precision positioning device attached to an iPhone, you can streamline location recording and on-site verification of candidate sites. By combining confirmation of meteorological conditions via MET files with the organization of on-site coordinate information using LRTK, it becomes easier to apply PVSyst simulations in a manner closer to actual practice.