What does PVSyst base its calculations on? A gentle explanation of the basic theory

PVSyst is simulation software used to predict the energy yield of photovoltaic (PV) systems and to organize design conditions and loss factors. However, for people using it for the first time in a professional setting, the impression that it is a convenient tool that produces results simply by entering inputs often comes first, and it can be difficult to see what those calculations are based on. Energy yield prediction is not just a matter of multiplying system capacity by solar irradiance. It is calculated by building on many theories and assumptions, from the position of the sun, mounting angle, and types of irradiance, to surrounding shading, module temperature, electrical losses, conversion efficiency, and output to the grid.

Table of Contents

• What PVSyst is used for

• The starting point for energy production calculations is solar irradiance

• Solar position and installation angles form the basis of the calculations

• Direct, diffuse, and reflected irradiance are considered separately

• The effect of shading is evaluated by geometric calculations and temporal changes

• Module temperature has a major influence on energy production

• Electrical losses are considered cumulatively, stage by stage

• Considerations for inverter conversion and output limitation

• Annual energy production is determined by accumulating calculations for each time step

• Points to note when reviewing PVSyst results

• Understanding local site conditions is important to improve accuracy

• Summary

What is PVSyst software used for?

PVSyst is simulation software that supports the planning, design, energy production forecasting, loss analysis, and report generation of photovoltaic power generation systems. In practice, it is used when planning a new plant, evaluating the energy output of existing installations, comparing design conditions, or explaining the validity of projected energy production.

The purpose of using such software is not simply to produce a number that answers “how many kWh will be generated.” It is important to verify why that amount of generation occurs, which losses are large, and which conditions, if changed, will improve the results. Predicted generation figures relate to business planning, equipment design, profitability assessments, construction planning, and operations management, so looking only at the results without understanding their basis can lead to incorrect decisions.

PVSyst's calculations combine a physical model of solar power generation with the input design conditions. Essentially, it determines how much solar energy reaches a given location, calculates the portion incident on the photovoltaic surface, and then subtracts losses such as temperature, shading, wiring, and conversion to estimate the final energy production.

In other words, it’s easier to understand PVSyst not as “software that will automatically give you the correct answer if you just enter the system capacity,” but as “a calculation environment for estimating energy production by organizing weather, topography, installation conditions, equipment characteristics, and loss conditions in accordance with theory.”

What practitioners should grasp first is that simulation results strongly depend on the quality of the input conditions. If there are errors in solar irradiance data, installation orientation, tilt angle, surrounding obstacles, module conditions, inverter conditions, or cable conditions, no matter how advanced the calculations are the results will deviate from reality. Therefore, understanding PVSyst is not merely learning how to operate the interface, but also understanding the approach to energy yield calculation.

The starting point for power generation calculations is solar irradiance

The first important factor in calculating solar power is how much solar irradiance reaches the location. Power output is the result of converting the light energy received by the solar cells into electrical energy. Therefore, solar irradiance is the starting point for power output calculations.

Solar irradiance refers to the amount of solar energy incident on a given surface. In general, it is treated as the amount of energy per unit area. In photovoltaic power generation, it is necessary to consider not only the irradiance reaching a horizontal plane but also the irradiance incident on the inclined surface where the solar modules are actually installed. The irradiance received on a surface horizontal to the ground is not the same as that received on a south-facing tilted surface.

In power generation forecasts like PVSyst, solar irradiance is handled based on meteorological data. Meteorological data may include solar irradiance, ambient temperature, wind speed, and so on. Of these, solar irradiance is the most fundamental factor that determines the overall magnitude of power generation. Regions with high solar irradiance tend to have higher power generation, while regions with low solar irradiance tend to have lower power generation.

However, a higher amount of solar irradiance does not necessarily mean a greater power output. In regions prone to high temperatures, module temperatures can rise and reduce generation efficiency. Snow, fog, shading from surrounding objects, and constraints on equipment layout also have an impact. Therefore, while solar irradiance is a starting point, it is not the actual power output.

In practical work, it is important to note that solar radiation data differ in type, period, and representativeness. Whether you use long-term averages, measured values from a specific year, or data from a nearby location will change the results. In power generation forecasting, because you cannot predict future generation exactly, it is common to perform calculations based on historical weather data while assuming average conditions.

Therefore, when reviewing PVSyst results, you should first confirm "which meteorological data are being used," "whether they are appropriate for that location," and "whether they can be regarded as long-term averages." If you leave these points unclear and only look at detailed loss rates, you cannot correctly assess the overall reliability of the prediction.

The solar position and installation angle form the basis of the calculations

The next most important factors after solar irradiance are the sun’s position and the mounting angle of the photovoltaic modules. The sun’s apparent position changes with the season and time of day. In summer it travels a high path across the sky, and in winter a low one. Also, it appears in the east in the morning, the south at midday, and the west in the evening. These changes in the sun’s position greatly affect the amount of light that reaches the surface of the solar panels.

Solar modules have an azimuth and a tilt angle. The azimuth indicates which direction the module faces. The tilt angle indicates how much the module is inclined from the horizontal plane. The more nearly perpendicular the sunlight strikes the module surface, the greater the energy received per unit area. Conversely, the more obliquely it strikes, the less effective energy reaches the surface from the same sunlight.

This approach is based on the calculation of the angle of incidence. The angle of incidence indicates the angle at which sunlight strikes the module surface. In power generation simulations, the sun’s position at each time is calculated, and the angle of incidence is obtained from the module’s azimuth and tilt angles. Then, the solar irradiance entering the tilted surface is estimated according to that angle.

The installation angle affects not only annual power generation but also seasonal generation trends. Increasing the tilt can be advantageous for the low solar elevation in winter. Conversely, it is not necessarily optimal for the high solar elevation in summer. Reducing the tilt can be beneficial for summer and around midday conditions, but practical factors such as how rain washes away dirt, snowfall, and installation density also play a role.

Also, due to equipment layout, you cannot always choose only the tilt that yields the highest power generation. You need to design taking into account roof shape, terrain, site boundaries, mounting conditions, wind loads, maintenance access routes, shadow avoidance, and so on. In PVSyst calculations, these installation conditions are entered and reflected in how the solar radiation is received.

In other words, the basis for power generation forecasts is not merely the installed capacity, but the relationship between the sun’s movement and the orientation of the module surface. When working with PVSyst, it is important not to treat azimuth and tilt angles as items to be simply transcribed from the design drawings, but to verify them as critical parameters that determine the fundamentals of energy yield.

Consider direct solar radiation, diffuse solar radiation, and reflected solar radiation separately

In solar power generation calculations, solar irradiance is not treated as a single lumped quantity; instead, it is broken down into components that have different characteristics. Typical ones are direct irradiance, diffuse irradiance, and reflected irradiance. Understanding this classification makes it easier to see why PVSyst’s calculations appear complex.

Direct solar radiation is the light that arrives directly from the sun. The light that casts strong shadows on clear days is largely due to this direct solar radiation. Because direct solar radiation has a clear direction from the sun, the amount received can vary greatly depending on the module’s orientation and tilt angle and on whether shadows are present.

Diffuse solar radiation is light that has been scattered by molecules, clouds, and fine particles in the atmosphere and arrives as illumination coming from the entire sky. Even on cloudy days the surroundings are bright and solar panels generate electricity because of diffuse solar radiation. Because diffuse solar radiation is less directional than direct solar radiation, the way shading and installation angle affect performance is different.

Reflected solar irradiance is the light reflected from the ground and surrounding surfaces that enters the module. Its effect varies depending on the reflectivity of the ground surface. For example, bright surfaces or snow-covered ground can increase the reflected component. Conversely, dark ground results in a smaller reflected component.

In simulations like PVSyst, these components are separated to calculate the irradiance incident on a tilted surface. Rather than using horizontal-plane irradiance data as-is, the solar position, the module surface orientation, the diffuse component from the sky, and the ground-reflected component are taken into account and converted into the irradiance actually received by the module.

This conversion is an important part of power generation forecasting. This is because the energy received by a solar cell is not the irradiance on the horizontal plane, but the irradiance incident on the module surface. If installation conditions change, the power generation will change even when using the same site's irradiance data.

In practice, in regions with a high share of direct solar radiation, shading and orientation effects tend to be more significant, while under conditions with a high share of diffuse solar radiation the influence of angle can be somewhat milder. Of course, actual results vary depending on meteorological and design conditions, but by considering solar radiation in its component parts, it becomes easier to interpret losses and power generation trends.

To understand what PVSyst bases its calculations on, the concept of solar radiation components is indispensable. Energy production is determined not only by whether solar irradiance is present, but by from which direction, what type of light, and how much of it reaches the module surface.

Evaluate the impact of shadows using geometric calculations and temporal variations

In solar power installations, avoiding the effects of shading is extremely important.

When modules are shaded by buildings, trees, utility poles, adjacent mounting racks, or terrain variations, the solar irradiance they receive in those areas is reduced. Furthermore, because photovoltaic modules are electrically connected, shading of one part can affect the overall output.

Shadow evaluation in PVSyst is basically performed based on the geometric relationship between the solar position and surrounding obstacles. At a given time, it calculates the direction and elevation of the sun and checks whether there are obstacles in that direction. If an obstacle is positioned so as to block sunlight, the direct solar irradiance during that period is reduced.

Shadows also change with the seasons. In winter, because the sun’s altitude is lower, the same obstacles tend to cast longer shadows. In summer, because the sun’s altitude is higher, shadow lengths tend to be shorter. Also, in the morning and evening, when the sun’s altitude is low, shadows can stretch out long horizontally. In this way, the effect of shadows is not constant throughout the year.

In power generation simulations, the presence or absence of shadows is evaluated on an hourly basis and reflected as a reduction in solar irradiance. Rather than simply deciding “this system is shaded so it loses X%,” the basic approach is to combine the sun’s movement with the shape of obstructions and the module layout to evaluate the effects including temporal variation.

However, the precision of the input data has a large impact on shadow calculations. If obstacle heights, positions, shapes, differences in ground elevation, the spacing between module rows, and so on are not entered correctly, the resulting shadow loss will also be off. In particular, for ground-mounted systems, shadows between racking rows tend to have a greater effect in winter and during mornings and evenings. For rooftop systems, shadows from surrounding buildings, parapets, equipment, antennas, trees, and the like can be overlooked.

Furthermore, shadows cause not only a reduction in solar irradiance but also electrical mismatch losses. When part of a module is shaded, it can affect the output of that module and the entire connected circuit. Therefore, when evaluating shading, it is necessary to consider not only how much solar irradiance is geometrically blocked but also how the reduction in output propagates through the electrical circuit.

When reviewing PVSyst results, it is important not only to check whether shading losses are large or small, but also to confirm during which periods, at what times of day, and from which directions the shading occurs. Because shading can potentially be improved through design changes, it is an aspect of the simulation results that can be readily used to inform design considerations.

Module temperature greatly affects power output

Photovoltaic (PV) modules generate electricity when exposed to light, but they are also influenced by temperature. In general, for crystalline solar cells, module output decreases as module temperature increases. This is because the voltage characteristics of solar cells are temperature-dependent. Therefore, power generation forecasting must take into account not only solar irradiance but also module temperature.

Module temperature is not the same as ambient temperature. The surface of a module exposed to sunlight is often hotter than the ambient temperature. Under conditions of strong solar radiation and weak wind, temperatures tend to rise more easily. Conversely, when wind is present and heat can be dissipated easily, the temperature rise is suppressed. Heat dissipation conditions also vary depending on the mounting structure, the gap to the roof, and the installation method.

In PVSyst's calculations, the module temperature is estimated from ambient temperature, solar irradiance, wind effects, mounting configuration, and so on. Then, using the module's temperature characteristics, the output reduction is calculated. This is the temperature loss.

Temperature losses can have a relatively large impact on power generation. Especially during high temperatures in summer, even with high solar irradiance the module temperature rises, so output may be lower than under ideal conditions. When looking at seasonal variations in generation, periods with high irradiance and periods when maximum output is likely do not always completely coincide. One reason for this is the effect of temperature.

What is important in practice is that module temperature calculations depend on the installation environment of the equipment. Installations that are tightly attached to the roof have poor heat dissipation, whereas installations like ground-mounted racks, where air can flow easily behind the modules, can dissipate heat more readily. Of course, in reality this varies with mounting height, ambient airflow, roofing material, ground surface, and the surrounding environment.

If temperature losses are not correctly understood, you can misinterpret why power generation does not increase as expected despite high solar irradiance. Rather than simply concluding that the equipment's performance is poor, the shortfall can sometimes be explained as a natural reduction in output caused by meteorological conditions and temperature characteristics.

PVSyst is software that incorporates such temperature effects into energy yield calculations to produce predictions that are closer to reality. Rather than judging energy production solely by irradiance, an important part of the basic theory is considering the temperature conditions under which the module actually operates.

Electrical losses should be assessed cumulatively at each stage.

Electricity generated by a photovoltaic module cannot be used in its entirety as is. Various losses occur at stages such as inside the module, in the connection circuits, in the wiring, in the power conversion equipment, and in output control. In PVSyst's calculations, these losses are accumulated step by step to determine the final energy yield.

First, modules have a rated output, but actual output varies with solar irradiance and temperature conditions. Furthermore, even modules of the same model exhibit unit-to-unit differences and do not all produce exactly the same output. When modules are connected in series or in parallel, these performance differences prevent outputs from matching and cause mismatch losses.

In addition, dirt on the module surface also causes losses. When sand and dust, pollen, bird droppings, fallen leaves, or exhaust-related grime adhere, incoming light is reduced and power generation decreases. Rain can naturally wash these away, but when the installation angle is small or the environment is prone to soiling, the effects of dirt tend to remain.

There are also losses in wiring. When electricity flows through a cable, some of it is lost as heat due to resistance. The longer the cable, the larger the current, and the greater the resistance, the higher the wiring losses. During the design phase, it is necessary to appropriately consider cable diameter, wiring distance, and circuit configuration.

Furthermore, small losses occur in the process of passing through junction boxes and protective devices. Although each loss may seem minor by itself, when accumulated over annual energy production it can have a non-negligible impact. Therefore, in power generation simulations it is important to be able to break down and inspect the losses at each stage.

PVSyst's loss analysis lets you see at which stages and by how much the energy yield is reduced. This is very useful for explaining the energy generation results. Looking only at the final annual energy yield does not reveal which conditions are suppressing the output. By examining losses stage by stage, you can separate items that can be improved from those that should be accepted as natural conditions.

For example, temperature losses and solar irradiance conditions cannot be completely eliminated by design, but wiring losses, shading losses, and excessive mismatch can potentially be reduced through careful design and installation. The purpose of using PVSyst is not simply to predict energy yield, but to visualize the structure of losses and to serve as a basis for design decisions.

Considerations for Inverter Conversion and Output Limitation

The electricity generated by a solar photovoltaic module is direct current (DC). On the other hand, to connect to the grid or to use it in equipment, it is generally necessary to convert it to alternating current (AC). The device responsible for converting this DC to AC is the inverter. In PVSyst’s calculations, conversion losses from the DC power produced at the module side until it becomes AC power through the inverter are also taken into account.

Inverters have a conversion efficiency. They cannot convert all of the input DC power into AC output; some is lost as heat and other forms. Conversion efficiency is not constant—it varies with the magnitude of the input power. Efficiency can decrease at low loads and may be higher in regions near their rated capacity. In power generation forecasting, it is important to take conversion efficiency into account according to the input power at each time.

Also, an inverter has input voltage and input current ranges. If the number of modules connected in series or the circuit configuration is not appropriate, the inverter may fall outside its optimal operating range under certain temperature or solar irradiance conditions. Because voltage rises at low temperatures and falls at high temperatures, it is necessary to verify this during the design phase, taking seasonal variations into account.

Output limiting is also an important concept. When the DC capacity on the module side is designed to be larger than the AC capacity of the inverter, the inverter can reach its output limit during periods of strong solar irradiance, and some of the potential generation may be curtailed. This is called peak cut. Designing to allow a certain degree of peak cut may be considered when evaluating the overall generation profile and equipment utilization of the facility, but if it is excessively large it results in a loss of generation opportunities.

In PVSyst, the AC output is calculated from the DC-side generation while taking into account such conversion efficiencies and output limits. Because the AC energy exported to the grid is often what ultimately matters for project planning and operation, it is necessary to correctly understand the losses occurring after the inverter.

A common misunderstanding in practice is judging power generation solely by module capacity. However, in reality inverter capacity, circuit configuration, voltage range, conversion efficiency, and output limitations affect the results. It becomes easier to understand PVSyst if you think of it as software that treats the flow from modules to inverters to grid output as a series of energy conversions and reflects the losses at each stage.

Annual electricity generation is determined by the cumulative sum of hourly calculations.

PVSyst's energy production forecast does not directly calculate only the annual total. Basically, it calculates hourly solar irradiance, temperature, sun position, shading, equipment operation, and losses, and by aggregating these it obtains the monthly and annual energy production.

This concept is extremely important. Annual energy production cannot be expressed by a simple average alone. The sun's altitude and solar irradiance differ in the morning, at noon, and in the evening. The sun's movement and temperatures differ in spring, summer, autumn, and winter. The components of solar radiation differ under clear, cloudy, and rainy conditions. Energy production is determined by the accumulation of these time-dependent variations.

For example, even at locations with the same annual solar radiation, the amount of electricity generated can vary depending on which seasons receive more sunlight and which times of day receive more sunlight. This is because the orientation and tilt of the installation can lead to designs optimized for strong morning generation, strong midday generation, or designs that are relatively advantageous in winter.

Also, the impact of shading varies hour by hour. Even if shading losses are only a few percent on an annual basis, there can be large shadows during particular winter mornings. Conversely, even if the effect on annual energy production is small, it can cause large fluctuations in output during specific time periods. Checking hourly calculations makes it easier to understand these characteristics.

The result for annual energy production is often presented as a single number, but behind that there are many time-step calculations. Therefore, when reviewing the results it is important to check not only the annual value but also the monthly energy production, the breakdown of losses, the distribution of output, and the behavior under specific conditions.

Regarding the question of what PVSyst uses as the basis for its calculations, it is better to understand that it does not "process annual average conditions all at once," but rather "builds up the physical conditions and system conditions on an hourly basis." With this understanding, the way you interpret the simulation results changes. If the annual energy production is lower than expected, you can track which months are low, which losses are having an impact, and during which time periods restrictions are occurring.

Points to note when reviewing PVSyst results

PVSyst's calculations are based on many theories, but it is not appropriate to treat the results as absolute truth. Energy yield forecasts are, at best, estimates based on the input conditions and the model. This is because future weather cannot be predicted perfectly, and it is difficult to reproduce all of the site's fine local conditions.

The first thing to pay attention to is the representativeness of the weather data. If long-term average weather data are used, the results assume an average year. After actual operation begins, variations will occur: one year may have more solar radiation, another year less. When comparing a single year’s actual results with simulation values, it is necessary to confirm whether that year’s weather conditions were typical.

Next are the errors in the input conditions. Errors in azimuth, tilt angle, surrounding obstacles, terrain, number of modules, circuit configuration, wiring distance, and so on will affect the results. In particular, shading, wiring, and installation angle are items that are likely to produce errors when the drawings and the actual site do not match.

Also, attention is required when setting the loss rates. Soiling, degradation, mismatch, downtime, maintenance conditions, and other factors vary depending on the equipment and the environment. Even if you enter standard values, forecasts will be inaccurate if they do not reflect the actual local conditions. Conversely, entering overly conservative values can lead to underestimating the energy production.

When reviewing PVSyst results, it is important to look not only at the final energy production but also at the balance of the loss breakdown. If a particular loss is unusually large, it may indicate errors in the input conditions or design issues. For example, if shading losses are large, check the obstacle model and layout; if wiring losses are large, check cable specifications and circuit configuration. If temperature losses differ significantly from expectations, review the installation method and the assumptions of the temperature model.

Furthermore, accountability for the results is important. In practice, there are occasions when you must explain power generation forecasts to internal staff, customers, financial institutions, construction stakeholders, operations personnel, and others. At such times, simply saying "because the software calculated it" is insufficient. You are expected to be able to explain which meteorological data were used, which design conditions were entered, which losses were taken into account, and under what assumptions the results were produced.

Understanding local conditions is important for improving accuracy

To increase PVSyst's calculation accuracy, it is essential not only to configure the software but also to accurately understand the site conditions. Since energy yield forecasts are made by inputting the site's environmental conditions and equipment conditions into the model, insufficient site information will reduce the reliability of the results.

First and foremost, the site's location and topography are important. If the latitude and longitude of the target point are not correct, it will affect solar position calculations and consistency with meteorological data. Undulating terrain also affects module layout, tilt, shading, drainage, and maintenance access. In particular, on reclaimed or sloped land, elevation differences that are not apparent from plan views can affect power generation and construction planning.

Next is identifying surrounding obstructions. Accurately determining the positions and heights of buildings, trees, slopes, transmission towers, and equipment brings shadow assessment closer to reality. If obstructions are underestimated, shadow losses will be underestimated; if they are overestimated, estimated power generation may be too low.

Also, checking the installation orientation and tilt angle is important. For roof installations, the orientation and pitch of the roof surface, and for ground-mounted installations, the orientation and inclination of the mounting structure directly affect power generation. It is desirable to identify the discrepancy between planned values and actual conditions not only from the figures on drawings but also through on-site positioning and surveying and post-construction verification.

Furthermore, local ground conditions and maintenance practices also affect power generation. Ground reflectance, weed management, dust generation, snowfall, drainage conditions, and the susceptibility to soiling from the surrounding environment can all influence solar irradiance and soiling losses. Because these factors cannot be adequately represented by meteorological data alone, it is valuable to identify them through on-site surveys.

In other words, to achieve high-accuracy calculations in PVSyst, carefully entering the on-screen settings alone is not sufficient. It is important to accurately organize the location information, topography information, obstruction information, and installation conditions obtained on site, and to reflect those in the input conditions. The accuracy of the simulation is closely linked to the accuracy of the site assessment.

Summary

PVSyst is a simulation software that calculates the energy production of photovoltaic installations based on fundamental theories such as solar irradiance, sun position, installation angle, shading, temperature, electrical losses, and conversion efficiency. The calculation is not based on a single factor; rather, it is grounded in each stage from when solar energy reaches the module surface, is converted into direct current (DC) power, and is finally extracted as alternating current (AC) output.

The starting point for power generation forecasts is solar irradiance, but generation is not determined by irradiance alone. The sun's position and the orientation of the modules, the difference between direct and diffuse solar radiation, shading from surrounding obstructions, module temperature, wiring and mismatch, inverter conversion, output limitations, and so on combine to determine the final annual power generation.

Therefore, when reviewing PVSyst results, it is important to check not only the final energy production figures but also what assumptions were used for the calculations, which losses are significant, and whether the input conditions match the site. Simulations do not automatically guarantee correct results; only by entering reasonable conditions do they become useful as a basis for practical decision making.

Especially important for practitioners is understanding the on-site conditions. To improve the accuracy of power generation calculations, it is necessary to accurately document latitude and longitude, site geometry, elevation differences, surrounding obstructions, installation orientation, and tilt angles. Because drawings and desk-based assumptions can lead to oversights, on-site inspection and the accuracy of positioning underpin the overall reliability of the simulation.

In solar power design and on-site surveys, obtaining accurate location information is becoming increasingly important alongside power generation simulations. If you want to efficiently capture coordinates and survey points in the field and link them to verification of design conditions, shading checks, and post-construction position confirmation, using an iPhone-mounted high-precision GNSS positioning device such as LRTK makes handling on-site information easier. To produce well-founded power generation forecasts in PVSyst, it is important to incorporate not only the simulation settings but also the accurate location information obtained on site into the design and verification process.