Seven Approaches to Determining System Capacity in the PVSyst Manual

Table of Contents

• Things to organize before determining system capacity in PVSyst

• Consider equipment capacity not as the "amount that can be placed" but as the "amount required for it to be viable".

• Approach 1: Determine the maximum capacity from the area available for installation

• Approach 2: Work backwards from the annual power generation target

• Approach 3: Reflect solar radiation conditions and regional differences in capacity assessment

• Approach 4: Consider the balance with inverter capacity

• Approach 5: Consider the effective capacity taking into account loss conditions

• Approach 6: Align with self-consumption, power sales, and grid constraints

• Approach 7: Compare multiple options and select the optimal equipment capacity

• Key points to check on the PVSyst results screen when assessing capacity

• Common mistakes when determining equipment capacity

• Summary

Points to clarify before determining system capacity in PVSyst

In planning a solar power generation system, the first major decision point is determining what system capacity in kW to install. The system capacity is directly linked to the number of modules, installation area, power generation output, inverter selection, construction costs, revenue from selling electricity, self-consumption rate, and grid interconnection conditions. Therefore, if you decide solely based on "how many panels can fit on the roof" or "what is the maximum kW that can fit on the land," you are likely to encounter problems later such as actual power generation not matching expectations, suboptimal combinations with inverters, significant output curtailment at peak times, or being oversized relative to electricity demand.

PVSyst is a widely used software for designing photovoltaic (PV) power generation systems and simulating their electricity output. Reading the PVSyst manual, you can sequentially organize the elements for considering system capacity—project creation, meteorological data settings, array design, system settings, loss settings, and reviewing simulation results. However, merely following each screen in the manual as an operational procedure can make it difficult to see the rationale behind why a particular capacity was chosen.

To determine system capacity, it is important not only to input the values into PVSyst but to read them in the context of design and project conditions. For example, even for the same 100 kW system, a region with good solar irradiance and a region with significant snow or shading will differ in annual generation and in how losses occur. Also, even with the same module capacity, how you combine inverter capacity will affect clipping losses and operational efficiency.

In this article, referring to the PVSyst manual, I organize seven key concepts you should grasp when determining system capacity. I explain them as closely as possible to design decision-making, covering everything from the fundamentals beginners should understand first to how to compare capacity proposals in practical work.

Consider facility capacity not as "the amount that can be placed" but as "the amount that can be realized"

When considering installed capacity, the first thing many people think about is “how many kW can be installed?” If you place as many modules as possible on the roof area or on the land area, the apparent installed capacity will increase. However, in solar power planning, the maximum capacity is not necessarily the optimal capacity.

There are two types of equipment capacity: a physical upper limit and a capacity that is viable economically, electrically, and operationally. The physical upper limit is the capacity that can be accommodated based on installation area, separation distances, racking layout, orientation, tilt, shading conditions, and so on. By contrast, the viable capacity is the capacity that can be judged reasonable when taking into account power generation, losses, demand, sales conditions, grid capacity, inverter configuration, maintainability, and the balance with initial investment.

When performing simulations in PVSyst, it is important not to treat the initially set capacity proposal as absolute; instead, create multiple capacity proposals while reviewing annual energy production, specific yield, PR, loss breakdown, and compatibility with the inverter. When reading the PVSyst manual, don’t view each screen in isolation—understanding the installation conditions, system settings, loss settings, and result screens as connected makes it easier to judge the system capacity.

One important point to note is that increasing installed capacity does not necessarily lead to a proportional increase in power generation. If you expand modules into areas with significant shading, the generation efficiency of the added capacity can be lower. If the DC-side capacity is made excessively large relative to the inverter capacity, output curtailment at peak times may increase. For self-consumption systems, even if capacity greatly exceeds demand, economic performance can decline depending on how surplus power is handled.

In other words, installed capacity is not a simple metric of “the bigger, the better.” The purpose of using PVSyst is not merely to input a capacity and obtain the energy production, but to evaluate which capacity is the most reasonable for the overall plan.

Approach 1: Determine the maximum capacity from the available installation area

The first step in determining system capacity is to ascertain the realistic maximum capacity from the available installation area. For roof installations, check roof shape, orientation, slope, obstructions, ridges, parapets, inspection walkways, fire separation clearances, and load conditions. For ground-mounted installations, consider site boundaries, slopes, access paths, fences, shading from adjacent properties, ground conditions, extent of site development, rack pitch, and maintenance space.

In PVSyst, you assemble the plant’s installed capacity by setting module types and quantities, string configuration, azimuth, and tilt angles. The important point here is not to fill every available installation area with modules, but to distinguish areas that are favorable for power generation from those that are not. If you force the use of places where shadows are long, where tilt or azimuth differ greatly, or where inspections are difficult, installed capacity may increase while actual generation efficiency and maintainability decline.

On roof-mounted installations, even within the same building, the power generation characteristics differ between south-facing, east- and west-facing, and north-facing surfaces. When evaluating system capacity in PVSyst, it is effective to split the arrays by surface and configure each separately, then check each one's energy production and losses. If you lump everything together as a single capacity, it becomes difficult to see which surface is contributing to generation and which is increasing losses.

In ground-mounted installations, the row spacing of the racking has a major impact on system capacity. If you reduce row spacing, you can place more modules on the same land, but shading from the front rows may increase. Conversely, widening the row spacing reduces capacity but suppresses shading losses and can improve generation efficiency. PVSyst can reflect these differences in layout conditions in its simulations, so you can judge capacity not only by simple area calculations but by looking at actual generation performance.

When considering the upper limit of a facility’s capacity, you need to take into account not only the modules’ nominal capacity but also the number of modules that can be placed, the conditions of the mounting surface, and the electrical connection units. For example, even if you can add an extra module to fill out the layout, it may create an awkward string configuration or fail to meet inverter input requirements. Therefore, even at the stage of deriving capacity from the installation area, it is important to keep the subsequent system design in mind.

Approach 2: Work backwards from the annual electricity generation target

Another basic approach to determining system capacity is to work backwards from a target annual energy production. For commercial solar, the annual amount of electricity sold and revenue planning are important. For self-consumption systems, not only the annual energy production but also how well it aligns with time-of-day electricity demand is important. When using PVSyst, you can compare the annual energy production for each system capacity to determine how much capacity is required to meet the target.

For example, if a facility has high annual electricity consumption and stable daytime demand, increasing the installed capacity may make self-consumption easier. On the other hand, at facilities where demand falls sharply on holidays or seasonally, making the capacity too large will increase surplus electricity. Whether surplus electricity can be sold, what the selling price will be, and whether there are output controls or grid constraints will affect the optimal capacity.

In PVSyst simulation results, you can check not only the annual energy production but also the monthly production and loss trends. When deciding system capacity, it is important not to look only at the annual total but to check which months have high production and which months have low production. In snowy regions, winter production can drop significantly. In high-temperature regions, even if there is a lot of solar radiation in summer, temperature losses can prevent production from increasing as much as expected.

When working backwards from annual generation, the concept of specific yield can also be useful. Specific yield refers to the annual generation per 1 kW of installed capacity. If you look only at simple total generation, options with larger capacity may appear more advantageous, but by looking at specific yield you can check whether efficiency declines as capacity is increased. If the generation efficiency of the added capacity is low, total generation may increase while investment efficiency could decrease.

When reading the PVSyst manual, it’s important not to view the energy production shown on the results screen as the “answer,” but to treat it as material for evaluating capacity proposals. By comparing how energy production and losses change when you slightly alter the system capacity, you can more easily find a capacity that has minimal excess or shortfall relative to the target energy production.

Approach 3: Reflect solar radiation conditions and regional differences in capacity assessment

Even with the same installed capacity, the annual energy yield can vary greatly depending on the solar irradiation conditions of the installation area. Therefore, when determining system capacity, it is necessary to consider the area's solar irradiance, temperature, snowfall, wind, surrounding terrain, and shading conditions. In PVSyst, you set the meteorological data and simulate the energy yield based on the local conditions.

In regions with good solar irradiation, it may be possible to achieve the target electricity generation with a relatively small system capacity. Conversely, in areas with frequent cloudy conditions or where snow has a significant impact, it may be necessary to increase capacity to obtain the same amount of generation. However, increasing capacity does not necessarily solve the problem. If downtime due to snow or the effects of shading are substantial, the added capacity will suffer the same losses, limiting the increase in generation.

When setting meteorological data in PVSyst, which data you use is also important. Depending on the choice of meteorological data, the simulation results may differ. If you are using it to determine system capacity, it is desirable to select data that is close to the target site and reflects long-term trends. Although selecting the data is a simple operation in the manual, in practice you need to verify that the data appropriately represents the conditions at the project site.

When looking at regional differences, you must also consider temperature conditions. Solar modules generate more power when irradiance is high, but their output declines as module temperature increases. Therefore, in hot regions, even if irradiance is abundant, it is necessary to consider the effective energy yield that accounts for temperature losses. Because PVSyst reflects temperature losses in its results, when comparing the energy yields for different capacity options you can assess not only irradiance but also the effects of temperature.

Also, in mountainous and urban areas, shadows cast by terrain and nearby buildings affect capacity determination. Adding modules in areas subject to shading increases the system capacity, but the gain in energy production is small. In particular, in locations where shadows occur at low solar elevation angles in the morning and evening, losses can accumulate over the year. When performing shading analysis in PVSyst, it is important to check how shading losses change for each capacity option and to avoid layouts with low generation efficiency.

Consideration 4: Balance with inverter capacity

When determining equipment capacity, it is essential to verify the balance not only of the module-side capacity but also with the inverter capacity. In solar power systems, the capacity of the solar modules on the DC side and the inverter capacity on the AC side are not necessarily exactly the same. In practice, designs sometimes make the DC capacity slightly larger than the inverter capacity, but if that ratio is not appropriate, it will affect losses and efficiency.

In PVSyst, you can configure combinations of modules, strings, and inverters and check voltage ranges, input currents, MPPT configurations, the effects of oversizing, and more. When determining system capacity, you should first calculate the DC capacity from the number of modules, then combine that with the inverter capacity to verify whether the system is feasible.

Increasing DC capacity can make it easier to operate the inverter efficiently during periods or seasons with low solar irradiance. On the other hand, during times of strong irradiance the output can exceed the inverter's rated capacity, causing clipping losses. If clipping losses are small, the design can still be reasonable overall, but if they are excessively large, the added module capacity may not be being fully utilized.

When evaluating the balance with inverter capacity, it is important not only to check the DC/AC ratio but also to review the loss results from PVSyst. By comparing how much inverter losses and clipping losses increase when you change the proposed capacity, you can more easily determine whether increasing module capacity is worthwhile.

Also, when roof surfaces are oriented in multiple directions or when generation peaks are dispersed—such as with east–west installations—the clipping behavior can differ even with the same DC/AC ratio. Systems concentrated on a single south-facing plane tend to produce output peaks around midday and are more likely to be limited by the inverter. Conversely, when generation is distributed east–west, the generation period widens and peaks can be smoothed. PVSyst can evaluate these differences in layout conditions, which helps when making capacity decisions.

When deciding system capacity, consider the number of inverters and the circuit configuration as well. Increasing capacity even slightly can raise the number of inverters, which may increase construction costs, required installation space, and maintenance demands. Conversely, a small adjustment to capacity can allow the design to fit an existing inverter configuration and simplify the overall system. Therefore, system capacity should be determined not only by the number of modules but together with the inverter configuration.

Approach 5: Consider effective capacity by accounting for loss conditions

An important factor in determining equipment capacity is the effective power-generation performance that accounts for expected losses. The nominal capacity of a solar module indicates its output under standard test conditions. However, in actual field conditions, power generation is reduced by various factors such as temperature losses, wiring losses, mismatch losses, soiling, shading, inverter losses, transformer losses, snow, and degradation over time.

The PVSyst manual places significant emphasis on loss settings and how to interpret the results screen. When determining system capacity, you need to check not only the kW value entered but also how much energy production can be expected after losses. In particular, when comparing capacity options, it is important to examine not only the total generation but also the breakdown of losses.

For example, if a proposal to increase capacity leads to a large increase in shading losses, the added portion may not be generating efficiently. Increasing wiring distances can also increase wiring losses. Densely arranging modules in a high-temperature environment can worsen ventilation conditions and result in greater temperature-related losses. If you increase capacity without accounting for these losses, an installation that appears large may not see its actual power output grow as much as expected.

In the PVSyst results, checking the loss diagram lets you understand how the incoming solar energy is converted through various processes into the final electricity output. When evaluating system capacity, it is useful to compare this loss diagram for each capacity option. If one option shows large temperature losses while another shows large shading losses, you can determine which design conditions should be reviewed.

Also, when accounting for losses, it is important to avoid overly optimistic settings. Underestimating dirt and degradation, snowfall, maintenance frequency, and similar factors may produce favorable simulation results but create a large gap with actual operation. For PVSyst simulations used to determine system capacity, it is desirable to use loss settings that reflect site conditions and, where necessary, consider conservative scenarios.

When assessing equipment capacity, what's more important than the nominal capacity in kW is how much value it will generate after losses are deducted. By using PVSyst, you can quantify this concept of effective capacity.

Approach 6: Align with self-consumption, electricity sales, and grid constraints

Installed capacity is not something that can be determined by the power generation equipment alone. The appropriate capacity varies greatly depending on how the generated electricity will be used, how it will be sold, and how much can be fed into the grid. In particular, in recent years the conditions affecting capacity decisions have diversified, including self-consumption solar, surplus power sales, battery storage integration, PPA, and output control.

In self-consumption systems, it is important to size the installed capacity to match demand. At factories and commercial facilities with high daytime electricity use, installing a relatively large capacity may still allow the generated power to be fully consumed. Conversely, at facilities with low operation on holidays or with large daytime demand fluctuations, making the capacity too large increases surplus power. If conditions for selling surplus power are unfavorable or reverse power flow is restricted, it is necessary to decide to limit capacity appropriately relative to demand.

In PVSyst, you can examine generation patterns for each installed capacity through generation simulations. When considering self-consumption, it is important to evaluate PVSyst’s results together with demand data. Even a proposal with a large annual energy production can have a low self-consumption rate if generation is concentrated during periods of low demand. Conversely, a proposal that slightly reduces installed capacity may be able to use generated power more efficiently, without waste.

In feed-in projects, check the relationship between installed capacity and revenue from electricity sales. Increasing capacity tends to increase annual generation, but construction costs, land use, equipment costs, and maintenance costs also rise. Also, depending on the grid connection’s available capacity and any output control conditions, installing a large capacity does not necessarily mean all generated electricity can be sold. PVSyst’s generation results form the basis for revenue calculations, but the final decision must consider the feed-in tariff, control risks, and equipment costs together.

When there are grid constraints, equipment capacity must be determined particularly carefully. If there are limits such as the capacity of the incoming power equipment or the interconnection point, whether reverse power flow is permitted, or contractual restrictions, it may be possible to install generation equipment but necessary to curtail output during operation. When considering equipment capacity in PVSyst, it is important to separately verify the electrical design and grid conditions and to reconcile the ideal capacity from the simulation with the capacity that can actually be interconnected.

When combining battery storage with the system, the way you think about equipment capacity also changes. If surplus power during the daytime can be charged into batteries and used after the evening, there can be a rationale for specifying a larger solar capacity. However, the optimal solar capacity varies depending on battery capacity, charge/discharge efficiency, and operational objectives. It is important to make decisions based on the overall energy flow including the battery, while checking the solar-side generation in PVSyst.

Approach 7: Compare multiple options and choose the optimal equipment capacity

When determining the installed capacity, the thing you most want to avoid is making a decision based only on the first capacity option that comes to mind. A major advantage of using PVSyst is that you can create multiple scenarios and compare them under the same conditions. By comparing options that change the installed capacity, change the module types, alter inverter configurations, or adjust the tilt angle and azimuth, the optimal capacity becomes easier to identify.

When comparing options, you should not simply choose the one with the highest annual generation; you need to consider multiple metrics together. Check annual generation, specific yield, PR, breakdown of losses, clipping losses, shading losses, monthly generation, inverter operating conditions, etc., and summarize the benefits and drawbacks of increasing capacity.

For example, suppose you compare an 80 kW option, a 100 kW option, and a 120 kW option. The 120 kW option may have the highest total power generation, but if its specific generation is low and shading losses and clipping losses are large, the investment efficiency of the additional 20 kW may be low. On the other hand, even if the 100 kW option is inferior to the 120 kW option in total power generation, it may have fewer losses and be better balanced in terms of inverter configuration and maintainability.

When comparing multiple options in PVSyst, it is important to clearly separate the parts of the model you change from the parts you keep fixed. If you change meteorological data, loss settings, installation orientation, tilt, and so on at the same time, it becomes difficult to know whether differences in energy production are due to capacity or to other conditions. When comparing capacities, first align all conditions except capacity as much as possible and confirm the results that arise from capacity differences. After that, adjust orientation, inverter configuration, and other parameters as needed to make the decision process easier to organize.

Also, when selecting equipment capacity, the technically most efficient option and the option that is easiest to adopt from a business standpoint do not always coincide. For example, even if an option with a slightly reduced capacity is advantageous purely in terms of generation efficiency, a different capacity may be chosen because of subsidy requirements, contracted capacity, equipment classification, or internal budget constraints. The role of PVSyst is to provide the technical basis that underpins such decisions.

The final system capacity is determined by a comprehensive consideration of the PVSyst simulation results, site conditions, electrical design, investment decisions, and operational policy. While configuring settings in accordance with the PVSyst manual, comparing multiple options and being able to explain "why this capacity was chosen" is an important practical goal.

Points to Check on the PVSyst Results Screen for Capacity Assessment

After running a simulation in PVSyst, carefully review the results screen. To determine the system capacity, you need to consider not only the annual energy production but also multiple indicators comprehensively.

First, what you should check is the annual generation. This is the basic comparative metric for each capacity option. However, since it is natural that options with larger capacity will have higher annual generation, you should not judge superiority based on that alone. Next, by looking at the generation per 1 kW of installed capacity, you can assess the efficiency when increasing capacity. If the generation per 1 kW falls significantly despite the increase in capacity, you should suspect effects such as shading, losses, or inverter limitations.

PR is also an important indicator. PR is a measure of how efficiently the system is generating power relative to solar irradiation conditions. If PR decreases in a proposal that increases installed capacity, the additional capacity may be lowering efficiency. However, because PR also changes with installation conditions and weather, comparing capacity proposals should be done under the same conditions.

Always check the breakdown of losses. By looking at which of temperature loss, wiring loss, mismatch loss, shading loss, inverter loss, clipping loss, etc., is large, you can identify weaknesses in the capacity proposal. For example, if clipping loss is large, there may be room to reconsider the balance between DC capacity and inverter capacity. If shading loss is large, you should consider reducing the number of modules placed in shaded areas, changing the layout, or splitting the arrays.

Don't overlook monthly power generation. Even proposals that appear to have similar annual generation can differ in how generation is distributed across seasons. For self-consumption systems, it's important whether generation is greater during periods of high demand. For systems that sell electricity, you also need to consider periods when output curtailment is likely and periods when snow will affect generation.

The PVSyst results screen is not merely a report output but a supporting document for explaining the installed capacity. When using it for internal review, explanations to the owner, materials for financial institutions, design reviews, and so on, ensuring you can numerically demonstrate which capacity option was chosen increases the plan’s credibility.

Common Mistakes When Determining Equipment Capacity

A common mistake when determining system capacity is simply adopting the maximum installable capacity. If you place as many modules as will fit on the site or roof, the apparent capacity will increase. However, if you use areas subject to heavy shading or with poor maintainability, power generation efficiency will decline and problems can arise during long-term operation. By running simulations in PVSyst, you can verify whether such increases in capacity truly produce the expected benefits.

Another common mistake is underestimating the balance with the inverter. If you decide the module capacity first and try to match the inverter afterward, the voltage range, input circuit, and DC/AC ratio can become unnatural. PVSyst allows you to verify the compatibility of the module and inverter at the system settings stage, so it is important to carry out capacity sizing and inverter selection simultaneously.

Being overly optimistic in loss assumptions can also lead to failure. If you do not sufficiently account for soiling, temperature, wiring, shading, snow, degradation, and so on, simulation results will appear better than they actually are. In simulations used to determine equipment capacity, it is important to obtain results that reflect site conditions rather than fabricate convenient numbers.

In self-consumption systems, a mismatch with demand is also a common problem. If capacity is determined based only on annual generation, surpluses can increase on days with low daytime demand and on holidays. If you prioritize the self-consumption rate, you need to check time-of-day generation trends together with the demand curve. It is important to consider not only the generation results from PVSyst alone but also to combine them with the facility’s actual electricity usage records.

Also, you should avoid making judgments while the comparison conditions for capacity options are not aligned. If one option uses new weather data and another uses a different loss setting, you cannot correctly interpret the differences in results. When conducting capacity comparisons, you need to make clear which conditions were changed and align the assumptions used for the comparison.

Failure to get equipment capacity right may look like a small difference at the initial design stage, but it can have a large impact over the entire operational period. While understanding the basic operations according to the PVSyst manual, it is important to adopt an approach that uses the results to inform design decisions.

Summary

When determining installed capacity in the PVSyst manual, you should not simply enter kW on the screen; instead, you need to take a comprehensive view of installation conditions, generation targets, solar irradiation conditions, inverter configuration, losses, demand, grid constraints, and project viability. Installed capacity is not something to "install as much as possible," but rather to choose "the capacity that is most reasonably feasible for that site, that use, and those conditions."

First, determine the maximum capacity from the available installation area, then back-calculate the required capacity from the annual generation and self-consumption targets. Then reflect local solar irradiance and temperature conditions and the effects of shading, and check the balance with inverter capacity. Furthermore, it is important to allow for temperature losses, wiring losses, shading losses, clipping losses, etc., and to evaluate the effective power generation performance.

In self-consumption systems, you need to match generation with demand; in feed-in systems, you must consider feed-in conditions and grid constraints; and in systems integrated with batteries, you also need to consider operational strategies for charging and discharging. Using PVSyst, you can compare multiple capacity options based on these conditions and quantify the differences in generation and losses.

When deciding on equipment capacity, what matters is not choosing the maximum capacity but choosing a capacity you can justify. If you can organize why you selected that capacity, how it is superior to other options, which losses you are willing to accept, and which conditions you prioritized, the persuasiveness of the design will be greatly enhanced.

When using the PVSyst manual, it is important not only to learn the operating procedures but also to be aware of how each setting affects decisions about system capacity. System capacity is the foundation of a solar PV plan and will have a long-lasting impact on subsequent design, financial performance, and operation. Carefully comparing multiple scenarios in PVSyst and choosing a capacity that matches site conditions and business objectives is the first step to a successful solar PV plan.