【How to Set the DC/AC Ratio in PVSyst｜6 Key Points to Avoid Mistakes】

Table of Contents

• First, understand what the DC/AC ratio is

• Input conditions to clarify before reviewing the DC/AC ratio in PVSyst

• Items to check when determining DC-side capacity

• Items to check when determining AC-side capacity

• How to interpret peak clipping when the DC/AC ratio is high

• Verify the appropriateness of the DC/AC ratio using simulation results

• Six practical items to organize to avoid mistakes

• Improve the accuracy of DC/AC ratio assessment by reflecting site conditions

• Summary

First, understand what the DC/AC ratio is

The DC/AC ratio is the ratio of the DC capacity, the total capacity of the photovoltaic modules, to the AC capacity, the output capacity on the AC side. For example, if the DC-side solar array capacity is 1,200 kW and the AC-side output capacity is 1,000 kW, the DC/AC ratio is 1.20. In Japanese practice, it is also referred to as the overloading rate.

The reason this ratio is important is that a solar module’s rated capacity does not always translate directly into its power output. Module capacity is a value evaluated under standard test conditions, and in real-world installations it is affected by irradiance, temperature, orientation, tilt angle, shading, wiring losses, equipment conversion losses, and so on. Therefore, even if DC capacity and AC capacity are made equal, the AC side will not always operate at maximum output.

On the other hand, if the DC capacity is increased too much, the AC side can reach its limit during periods of strong sunlight and the output can be clipped. This is called peak cut. Even if a small amount of peak cut occurs, increased generation in the mornings, evenings, and on cloudy days can be advantageous for annual energy yield. However, if peak cut becomes excessive, the energy obtained relative to the added module capacity becomes small, increasing design waste.

When dealing with the DC/AC ratio in PVSyst, it is important to understand this concept. The DC/AC ratio is not a figure that can be judged as good or bad on its own. It must be evaluated together with the power plant's location, meteorological data, module characteristics, equipment capacity, azimuth, tilt angle, shading conditions, grid interconnection conditions, and operational policy.

A common stumbling block for beginners is to simplify the DC/AC ratio as “the higher the better” or “the lower the safer.” In reality, if it is too high or too low, either extreme will produce results that do not match the design intent. When the DC/AC ratio is low, the DC side is insufficient relative to the AC-side capacity, and the time during which equipment cannot be fully utilized may increase. Conversely, when it is high, the time spent at the AC-side upper limit increases, and peak-cut losses may become larger.

PVSyst is a simulation tool for checking such balances numerically. Rather than finishing by directly entering the DC/AC ratio, it is easier to understand if you think of the DC/AC ratio as being determined as the result of configuring the number of modules, string arrangement, number of connections, equipment capacities, and operating conditions, and that its effects are reflected in the loss diagram and the energy production results.

Input conditions to organize before checking the DC/AC ratio in PVSyst

Before checking the DC/AC ratio in PVSyst, you first need to clarify the design conditions. The DC/AC ratio is the ratio between the module capacity and the AC-side capacity, so if either of those conditions remains vague, the meaning of the simulation results will also be ambiguous.

The first thing to check is the rated capacity of the photovoltaic modules. In PVSyst, you select or register the electrical characteristics of the modules you will use, and the total DC-side capacity is determined by how many of those modules you use. What you should pay attention to here is not simply matching the total kW, but simultaneously verifying that the number of modules in series per string and the number of parallel strings fall within the voltage and current ranges.

Next, check the output capacity of the AC-side equipment. The AC-side capacity is determined by the equipment’s rated output, the number of units, grid interconnection limits, the approach taken in equipment certification, and other factors. In PVSyst, the AC-side capacity is reflected by the selection of equipment and the configured number of units. Even if you increase only the DC-side here, because there is an upper limit on AC-side output, you cannot produce more than a certain amount of generation.

Also, site configuration and meteorological data are important. In regions with high solar irradiance, the time spent reaching the AC-side limit may increase. Conversely, in areas with poor insolation conditions or frequent cloudiness, slightly increasing DC capacity may result in only limited peak clipping and could raise annual energy production. In other words, the same DC/AC ratio can produce different outcomes depending on the region.

Azimuth and tilt angle also affect the evaluation of the DC/AC ratio. When a system faces south and is close to an appropriate tilt, the daytime peak tends to be more distinct, and times when the AC-side limit is reached are more likely to occur. Conversely, for east–west orientations or arrays with multiple azimuths, generation peaks are dispersed, and even with the same DC capacity the concentration on the AC side can be more gradual. Therefore, it is necessary to consider not only the DC/AC ratio but also the shape of the generation curve.

Additionally, verify the shading conditions. If power generation is reduced by surrounding buildings, trees, terrain, shading between mounting structures, or obstacles within the facility, increasing the DC-side capacity may not yield the expected generation. In PVSyst, shading losses can be evaluated using Near Shading and 3D scene settings, but judging the DC/AC ratio without configuring shading conditions can produce results that are more optimistic than reality.

Thus, before examining the DC/AC ratio, it is necessary to organize the module conditions, string configuration, AC-side equipment capacity, location, meteorological data, azimuth, tilt, and shading conditions. Only after entering these will the DC/AC ratio in PVSyst have practical meaning.

Items to check when determining DC-side capacitance

The DC-side capacity is determined by the rated capacity of the solar PV modules and the number of modules. In PVSyst, by selecting the modules to be used and setting the number of modules in series and the number in parallel, the total DC capacity of the array is calculated. If you want to adjust the DC/AC ratio, in many cases you will do so by changing the module count or the number of strings.

When deciding the DC-side capacity, the first thing to check is not the simple total capacity but the voltage range. The voltage of a solar PV module changes with temperature. At low temperatures the open-circuit voltage increases, and at high temperatures the operating voltage decreases. Therefore, if the number of modules in series per string is too large, the equipment's maximum allowable voltage may be exceeded at low temperatures. Conversely, if the number of modules in series is too small, the required operating voltage may not be met at high temperatures, and efficient operation may not be possible.

In PVSyst you can check the voltage and current compatibility for combinations of modules and equipment. If warnings appear, you should review the basic electrical design before addressing the DC/AC ratio. Even if the DC/AC ratio is as intended, if the voltage range is inappropriate it cannot be regarded as a valid practical design.

Next to check is the relationship between the number of parallel strings and the input circuits. AC-side equipment has a specified number of DC input circuits and allowable current. If you increase modules to raise DC capacity, the number of parallel strings will increase and the input current will also rise. Because configurations that exceed the allowable current cannot be used, even if you want to raise the DC/AC ratio you must adjust it so it remains within the equipment’s input specifications.

Also, increasing module capacity will increase generation, but not all of it will be used effectively. The portion that exceeds the AC-side limit during periods of strong solar irradiance is clipped. Therefore, when increasing DC-side capacity it is important to check the increase in annual generation together with the increase in peak-clipping losses.

Beginners tend to judge solely by the annual energy production in the simulation results when increasing DC capacity. However, in practice you also examine loss diagrams and monthly results to check in which seasons and to what extent losses occur. For example, whether peak clipping is increasing on sunny days in spring and autumn, whether output is being suppressed by high temperatures in summer, or whether effects appear under winter insolation conditions will change the design decision.

When setting DC-side capacity, the available installation area cannot be ignored. Depending on local site conditions, roof area, clearances, maintenance aisles, shadow avoidance, and fire safety and inspection requirements, there is an upper limit to the number of modules. Even if PVSyst shows an ideal DC/AC ratio, it cannot be used in the detailed design if the modules cannot be placed on site. Therefore, DC-side capacity should not be decided solely from the simulation screen but must be coordinated with the layout plan.

Items to check when determining AC-side capacity

AC-side capacity is the capacity on the output side that converts the generated DC power into AC for output. When considering the DC/AC ratio, it is an important value that serves as the denominator. In PVSyst, the AC-side capacity is determined by setting the type of AC-side equipment, the number of units, output capacity, input conditions, and so on.

The first thing to check when deciding the AC-side capacity is the grid interconnection conditions. Power plants and facilities may have limits on the output capacity that can be connected or contractual restrictions. Because the AC-side output capacity cannot always be freely increased, the AC-side upper limit is often determined first, and the DC-side capacity is adjusted within that range.

Next, check the equipment’s rated output and the number of units. The AC-side capacity is generally treated as the value obtained by multiplying the rated output of a single unit by the number of units. However, in actual operation the output may vary due to temperature conditions, protective controls, input voltage, conversion efficiency, and other factors. In PVSyst, loss calculations are performed based on the characteristics of the selected equipment, so you need to verify not only the simple rated values but also the effects of conversion losses and output limitations.

When the AC-side capacity is small, the DC/AC ratio is higher. This means that, for the same module capacity, the available AC-side capacity is small. While peak clipping increases, the utilization rate of AC-side equipment tends to rise, and the equipment may be used relatively often in the morning, evening, or during low irradiance. Conversely, when the AC-side capacity is large, peak clipping decreases, but there may be more periods when generation output is low relative to equipment capacity.

What’s important here is that increasing the AC-side capacity does not necessarily produce a better design. If the AC side is enlarged solely to reduce peak clipping to near zero, equipment may end up being used inefficiently. Conversely, if the AC side is made too small, significant output limitations will be imposed during the periods when generation is possible, reducing the benefit of adding modules.

In PVSyst, it is practical to create multiple cases with different AC-side capacities and compare annual energy production, losses, peak clipping, and generation curves. Even with the same DC capacity, changing only the AC capacity alters the DC/AC ratio and thus the results. Especially in the early design stage, it is important not to fix on a single value but to compare several options to understand the trends.

Also, the AC-side capacity affects downstream processes. It must be coordinated with substation and transformer equipment, protection devices, cables, metering, grid-connection coordination, monitoring systems, and so on. Even if simulations in PVSyst show no issues, it cannot be adopted if it does not match peripheral equipment or application/permitting conditions. Therefore, the AC-side capacity needs to be checked not only against power generation but also as a design requirement for the entire facility.

How to Interpret Peak Clipping That Occurs When the DC/AC Ratio Is High

When the DC/AC ratio is set high, during periods of good solar irradiance the generation capacity on the DC side can exceed the AC side’s output limit. In that case, the portion of power that cannot be delivered to the AC side is clipped. This is called peak cut. When evaluating the DC/AC ratio in PVSyst, an important decision point is how much of this peak cut you are willing to accept.

Peak clipping is not necessarily a bad thing. In solar power generation, the periods when output is close to rated capacity are limited to a portion of the year. Therefore, even if the AC-side capacity is set slightly smaller, the impact on annual generation can be limited when viewed over the course of a year. Rather, increasing the DC-side capacity can boost output in the mornings, evenings, and on cloudy days, which can increase annual generation.

However, if peak clipping becomes too large, the amount of time during which the added module capacity is not used effectively increases. In PVSyst, you check losses related to output limiting on the loss diagram and the results screen after the simulation. There, confirm how much loss equivalent to peak clipping is occurring and determine whether the DC/AC ratio is excessive.

What beginners should be careful about is that if they only look at annual energy production, they can easily overlook peak clipping issues. Increasing DC capacity may increase annual energy production. However, whether that increase is efficient is another matter. For example, if you greatly increase DC capacity but see only a small rise in annual energy production, peak clipping or other losses may have increased.

In PVSyst, using case comparison to check results while gradually changing the DC/AC ratio makes it easier to see how peak clipping increases. For example, comparing DC/AC ratios of 1.10, 1.20, and 1.30 can show that, beyond a certain point, annual energy yield gains become marginal while losses increase noticeably. Identifying this inflection point helps inform practical decision-making.

Also, peak cut occurs differently depending on the season and time of day. In spring and autumn, temperatures are relatively low and, under conditions of good solar irradiance, output tends to increase, so peak cut may be more likely to occur. In summer, even if solar irradiance is strong, module temperatures become high and DC output decreases, so it does not necessarily result in the largest peak cut. Checking these seasonal differences also allows a more accurate understanding of the meaning of the DC/AC ratio.

When evaluating peak clipping, you should not simply think “it’s bad because there are losses”; instead, examine the balance between the energy produced by the added DC capacity and the energy that is clipped. The results from PVSyst provide data to numerically compare this balance. The designer must determine an acceptable DC/AC ratio by considering generation, losses, equipment conditions, and operating conditions together.

Verify the validity of the DC/AC ratio using simulation results

After setting the DC/AC ratio in PVSyst, always check the simulation results. Even if the ratio shown on the settings screen is close to the target, the results may show excessive losses or only a small increase in energy production. It is important to see how the DC/AC ratio is reflected in the simulation results, rather than relying solely on the value entered.

First, what I want to check is the annual energy production. Annual energy production is a basic metric for assessing the overall performance of the design proposal. However, it is insufficient to judge the appropriateness of the DC/AC ratio based solely on annual energy production. Increasing DC capacity can raise annual energy production, but you must verify whether that increase is achieved efficiently.

Next, review the loss diagram. The loss diagram displays, step by step, the factors that reduce power generation—such as solar irradiance, temperature, wiring, mismatch, equipment conversion, and output limiting. In designs with a high DC/AC ratio, losses related to output limiting can become conspicuous. By checking this, you can determine whether the DC side is oversized relative to the AC-side capacity.

Monthly generation is also important. Annual values alone do not show which season concentrates peak cuts or losses. By looking at monthly results, you can determine whether output limitations are more frequent in spring or autumn, whether temperature-related losses in summer are large, or whether winter’s lack of solar radiation is dominant. When adjusting the DC/AC ratio, looking at both annual and monthly values enables a more practical decision.

Additionally, check the performance ratio and specific yield. The performance ratio is an indicator that shows how efficiently the system is generating power relative to irradiance conditions. If the DC/AC ratio is set too high, output clipping can affect the performance ratio. However, it is important not to judge based solely on the performance ratio; you should consider it together with the energy yield and loss components.

Case comparisons are also effective. In PVSyst, you can create multiple scenarios with slightly different design conditions and compare the results. When changing the DC/AC ratio, prepare multiple cases that alter the number of modules or the AC-side capacity, and compare the annual energy production, losses, peak clipping, and monthly results. This makes it easier to determine which ratio has the best balance.

Note that PVSyst results depend on the input conditions. If the meteorological data, shading conditions, equipment characteristics, or layout conditions deviate from reality, evaluations such as the DC/AC ratio will also be off. For example, if there is significant shading on site but it is left unset in the simulation, peak clipping and energy yield may not match the actual situation. To have confidence in the results, the accuracy of the input conditions needs to be improved.

Organizing 6 items from a practical perspective to avoid failure

To avoid failures when setting the DC/AC ratio in PVSyst, it is effective to check six items in order. Here, we summarize the points that practitioners tend to overlook during design.

The first point is to align the definitions of DC capacity and AC capacity. DC capacity is treated as the total rated capacity of the PV modules, and AC capacity is treated as the output capacity on the AC side; however, internal documents and application materials may express them according to different criteria. If you do not confirm that the values in PVSyst match the values in the design documents, discrepancies in the understanding of the DC/AC ratio will occur.

The second is to check the suitability of the string configuration. Even if you adjust the number of modules to bring the DC/AC ratio closer to the target value, it cannot be used if the voltage or current fall outside the equipment’s allowable range. You need to check the open-circuit voltage at low temperatures, the operating voltage at high temperatures, the input current, and the number of circuits, and ensure the configuration is electrically feasible.

The third point is to always check the peak clipping loss. Designs with a high DC/AC ratio are more likely to experience peak clipping. Check the losses due to output limiting in PVSyst’s loss diagram and results screen to determine whether they fall within an acceptable range. Even if the annual energy yield increases, excessive peak clipping means the design may not be optimal.

The fourth point is to check seasonal differences using the monthly results. Annual values alone do not reveal in which seasons losses are occurring. By seeing whether peak shaving is concentrated in spring and autumn, whether temperature-related losses in summer are large, or whether generation does not increase in winter, you can more accurately assess the impact of the DC/AC ratio.

The fifth point is to account for shading and layout conditions. If you evaluate the DC/AC ratio while ignoring on-site shading, estimates of energy generation and peak shaving can deviate from reality. It is important to incorporate, as far as possible, inter-rack shading, nearby obstacles, terrain, buildings, trees, and so on, to bring conditions closer to actual generation conditions.

The sixth point is to compare multiple cases. The DC/AC ratio is not something to decide in a single step. Create several candidate ratios and compare generation, losses, peak clipping, performance ratio, and monthly generation to more easily find a value that matches the design intent. In particular, during the initial study it is important not to fix on a single option but to line up multiple options to grasp the trends.

By covering these six items, you can greatly reduce failures when handling the DC/AC ratio in PVSyst. The important point is not to make the ratio itself the objective. The DC/AC ratio is an indicator for checking the balance between energy generation and losses, system capacity, and site conditions. You need to take the stance of assessing whether that ratio is reasonable for the design objectives.

Improve the accuracy of DC/AC ratio assessments by incorporating on-site conditions

When evaluating the DC/AC ratio in PVSyst, judging solely from desk-top settings can lead to discrepancies with actual field conditions. In particular, terrain, shading, installation azimuth, tilt, nearby obstacles, and constraints on module layout directly affect the simulation results. Accurately assessing the DC/AC ratio requires understanding the on-site conditions as precisely as possible.

For example, even with the same DC/AC ratio, results can differ between a flat, unobstructed site and a site surrounded by buildings or trees. At sites with significant shading, increasing DC-side capacity may not produce the expected increase in energy output. Also, when the terrain is uneven, the modules’ tilt and azimuth can vary slightly by location, which affects the generation curve.

Entering conditions into PVSyst without sufficient on-site surveying and layout verification will make judgment of the DC/AC ratio unstable. Even if the design screen looks ideal, in practice you may find that modules cannot be placed as planned, shading is larger than expected, or the number of modules is reduced once maintenance aisles are provided. Therefore, consideration of the DC/AC ratio should be conducted together with a thorough understanding of the site conditions.

In practice, survey data for candidate sites, on-site photographs, obstacle locations, differences in ground elevation, and positional information of existing structures are particularly important. If this information is accurate, the assumptions for shading analysis and layout planning in PVSyst will be stable, and comparisons of DC/AC ratios will be more convincing. Conversely, if the site information is vague, even comparing detailed simulation figures will produce results that are difficult to use in detailed design.

A positioning environment that enables easy acquisition of high-precision location information on site is useful here. LRTK is a GNSS high-precision positioning device that can be attached to an iPhone, making it easy to apply on-site position data to design and verification work. In candidate site surveys for solar power plants, recording site boundaries, obstacles, planned racking positions, inspection paths, and terrain change points helps improve the accuracy of the assumptions entered into PVSyst.

The DC/AC ratio is not determined solely by the on-screen ratio. Only by understanding how many modules can actually be installed on site, how much they will be affected by shading, and what the power generation curve will look like according to the actual terrain can you make an appropriate decision. By combining simulations with PVSyst and on-site positioning such as LRTK, you can reduce the discrepancy between desktop design and field conditions and enable design studies that are more practical.

Summary

To understand how to set the DC/AC ratio in PVSyst, it is important to consider not only the ratio of DC capacity to AC capacity but also how that ratio affects energy production and losses. The DC/AC ratio is an indicator of the balance between the total capacity of the photovoltaic modules and the AC-side output capacity, but its optimal value is not determined in isolation.

In practical work, we first organize module capacity, string configuration, AC-side equipment capacity, site, meteorological data, azimuth angle, tilt angle, and shading conditions. Then we run a simulation in PVSyst and check annual energy production, loss diagram, peak clipping, monthly generation, and performance ratio. In particular, when the DC/AC ratio is high, it is essential to check peak clipping losses.

To prevent failures, it is important to align the definitions of DC capacity and AC capacity, check voltage and current conditions, not overlook peak clipping, verify seasonal differences using monthly results, account for shading conditions, and compare multiple cases. By checking these items in order, you can reduce the risk of making a major error when setting the DC/AC ratio.

Additionally, the accuracy of PVSyst simulations is largely determined by the accuracy of the site conditions entered. If the site’s topography, obstacles, available installation area, and shading factors can be accurately understood, the assessment of the DC/AC ratio becomes more persuasive. By using LRTK, an iPhone-mounted GNSS high-precision positioning device, you can easily obtain the positional information needed on site and improve the accuracy of candidate site surveys and layout studies. Combining PVSyst power generation simulations with on-site information acquired via LRTK makes it easier to advance solar power system designs that are grounded in actual field conditions rather than relying solely on desk-based figures.