How to Read PVSyst Wiring Losses: 4 Things Often Overlooked

When analyzing the energy production of a solar power plant, attention often focuses on irradiance, temperature conditions, shading effects, and panel performance, but in practice how wiring losses are read is also extremely important. In PVSyst reports, wiring losses can at first glance appear as small numbers. However, unless you check them in relation to the DC side, AC side, around transformers, equipment layout, and output limits, you may not be able to correctly judge whether the analysis conditions are reasonable.

For practitioners searching for “how to read PVSyst,” what matters is not simply reading the loss percentage, but confirming which electrical section that number represents, under what assumptions it was calculated, whether it overlaps with other loss items, and whether it matches the on-site wiring plan. This article explains four items that are easy to overlook when reading PVSyst wiring losses, from a perspective that is practical to verify in the field.

Contents

• Where to look for wiring losses in PVSyst

• How to read DC wiring losses and common overlooked assumptions

• How to read AC wiring losses and the ranges you should check

• How to interpret transformer and high-voltage side losses

• Wiring losses in relation to output curtailment and PR

• Procedures to verify wiring losses in practice

• Importance of reflecting site information in analysis conditions

• Summary

Where to look for wiring losses in PVSyst

When reading PVSyst wiring losses, the first thing to confirm is which section the loss value in the report refers to. The electrical path of a solar power plant continues from the PV modules to the combiner boxes, collection panels, power conditioners (PCS), transformers, receiving equipment, and the point of grid interconnection. “Wiring losses” can refer to multiple sections: DC-side low-voltage or high-voltage DC wiring, AC wiring downstream of the PCS, and high-voltage side wiring beyond the transformer.

In PVSyst reports, losses related to wiring are shown in items such as the Loss diagram or Detailed losses. What’s important here is not only the magnitude of the numbers but also which stage of energy those loss rates are expressed against. For example, the meaning of a 1.5% loss differs slightly depending on whether it is shown as a percentage of “theoretical energy before generation” or as a percentage of “energy after other losses have already been subtracted.”

Wiring losses strongly depend on the plant layout. A configuration with short distances from modules to the PCS tends to have smaller DC wiring losses, while a wide site with long collection distances tends to have larger losses. Conversely, if collection distances after the PCS are long, AC-side or high-voltage-side losses may become non-negligible. Therefore, when reading PVSyst numbers, you should not simply judge them as “typical and therefore reasonable”; instead compare them with the actual layout, voltage class, cable sizes, wiring lengths, and collection method.

Wiring losses are items that are often overlooked compared to other losses. Shading losses and temperature losses are more conspicuous in the report and their impact on generation is intuitive, while wiring losses are small numbers and are sometimes treated as fixed values in the analysis. However, because thousands to tens of thousands of kW of energy flows through the plant, even a 0.5% difference affects annual generation and PR evaluation. Especially when comparing third-party reports, internal analyses, design values, and measured values, differences in wiring loss settings alone can produce discrepancies.

Practitioners reading PVSyst should first look at the loss diagram to confirm how DC wiring losses, AC wiring losses, transformer losses, and high-voltage side wiring losses are each handled. Then check whether each loss is an input value, calculated from electrical design conditions, or entered as an estimate. Understanding this clarifies how to read wiring losses.

How to read DC wiring losses and common overlooked assumptions

DC wiring losses occur in the DC wiring from the PV modules to the PCS input. When looking at PVSyst wiring losses, DC-side items are often the first that many practitioners focus on. DC losses vary with string configuration, the presence or absence of combiner boxes, PCS placement, cable sizes, wiring lengths, and voltage conditions. Therefore, when reading DC wiring losses you need to judge them based on an understanding of the equipment configuration, not only the numerical value.

For example, if the PCS is distributed close to each array, DC wiring distances from modules to the PCS are relatively short. In such configurations DC wiring losses tend to be set lower. On the other hand, if multiple strings are collected to remote collection equipment, DC wiring distances increase and losses may grow. In other words, even for the same plant capacity, appropriate DC wiring losses depend on the PCS placement strategy.

A commonly overlooked concept in DC wiring losses is the “average wiring length.” On drawings the length of the farthest string stands out, but in loss calculations not all strings are at the same distance. Some strings are close and others are far. Therefore, it is important which wiring length is used as the representative value. Using only the longest distance to set losses can be overly conservative, while using only short distances can underestimate losses.

DC wiring losses are related to the magnitude of current and cable resistance. Cable resistance varies with cross-sectional area, material, temperature, and length. Losses increase during periods of high current while generating, so sometimes a simple fixed percentage is acceptable and other times a more detailed calculation is required. PVSyst can calculate losses if design conditions are entered, but if inputs do not match the actual design, the results will naturally be off.

When reading DC wiring losses, confirm whether the loss rate is a design target or calculated from actual cable specifications. For example, if the report shows DC wiring loss as 1.0% or 1.5%, the confidence differs depending on whether that was entered as a standard assumption or calculated from wiring lengths and cable cross-sections. In practice it’s desirable to verify against plant drawings, single-line diagrams, and cable lists.

Also, it’s important to separate wiring to the combiner boxes or collection panels and wiring from there to the PCS. On some sites the cables from strings to combiner boxes are thin and the cables from combiner boxes to the PCS are thick, with different current and distance conditions. If these two sections are lumped into a single loss rate, it can be unclear which section is dominant. This is especially true when the PCS is distant or combiner boxes are unevenly located; in such cases it’s easier to judge if you check section-by-section.

DC wiring losses may seem small within a generation analysis, but they are crucial for confirming the overall design validity. Large wiring losses may indicate opportunities to review cable selection or equipment layout, while extremely small losses may mean the input assumptions are overly optimistic.

How to read AC wiring losses and the ranges you should check

AC wiring losses occur on the AC side after the PCS output. In PVSyst reports they may attract less attention than DC wiring losses, but they significantly affect the overall plant evaluation. AC-side losses vary depending on the distance from the PCS output to the transformer, the distance from the transformer to the receiving equipment, and whether collection is low-voltage or high-voltage.

The first point when reading AC wiring losses is to confirm what range those losses include. Do they cover from the PCS output terminal to the transformer primary, from the transformer secondary to the receiving point, or even wiring beyond the receiving point? When comparing reports, if these ranges are not aligned, you cannot judge the reasonableness by loss rates alone.

For example, one analysis may treat only the low-voltage AC wiring from the PCS to the transformer as AC wiring loss and list high-voltage side post-transformer losses separately. Another analysis may combine them together as AC loss. If you compare only the item names across reports, you might think you are comparing the same loss type when in fact the ranges differ.

AC wiring losses are affected by the PCS output voltage and current and by power factor conditions. On the AC side you may need to consider currents that include reactive power as well as active power. If the power factor changes the wiring current, it also changes wiring losses. However, in generation analysis reports it may be hard to see how much power factor or output curtailment is reflected. Therefore, when reading AC wiring losses, check power factor conditions, PCS ratings, output limitation settings, and grid connection conditions together.

AC wiring losses are also influenced by equipment layout. If the PCS and transformer are located close together, low-voltage AC wiring is short and losses are small. If multiple PCSs are deployed and each output is collected to remote collection equipment, AC wiring distances can be long. Especially on narrow sites, layouts following terrain, or when roads and construction force cable routes to detour, actual cable lengths can be longer than straight-line distances on drawings.

An often-overlooked point for AC wiring losses is the basis for wiring length. A single-line diagram alone may not show actual installation routes, slack length, internal panel wiring, or riser/drop portions. In analysis stages estimated distances are sometimes used, but as design progresses you should update loss conditions by checking against plan views and cable route diagrams. If wiring loss settings remain as initial design values, they may diverge from the as-built situation.

When reading AC wiring losses, also look at the balance with DC-side losses. If DC losses are low but AC losses are high, that may indicate a design with long collection distances after the PCS. Conversely, high DC losses and low AC losses may indicate long DC collection distances to the PCS. Which is better cannot be said universally, but it is important that the design philosophy aligns with how losses appear.

How to interpret transformer and high-voltage side losses

Sometimes people stop after checking only DC and AC losses in PVSyst, but in practice you must also check transformer losses and high-voltage side wiring losses. In solar plants the power output from the PCS is stepped up by transformers and sent to the receiving equipment and the grid. Which item these losses are included in can vary depending on analysis conditions.

Transformer losses include losses that vary with load and losses that occur simply because the transformer is energized. Some losses increase with higher generation, while some occur to a certain degree even at night or low output. When viewing transformer losses in PVSyst reports, it’s important to check how these two types of loss are set.

A common overlook is the overlap between transformer losses and AC wiring losses. In one analysis, transformer losses may be set separately while another item inadvertently includes similar losses. Conversely, one may intend to treat them as part of AC-side losses but in reality transformer losses are not fully included. When comparing reports, organize which equipment generates which losses rather than relying on item names.

High-voltage side wiring losses are similarly important. If the distance from the transformers to the receiving point is short, the impact may be small, but on wide sites with long high-voltage mains the losses can be non-negligible. Especially on plants with multiple transformers, distances to collection equipment differ and a simple uniform setting poorly represents reality.

When reading high-voltage side losses, be mindful of the voltage class. For the same transmitted power, higher voltage results in lower current and thus easier suppression of wiring losses. Therefore, high-voltage does not automatically mean larger losses. However, long distances, insufficient cable sizing, multiple stages of collection, and losses around panels and transformers can increase losses.

In PVSyst loss diagrams, transformer and high-voltage losses are sometimes shown close to the final grid-injected energy. Without understanding this positional relationship you may not know which stage subtracts what. When reading generation analysis, it is important to trace the energy flow in order from solar irradiance incident on the modules, module output, DC-side, PCS conversion, AC-side, transformer, high-voltage side, to grid injection.

Especially when evaluating PR, it is crucial to know up to which point losses are included. PR that includes post-transformer losses is not the same as PR up to PCS output. For plant performance evaluation, contracts, and third-party assessments, be clear about the evaluation point. When viewing PVSyst reports, confirm which point the final output energy represents.

Wiring losses in relation to output curtailment and PR

Wiring losses affect PR. PR is an indicator that compares the actual energy obtained to the theoretically attainable energy. Larger wiring losses reduce final output energy and lower PR. However, when reading PVSyst you should pay attention to how wiring losses relate to other losses and to output curtailment.

For example, in plants with strict PCS output limits, part of the power generated on the DC side during high irradiance may not be converted to AC output and may be treated as clipping loss. In such cases, the appearance of wiring losses is not straightforward. Wiring losses increase with higher current during high output, but if output is curtailed and some energy is cut, the composition of final losses can change.

Also, because wiring losses depend on current, their impact differs between low-output and high-output periods. Even if annual average loss is in the 1% range, losses concentrate during times of high instantaneous generation. Annual reports in PVSyst may not show this time-of-day behavior clearly. Therefore, in plants with significant output curtailment or peak cuts, looking at time-series output and loss relationships deepens understanding.

Be careful when comparing PRs. If you compare multiple analysis results and one PR is higher while another is lower, the difference might stem not only from irradiance and temperature data but also from differences in wiring loss settings. Especially if DC wiring losses, AC wiring losses, and transformer losses each differ slightly, the total difference becomes non-negligible.

When reading PVSyst wiring losses together with PR, it is important to look not only at the total loss but also at the breakdown of each item. Small differences, such as DC wiring loss differing by 0.5%, AC wiring loss differing by 0.5%, and transformer loss differing by 0.3%, can cumulatively affect PR comparison results. When comparing third-party reports and internal analyses, do not judge solely on PR; confirm whether wiring loss conditions are aligned.

Moreover, PR depends on the evaluation point. Whether you base it on energy at the receiving point, on PCS output, or include post-transformer losses changes how wiring losses are included. If this is unclear during comparison, you may unintentionally be comparing different metrics even though both are called PR.

In plants with output curtailment, separating wiring losses from curtailment losses is important. If generation is lower than expected, determine whether it is due to resistive wiring losses, output limits, equipment outages, or irradiance conditions. PVSyst analysis can separate losses to some extent by following the loss diagram, but comparing with measured data requires on-site data as well.

Procedures to verify wiring losses in practice

To verify PVSyst wiring losses in practice, you should cross-check materials rather than just glance at the report. The first item to check is the single-line diagram. The single-line diagram helps you understand the electrical flow from modules to PCS, transformer, and receiving equipment. Next, check the layout plan and cable route diagrams to confirm actual distances.

At this point, map which sections of the single-line and layout correspond to the wiring loss settings in PVSyst. On the DC side, confirm whether the setting covers string to combiner box, combiner box to PCS, or modules to PCS all combined. On the AC side, confirm whether it covers PCS to transformer, transformer to receiving equipment, or up to the high-voltage main.

Then check cable sizes and wiring lengths. Larger cable cross-sectional areas reduce resistance and suppress wiring losses. On long-distance wiring, losses can be significant even with large cross-sections. Even if analysis conditions use a uniform loss rate, checking whether that value aligns with cable specifications helps you assess reasonableness.

Choosing representative values is crucial in wiring loss verification. It is fine if you can input every wiring length in detail, but in practice average or representative values are sometimes used. In that case, confirm how the average was taken. Results differ depending on whether it is a simple mean or weighted by capacity or number of strings. If wiring conditions vary greatly by zone, analyze by zone and use capacity-weighted averages to better represent reality.

When comparing with third-party reports, align the naming of wiring loss items. A report may label an item “DC wiring loss” while another uses a different name for a similar item. Do not judge by names alone; confirm the loss occurrence sections. If necessary, tabulate loss items from each report and compare corresponding sections.

Watch out for extreme values when checking reasonableness. For example, if a design has short DC wiring distances but DC wiring loss is large, inputs may be overly conservative. Conversely, if a site has long-distance collection but wiring loss is very small, there may be missing wiring lengths or sections. Apply the same checks for AC and high-voltage side losses against equipment layout.

In practice, do not check wiring losses in isolation but view them within the overall loss structure. Check shading loss, temperature loss, mismatch loss, PCS loss, transformer loss, etc., together to see what share wiring losses occupy. Large wiring losses may indicate potential design improvements, while very small losses require checking input conditions.

Importance of reflecting site information in analysis conditions

To correctly read PVSyst wiring losses, it is essential to reflect site information, not only inputs in the analysis software. Solar plants may not allow ideal straight-line wiring due to terrain, earthworks, roads, drainage, existing equipment, fences, and maintenance access. Even equipment that appears close on drawings may require detoured cable routes on site.

When site information is lacking, wiring loss values tend to be estimates. In basic design you may use standard loss rates, and update them based on cable routes and sizes during detailed design. However, if you see an analysis report later, it may be unclear which design stage the numbers reflect. Therefore, when reading PVSyst, confirm whether wiring loss values are based on initial assumptions or on detailed design.

Understanding wiring losses is also important when comparing with on-site measured data. For example, comparing irradiance, PCS output, and receiving point energy, you may find PCS output matches expectations but receiving point energy is slightly lower. In that case transformer losses, high-voltage side wiring losses, or differences at the meter point might be affecting the results. Conversely, if PCS input power is lower than expected, consider not only DC wiring losses but also string faults, soiling, shading, and module degradation.

Location information also helps verify wiring losses. If you can identify positions of on-site equipment, racks, combiner boxes, PCS locations, and cable routes, the accuracy of wiring length estimates improves. Traditionally much of this relied on drawings and site checks, but recently high-precision surveying, local point-cloud data, and overlaying drawings make it easier to accurately capture equipment positions and cable routes.

Reflecting site information is particularly important for large plants or those divided into multiple zones. If PCS placement and wiring distances differ by zone, treating the whole site with one average obscures reality. Organize DC capacity, wiring distances, and equipment layout by zone, confirm each zone’s loss conditions, and then aggregate to produce results that are easier to explain.

Also be aware of post-construction changes. Panel board relocations, cable route adjustments, and minor equipment layout tweaks may occur that were not anticipated in the design. If these changes are not reflected in PVSyst analysis conditions, discrepancies between analysis and actual generation can arise. It is desirable to revise wiring loss settings as needed based on as-built drawings and site surveys.

Reflecting site information in the analysis conditions is not only about making generation forecasts more accurate. When explaining analysis results to customers or third parties, being able to show the basis for wiring losses is important. Saying “we used a standard value” is less persuasive than explaining “this setting is based on the distance in this section, the cable size, and the equipment layout,” which increases trust in the analysis results.

Summary

When reading PVSyst wiring losses, first confirm how DC wiring losses, AC wiring losses, transformer losses, and high-voltage side wiring losses are segregated. Wiring losses may look like small numbers, but they certainly affect total plant generation and PR comparisons. Especially when comparing third-party reports with internal analyses, you must organize not only loss item names but also which sections they include.

For DC wiring losses, check distances from modules to the PCS, combiner box placement, cable sizes, string configuration, and the approach to representative wiring length. For AC wiring losses, confirm whether the measure covers PCS-to-transformer, up to the receiving point, and whether power factor or output conditions are involved. For transformer and high-voltage side losses, be mindful of load-dependent vs. no-load losses, collection distances, and the evaluation point.

Wiring losses are also related to PR and output curtailment. When comparing PRs, check wiring loss settings as well as irradiance and temperature inputs. In plants with output limits, separating wiring losses from clipping, transformer, and high-voltage losses is more important.

In practice, do not judge solely by PVSyst reports. Cross-check single-line diagrams, layout plans, cable route diagrams, cable specifications, zone DC capacities, and on-site equipment positions. Being able to explain the basis for wiring losses improves the credibility of analysis results and makes it easier to communicate with clients and stakeholders.

To accurately grasp on-site equipment positions and cable routes, site location information is useful. Using a high-precision GNSS positioning device that can be attached to an iPhone, such as LRTK, makes it easier to efficiently record equipment positions, racking, panels, and cable routes on site. When you want to bring PVSyst analysis conditions closer to the actual site or to verify as-built equipment locations and organize the basis for wiring losses, combining high-precision on-site records like LRTK helps improve the accuracy of generation analysis and site management.