What percentage is a guideline for PVSyst wiring loss settings? 5 ways to judge

When setting wiring losses in PVSyst, what many practitioners first struggle with is, "After all, what percentage should I enter?" If the loss rate is set too low, the generation estimate will be overly optimistic; if set too high, the project assessment will be unnecessarily strict. Moreover, wiring loss is not an item that can be treated as a fixed rate like soiling or aging; it varies greatly depending on design conditions such as wiring length, conductor size, how sub-arrays are gathered, inverter placement, and distance to the grid connection point.

To conclude up front, a practical starting point for initial studies is around 1.5%. PVSyst also treats 1.5% based on nominal test conditions as a guideline initial value for PV-side wiring losses, and the entered percentage is internally converted to an equivalent resistance for calculation. In other words, the percent is only an entry expression; the essence is estimating the wiring resistance.

However, it is dangerous to treat 1.5% as the fixed correct answer. Wiring loss should be considered in stages according to project maturity: a coarse value for early-stage studies and a finalized value based on the actual wiring. This article organizes five practical perspectives to help you remain consistent when in doubt about PVSyst wiring loss settings.

Table of contents

• Premises to grasp first when setting wiring losses

• Approach 1 Start from around 1.5%

• Approach 2 The entered percentage does not directly equal the annual loss

• Approach 3 Judge by the overall wiring configuration rather than distance alone

• Approach 4 Treat DC side and AC side separately

• Approach 5 Finalize based on the actual wiring

• Summary

Premises to grasp first when setting wiring losses

In PVSyst, wiring losses appear as a % input, but the calculation’s core is the equivalent resistance. The wiring loss rate does not act as a constant fixed value throughout the year; it changes moment to moment depending on the current. The official explanation also states that loss power is proportional to the square of the current, and the entered percentage is evaluated at each simulation step according to the current operating state.

If you do not understand this premise, you may look only at numbers such as 1.0% or 1.5% on the settings screen and jump to conclusions like “this project has small wiring losses” or “this project has large losses.” In reality, the annual appearance will differ even for the same 1.5% input between a project with short wiring and many high-irradiance hours and a project with long wiring and a gradual power ramp-up. The important thing is not to memorize the input value as the correct answer, but to be able to explain which design conditions that number represents.

Also, PV-side wiring losses cannot be fully summarized by a simple voltage drop rate as in general power distribution equipment. PVSyst’s approach explains that, on the PV side, current and voltage tie into maximum power point tracking, so it is more natural to treat losses as output loss rather than voltage drop. In other words, rather than applying field intuition like “this distance causes this voltage drop,” it is more consistent with PVSyst’s calculation philosophy to consider how much resistance exists in each section and how that affects the operating point.

Furthermore, wiring loss reflects design quality. Design inefficiencies—such as modules and inverters placed far apart, awkward combiner box placement, overly long AC main feeders, or bulky routing to transformers and grid connection points—will ultimately show up here. Therefore, wiring loss settings should be considered not as a number you “just enter,” but as a number that checks the reasonableness of the layout. Having this perspective alone will significantly stabilize your use of PVSyst.

Approach 1 Start from around 1.5%

PVSyst suggests 1.5% based on nominal test conditions as an initial guideline for PV-side wiring loss. This is a standard placeholder for preliminary studies before detailed design and is practical for initial comparisons and project screening. When wiring routes are not yet fixed, starting from around 1.5% reduces estimate variability better than immediately applying extreme values like 0.3% or 2.8%.

From a practical sense, a very compact layout where each string is close to the inverter and wiring is easy to organize may ultimately converge toward around 1%. Conversely, for a wide site that consolidates combiner boxes in multiple stages, has large variations in string lengths, and concentrates inverters in one place, it is not surprising to exceed 1.5%. Thus, it is helpful to view 1.5% as “a starting point for a normally arranged design.”

It is crucial not to conflate initial and finalized values. In preliminary studies you need to evaluate under somewhat aligned conditions to compare layouts and equipment choices; 1.5% is convenient for that stage. However, for detailed design, bid submission, financial evaluation, or owner briefings—stages where you must be able to justify numbers—you should not leave wiring loss at the initial 1.5% by default. It is a waste of PVSyst’s capability to advance the design while leaving the wiring loss at the preliminary value.

To organize a viewpoint: less than 1% indicates very favorable wiring conditions, around 1.5% is the standard initial placement for preliminary studies, and settings above 2% are a level at which you should question the routing. Of course, this is not an absolute standard. The essence is whether you can explain the number with wiring length, collection scheme, conductor size, and placement density. The quality of the setting is determined not by chasing numbers but by whether you can explain “why this number.”

Also, if you enter an excessively small number in the initial study, the estimated generation will look too favorable. At the initial stage, a middling value that is easy to explain is more manageable; it is healthier to refine and see the value decrease later. Conversely, starting with an overly large loss rate distorts the comparison of equipment configurations and layout options. The reasoning to start from around 1.5% helps strike a good balance in early design.

Approach 2 The entered percentage does not directly equal the annual loss

A common misunderstanding about PVSyst wiring losses is the relationship between the setting and the annual result. The official explanation states that wiring losses are calculated proportional to the square of the current, and the annual energy loss from wiring generally corresponds to around 60% of the nominal loss rate specified under nominal test conditions. In other words, entering 1.5% does not mean the annual result will be exactly 1.5%.

Understanding this prevents misreading results. For example, a project with an initial setting of 1.5% may well appear as around 0.9% annual loss. This is not an input error; during low-irradiance periods like mornings and evenings when current is small, the percentage of wiring loss is also small. PV systems do not operate at nominal test conditions all day, so the annual appearance naturally differs.

If you look only at the results without this understanding, you might think, “The setting is 1.5% but the result is below 1%—shouldn’t I raise the input?” That line of thought is dangerous. PVSyst accumulates losses based on the operating state at each time step, so an annual result lower than the set value is natural. The setting is the loss rate relative to nominal design conditions; the annual result is the energy loss through real weather conditions—these are different things.

This distinction is also important when explaining to stakeholders. Saying “the wiring loss is set to 1.5%” and saying “the annual impact on generation is this much” mean different things. The former is an input design condition; the latter is an annual simulation outcome. Being able to separate and explain these improves trust in PVSyst numbers.

Also be careful when comparing wiring loss with other loss items. Unlike relatively steady items like soiling or downtime, wiring loss varies with the operating point. Therefore, check not just “what percentage is large,” but “under what assumptions that percentage was set.” The first step to handling PVSyst wiring losses correctly is to have a feel for this nonlinearity.

Approach 3 Judge by the overall wiring configuration rather than distance alone

When thinking about wiring loss, it’s easy to focus only on whether distances are long or short, but in PVSyst the whole configuration matters. In the detailed calculation function, the approach is to input average round-trip lengths per wiring section, conductor sizes, number of branch circuits, etc., to compute the equivalent resistance of the entire lower-level collection. Evaluate circuit length including round-trip distance and the overall configuration including parallelism, not just one-way distance.

For example, even with the same “50 m (164.0 ft) to the inverter” condition, the reality differs if strings are almost evenly distributed versus if a few are extremely far. Whether combiner boxes consolidate along the way or each string is run individually also changes the result. Moreover, whether conductor sizes are sufficient and where multiple strings merge affect how losses manifest. Therefore, when thinking about wiring loss guidelines, you must look at “what kind of collection configuration exists overall,” not just “what is the longest distance.”

A practical viewpoint that helps is to consider not distance per se but how long sections carrying large currents are routed. Current per conductor near the PV strings is relatively small, but on the feeder side after merging currents combine; the same distance becomes more impactful. Conversely, even if branch wiring before merging is somewhat long, adequate conductor sizing can keep losses from being as large as expected. PVSyst wiring loss accuracy improves the more carefully you examine “where how many conductors come together and where large currents occur.”

Also, adding combiner boxes does not necessarily make things worse. If combiner box placement is appropriate, consolidating mid-run can reduce overall resistance compared to forcing a single run for distant strings. The important thing is to be able to explain the wiring scheme on drawings. If string-to-string variation is large, only the edges have long runs, or inverters are biased to the site edge, then rather than fixing 1.5% you may need to consider a slightly higher value.

Conversely, if racking layout is well organized, inverters are near the center of sub-arrays, and feeders are short, the wiring loss may well be lower than 1.5%. Even then, don’t lower the number based solely on a feeling; you must be able to explain why it can be reduced from the wiring configuration. Don’t move numbers by impressions of distance; judge based on the assembly of the entire circuit—this is the PVSyst-consistent approach to wiring loss settings.

Approach 4 Treat DC side and AC side separately

In PVSyst, DC-side wiring loss and AC-side wiring loss have different characteristics. The DC side is treated as the resistance of each sub-array, while the AC side is treated as the loss from inverter output to the grid connection point. Also, the AC-side percent can be interpreted relative to different reference powers depending on project settings. Therefore, placing 1.5% on both DC and AC sides with the same intuition may inadvertently line up numbers that mean different things.

In practice, which side dominates varies by project. If inverters are distributed close to PV arrays, DC-side losses are easier to control while AC main feeders can be long. Conversely, clustering inverters near the substation to shorten AC routing tends to lengthen DC runs. Optimism about only one side does not lower total losses.

Thus, when thinking about wiring loss guidelines, mentally split the total first. For example, if you want to slightly suppress total wiring loss, decide whether to view DC as around 1% and AC as around 0.5%, or DC 0.7% and AC 0.8%—the design focus changes accordingly. Of course, this is conceptual organization; ultimately you must refine with actual wiring. Still, separating DC and AC in your thinking makes the rationale for “why this percent” much clearer.

Also pay attention to separating out transformer losses. PVSyst allows treating transformer iron and copper losses separately and defining AC wiring to the grid connection point separately. If you account for transformer copper losses in a separate item but also include them broadly in the wiring loss, you risk double counting. Unless you clarify what is treated as wiring and what as transformer, you may appear conservative but actually overstate losses.

Furthermore, in projects with a far grid connection point or a weak grid, AC feeder conditions become important. PVSyst notes that wiring conditions up to the grid connection point can, in some cases, affect injection constraints. In wide-area sites or distant interconnections, focusing only on DC-side detail while leaving AC-side crude will make the overall assessment too optimistic. To truly master wiring loss settings, manage DC and AC as separate discussion points.

Approach 5 Finalize based on the actual wiring

PVSyst’s official guidance also states that in final design, wiring losses should be evaluated from actual lengths and conductor sizes to determine the equivalent resistance. Its dedicated optimization functions can compute the equivalent resistance of the entire lower-level collection from average round-trip lengths and conductor sizes per wiring section. In other words, percent input is a convenient initial method; the orthodox approach is to move to actual wiring.

In practice, adopt a three-stage flow: initially set around 1.5% for early studies, then reflect string lengths and inverter positions once the layout plan is fixed, and finally move to actual-wiring-based values at the estimate and detailed design stage. Doing too much detail too early increases rework with every design change, while staying coarse until the end undermines the credibility of the numbers. It is realistic to increase precision step by step.

One useful rule of thumb is whether the loss exceeds 2%. If, after reflecting actual wiring, the total wiring loss greatly exceeds 2%, don’t just accept it as “that’s the nature of the project”—look for opportunities to improve routing or conductor size. Changing inverter placement slightly can improve things, and moving combiner boxes or altering main feeder routes can reduce losses. Wiring loss can serve as an early warning to find design improvement opportunities.

Conversely, be cautious if you find yourself wanting to enter an extremely low value even at the final stage. True, very short and well-organized wiring can legitimately settle at a small value, but lowering it without solid justification makes the generation forecast look better than reality and leads to mismatches during construction. Especially while site conditions and equipment placement are still uncertain, it is safer to be somewhat conservative. PVSyst is not a box to produce convenient numbers; it is a tool to reflect design reality.

If you enforce finalization based on actual wiring, internal design quality becomes consistent. Rather than letting each project member decide 1% or 2% by feel, define when to use provisional values, when to switch to actual wiring, and when to finalize. That improves comparability of PVSyst results. Wiring loss settings may appear minor, but they reveal the maturity of the design process. For that reason, having an operational rule to always finalize with actual wiring is important.

Summary

There is no absolute correct percent for PVSyst wiring loss settings, but the judgment axes are clear. Use around 1.5% as a starting point for preliminary studies, then revise according to wiring configuration, distances, conductor sizes, how sub-arrays are gathered, inverter placement, and routing to the grid connection point. Also remember that the entered percentage does not directly become the annual loss; the actual annual result varies over time with operating conditions.

What matters is not memorizing numbers but being able to explain the number’s background in design terms. Short-distance, well-organized projects have reasons to be small; wide sites with heavy collection configurations have reasons to be large. Separate DC and AC sides, split out transformer losses when needed, and finalize based on actual wiring. Following this flow makes wiring loss settings in PVSyst much easier to handle.

To improve design accuracy for PV projects, it is important not only to set input conditions at the desk but also to capture site layout information and distances accurately. Especially when inverter positions, combiner box positions, distance to interconnection equipment, and actual equipment placement are unclear, wiring loss estimates will vary. In such cases, using iPhone-mounted GNSS high-precision positioning devices such as LRTK to align equipment positions and distance information precisely and reflect them in the design is effective. Building a system to capture accurate on-site information contributes to improving the reliability of generation forecasts by optimizing wiring loss settings.