What causes low power output? 6 checks to perform before making improvements

When you find that a solar power system is producing less power than expected, it's common to immediately consider replacing equipment or undertaking major renovations. However, reductions in output can be caused by multiple factors, including solar irradiance, the season, shading, soiling, equipment shutdowns or settings, measurement methods, and assumptions made during design. If you proceed with countermeasures without isolating the causes, you may end up working on areas that will have little effect and risk making ambiguous judgments about whether improvements have been achieved.

For practitioners responsible for increasing power generation, the important thing is to first check in order: "Is it really abnormal?", "Where are the losses occurring?", and "Is the extent recoverable through improvement?". This article organizes six checks to review before making improvements when power generation is low, covering both on-site inspections and data verification.

Table of Contents

• Align the baseline before concluding that power generation is low

• Check 1: Confirm solar irradiance and seasonal variations

• Check 2: Confirm shadow locations and time periods

• Check 3: Confirm soiling and obstructions on panel surfaces

• Check 4: Confirm equipment shutdowns, output curtailments, and fault history

• Check 5: Confirm variations in wiring, connections, and across strings/systems

• Check 6: Confirm deviations between design assumptions and actual conditions

• Begin improvements to increase power generation by prioritizing causes

• Summary: Improvements in power generation should be judged by measuring and comparing

Standardize the criteria before determining that power output is low

When you feel that your solar power generation is low, the first thing you should do is not just look at the generation numbers, but align the basis for comparison. Even with the same installed capacity, output can vary depending on installation location, orientation, tilt, surrounding environment, weather, temperature, equipment configuration, and years in operation. Therefore, simply saying "less than last year" or "less than neighboring systems" may not allow you to correctly determine the cause.

For example, even if monthly power generation has decreased, if the solar irradiance for that month was lower than the previous year, differences in weather rather than equipment faults may be the primary reason. Conversely, if irradiance was sufficient but generation has dropped significantly, you should check for shading, soiling, equipment shutdowns, wiring faults, output curtailment, and similar issues. In other words, when evaluating generation it is important not only to look at the actual values but also to assess how much generation is being achieved relative to the solar irradiance conditions.

In practice, separating power generation by month, by day, and by time of day makes it easier to narrow down the causes. If generation is declining on a monthly basis, check for seasonal variations in solar irradiance, long-term degradation, and deviations from design conditions. If it suddenly drops on a daily basis, equipment shutdowns, communication failures, weather, or grid-side restrictions may be involved. If it falls only during specific times of day, shading, output curtailment, or the effects of temperature rise may become apparent.

Also, it is necessary to standardize the units of energy generation. Looking only at the total generation of a system makes it difficult to compare projects with different system capacities. Converting to generation per unit capacity, for example kWh/kW, makes it easier to compare with other power plants or different sections. However, even after normalizing by capacity, differences will arise if azimuth, tilt, irradiance conditions, or shading conditions differ. Therefore, it is important not to draw conclusions based solely on the numbers, but to verify them together with the on-site conditions.

Before considering ways to increase power generation, you first need to put the impression that "power generation is low" into a form that can be explained with data. If you can sort out when it became low, which times of day it is low, whether the whole installation is low or only some systems are low, the next places to check will become clear. Conversely, if you begin on-site responses without this整理, it will be difficult to judge the effects of cleaning, inspection, replacement, or setting changes.

Improving power output is not a matter of deciding causes by mere conjecture; it’s about narrowing down anomalies by standardizing comparison conditions. The more carefully the initial criteria are established, the easier subsequent on-site verification and improvement decisions will be.

Check 1: Confirm solar radiation and seasonal variation

When checking the cause of low power generation, the first thing to look at is solar irradiance. Because solar power generation converts sunlight into electricity, if solar irradiance is low, power output will also be low. Months with few sunny days, periods of prolonged rain or cloud cover, or times affected by snow accumulation or yellow sand can see reduced output even if the equipment is functioning normally. Therefore, you should not judge based only on power output; you need to check it together with the solar irradiance conditions for the same period.

What you should pay particular attention to is the comparison with the same month of the previous year. If generation is lower than the previous year, you may suspect an equipment fault, but if solar irradiance in the same month last year was higher and this year’s solar irradiance is lower, a difference is natural. Conversely, if solar irradiance is about the same as last year, or higher this year, yet generation has decreased, it is more likely that the cause lies with the equipment or the surrounding environment.

Seasonal variations are also important. In summer, longer daylight hours tend to extend generation time, but higher temperatures can reduce the output of solar panels. In winter, temperatures can be advantageous, but shorter sunlight hours and a lower solar altitude make systems more susceptible to shading. Spring and autumn may appear relatively stable, but power generation can vary depending on weather and atmospheric conditions. Therefore, you should avoid judging performance based solely on a single month’s generation without understanding seasonal trends.

In practice, we compare power generation and solar irradiance side by side over the same period. If the decrease in solar irradiance and the decrease in power generation are similar, weather-related factors are likely. If power generation falls more than the decline in solar irradiance, suspect other causes such as shading, soiling, equipment shutdowns, temperature-related issues, or poor connections. If power generation does not increase during periods when solar irradiance is sufficient, focus primarily on the condition of the equipment.

Also, when extracting days with low power generation, it is important not to judge solely by rainy or cloudy days. If you select days close to clear skies and look at generation curves by time of day, it becomes easier to understand the condition of the equipment. If you see patterns such as output being low only in the morning, leveling off around midday, or dropping suddenly only in the afternoon, the direction of the cause changes. If the drop is biased toward the morning or the afternoon, orientation or shading may be involved, and if a plateau is seen at midday, you need to check equipment capacity, output control, and temperature effects.

What’s important when checking solar irradiance is not to immediately assume a drop in power output is an abnormality. By distinguishing between declines that can be explained by solar irradiance conditions and those that cannot, inspection priorities change significantly. Especially when you want to increase power output, it is crucial to first isolate the influence of natural conditions and determine where there is room for improvement on the equipment side.

Check 2: Confirm where and when shadows occur

One often-overlooked cause of low power generation is shading. In solar power generation, even if only part of a panel is shaded, it can affect the power output of the corresponding circuit or system.

Shadows don't necessarily fall in the same place all day; they may occur only in the morning, only in the evening, only in winter, or only during certain seasons. For that reason, it's safer to avoid concluding that "there is no shading" based on a single on-site inspection.

Causes of shading include nearby buildings, trees, utility poles, mounting racks, fences, adjacent equipment, rooftop equipment, signs, piping, and lightning protection systems. In ground-mounted power plants, surrounding trees can grow and create shading that was not a problem a few years earlier. In roof-mounted installations, chimneys, air-conditioning equipment, railings, or additions to adjacent buildings can cause shading. Even if shading was minimal at the design stage, changes in current conditions can affect energy output, so regular inspections are necessary.

When checking shadows, it's important not only to determine whether shadows are present, but also to see when, where, and over what extent they occur. Whether morning shadows are affecting the eastern array, evening shadows the western array, or racking and equipment cast shadows across panel rows around midday will change the direction of countermeasures. Cross-referencing time-of-day generation with the times shadows occur makes it easier to link dips in the generation curve to on-site conditions.

Pay attention to seasonal changes in sun altitude. Even if shadows are short in summer, the sun altitude is lower in winter and shadows extend much longer. Even if an on-site check in summer shows no issues, power output can drop on winter mornings or afternoons. In particular, if generation is lower than expected only in winter, checking how shadows fall around the winter solstice can help reveal the cause.

When considering measures against shading, do not immediately think of relocating equipment or undertaking large-scale renovations; first clarify the cause of the shading and the extent of its impact. If tree branches or foliage are the cause, consider pruning or reviewing management practices. If surrounding structures are the cause, you need to identify the times and areas when power generation is most affected and then select realistic countermeasures. If the panel layout or system configuration is particularly susceptible to shading, you may also consider reviewing the connection units or layout during renovations.

Shading is one of the factors for which it is relatively easy to verify the effectiveness of measures to increase power generation. However, rather than judging the presence or absence of shading by eye, it is necessary to check it together with time-of-day generation data, on-site photographs, the sun's position, and the system configuration. If the impact of shading can be explained quantitatively, it becomes easier to determine the priority and scope of countermeasures.

Check 3: Inspect the panel surface for dirt and obstructions

When power generation is low, you should also check the condition of the panel surface. Because solar panels are installed outdoors, they accumulate various types of dirt such as dust, sand, pollen, bird droppings, fallen leaves, sap, exhaust-related grime, and deposits containing salt in coastal areas. When dirt adheres to the surface, the light reaching the panels is reduced, which can affect power generation.

The impact of soiling cannot be judged solely by whether the surface looks slightly cloudy. If the entire surface is thinly soiled, overall power output may decrease. Conversely, localized soiling such as bird droppings or fallen leaves can have a biased impact on certain cells or circuits. Even if localized soiling seems small, it can affect power production, so you should not underestimate it based only on a visual inspection.

Particular attention should be paid to installations with a shallow tilt. When the tilt is sufficient, rain can wash away dirt easily, but with a shallow tilt, dirt and water stains tend to remain. Mud and dust can accumulate at the lower edges of the panels, forming banded soiling. Such soiling not only affects power output but can, over the long term, cause local hot spots and degradation.

On rooftop installations and equipment around factories, differences in how dirt accumulates due to the surrounding environment are also observed. If there are roads, development sites, fields, factory exhausts, trees, or places where birds tend to gather nearby, the frequency of dirt accumulation can increase. In mountainous areas or around farmland, dust and pollen may be present; in coastal areas, deposits containing salt; and near trees, fallen leaves and tree sap can have an impact. Knowing the type of dirt makes it easier to revise cleaning frequency and inspection timing.

However, performing cleaning does not necessarily lead to a large increase in power generation. If the soiling is minor and not the main cause of the power loss, the improvement from cleaning will be limited. Therefore, it is important to record the cleaning date, the area cleaned, the weather, and the solar irradiance conditions so that power generation before and after cleaning can be compared. Without such records, it becomes impossible to determine whether changes are due to the cleaning or to differences in the weather.

Also, cleaning and inspection require attention to safety. On roofs, sloping ground, at height, or with energized equipment, incorrect working methods can lead to accidents or equipment damage. During on-site checks, it is important not to approach the panels forcefully and to choose methods that allow safe inspection. As necessary, combining ground-level checks, telephoto photography, records made during maintenance inspections, and aerial inspections makes it easier to grasp the condition.

Dirt and obstructions on the panel surface are relatively easy to identify as causes of low power output. However, rather than drawing conclusions based on appearance alone, organizing and assessing the dirt’s location, extent, type, its impact on power output, and the changes before and after cleaning will help improve power generation.

Check 4: Verify equipment shutdown, output restrictions, and history of abnormalities

Checking the operational status of equipment is indispensable when investigating the causes of low power generation. In photovoltaic systems, even if the panels are functioning normally, problems with the power conditioner (inverter), junction boxes, combiner boxes, circuit breakers, monitoring devices, communications equipment, or connections can reduce power output. Looking only at the facility’s total output can make issues hard to detect, but checking by individual device or by system can reveal that only part of the system has stopped or that output is not increasing as expected.

The first thing to check is the downtime and fault history. If the equipment was temporarily stopped on a given day, the power generation for that day will be reduced. Even if the stoppage was short, if it occurred during sunny hours its impact on generation can be significant. The fault history may record overvoltage, overcurrent, temperature rise, insulation-related warnings, communication errors, and grid-side fluctuations. When reviewing the history, it is important to check whether the date and time when generation dropped match the date and time when anomalies occurred.

Next, check whether output limitation is present. If solar irradiance is sufficient but the power output does not rise above a certain level and the top of the generation curve becomes flat, restrictions due to equipment capacity, control conditions, or grid-side conditions may be involved. Output limitation is not necessarily a fault of the equipment. It can occur because of design capacity allocation, grid interconnection conditions, or operational controls. However, if output is being limited at unexpected times or frequencies, you should check the settings and the grid conditions.

Be aware of communication failures. The power output may only appear low because monitoring data is missing, even though generation is actually occurring. Even if the monitoring screen shows zero output, checking the local equipment display and cumulative readings may reveal that generation is occurring. As an initial check, it is important to distinguish whether a drop in power output is due to record loss from communication problems or an actual stoppage of generation.

Comparing power generation by device is also effective. Within the same plant, when there are multiple systems with similar azimuths and tilts, compare their respective generation. If only a particular device shows low generation, check the panels, wiring, junction boxes, settings, and internal condition connected to that device. If all devices are similarly low, suspect solar irradiance conditions, overall soiling, grid-side restrictions, or deviations from the design conditions.

Equipment is also affected by temperature. In poorly ventilated locations, places exposed to direct sunlight, or where heat accumulates around the equipment, the device may reduce its output due to rising temperatures. If power generation is reduced only around midday on clear summer days, it is worth checking the temperature conditions and the installation environment. Check whether objects are placed around the equipment that obstruct ventilation, whether the intake or exhaust ports are dirty, and whether the installation location has become hotter than expected.

When inspecting equipment, it is important to look at the on-site displays, monitoring data, fault history, and daily and time-of-day power generation together. Before considering equipment replacement as a measure to increase power output, checking for shutdowns, curtailments, communications, settings, temperature, and variations at the system level will make it easier to narrow down the cause.

Check 5: Verify variations by wiring, connections, and circuits

The cause of low power generation may be hidden not in the entire installation but in some wiring, connections, or circuits. A solar power generation system is configured by combining multiple panels and collecting electricity through junction boxes and equipment. Therefore, if there are partial connection faults, broken wires, loose terminals, or circuit abnormalities, the overall power output can decrease.

Problems like this are hard to detect by looking only at the installation’s total monthly power generation. Even if the overall drop appears to be only a few percent, some strings may have fallen significantly. Conversely, even if the total generation seems low, viewing it by string can show the same trend across all strings, indicating that irradiance, shading, or soiling—not wiring or connections—are the cause. In other words, comparing strings is an important clue for isolating the cause.

The basic principle of inspection is to compare systems under the same conditions. If they have the same orientation, the same tilt, a similar number of panels, and similar shading conditions, it is natural that their power output trends will be similar. If only one system has a lower power output, check the panels, wiring, junction boxes, terminals, breakers, input circuits, and other components connected to that system. If you can compare the voltage and current values for each system, variations in those values make it easier to judge the possibility of an abnormality.

Problems with connection points can be difficult to detect from appearance alone. Loose terminals, increased contact resistance, insufficient connector insertion, cable damage, insulation deterioration, and moisture ingress can lead to reduced power output and safety risks. Even if no abnormalities are visible to the naked eye, if there are deviations in power generation data, a professional inspection should be considered.

Checking wiring routes is also important. Outdoor wiring is exposed to ultraviolet (UV) light, wind and rain, temperature changes, damage from animals, and the effects of bending or fastening during installation. For ground-mounted installations, damage from mowing and maintenance work can be a problem, while for roof-mounted installations cable sagging and deterioration of attachment points can occur. If wiring is in contact with equipment or mounting structures and rubbing, it can lead to long-term degradation.

System-to-system variations are also checked in relation to shading and soiling. If a system with low power output coincides with locations that receive shade or have heavy soiling, environmental factors rather than wiring faults may be the primary cause. Conversely, if a particular system shows reduced output despite no apparent shading or soiling, the priority of electrical checks increases. By overlaying the on-site positional relationships with the generation data in this way, you can narrow down the causes more realistically.

Also, care is needed when records of the system configuration are outdated. If drawings and management documents have not been updated after renovations, expansions, equipment replacements, or wiring changes, it can become unclear which system covers which area. In that situation, it takes time to identify the areas with low power generation. Before making improvements, it is important to confirm that the current connections match the management documents.

To increase power generation, it is essential not only to have an overall impression but also to understand the condition of each system. By separating where things are normal and where they are underperforming, you can reduce unnecessary measures and more easily focus improvements on the areas most likely to deliver results.

Check 6: Verify discrepancies between design assumptions and current conditions

When considering the causes of low power generation, it is also important to verify whether the generation predicted at the design stage matches the actual conditions. In the design of a solar power system, expected generation is estimated based on assumptions such as the orientation and tilt of the installation surface, panel layout, equipment capacity, solar irradiance conditions, nearby shading, and loss conditions. However, on actual sites, conditions that differ from the design assumptions may be discovered later.

For example, if the on-site roof pitch or installation angle differs from the design documents, the estimated power generation may differ. For ground-mounted installations, the terrain after site development, the height of the mounting structures, and the positions of surrounding structures may differ from the initial assumptions. For roof-mounted installations, level differences, equipment, handrails, and shadows from surrounding buildings that were overlooked on the drawings may actually affect power generation.

There are also changes in the surrounding environment that could not be fully considered at the design stage. For example, a building may have been erected on adjacent land, trees may have grown, new equipment may have been installed nearby, or the terrain or fences may have changed; over time these factors can alter shading, ventilation, and soiling conditions. When checking the difference between predicted values at the design stage and actual results, you need to confirm whether the conditions at the time of design and the current conditions are the same.

Design loss assumptions are also subject to review. In power generation forecasts, various losses are anticipated, such as temperature losses, wiring losses, equipment conversion losses, soiling losses, and shading losses. If these assumptions were more optimistic than the actual conditions, actual generation may appear lower than predicted. Conversely, if losses that were estimated to be larger at the time of forecasting turn out to be smaller in reality, actual generation may exceed the forecast.

What's important here is not to immediately assume that a difference between predicted and actual values is due to equipment failure. Predictions are estimates based on assumed conditions, and differences can arise if on-site conditions change. Before making improvements, reconcile the conditions used for the prediction, the current on-site conditions, and the actual performance data to determine which differences can be explained as a decrease in power generation.

For a site condition survey, it is effective to organize survey data, site photographs, equipment layout drawings, records of shading, equipment configurations, and system diagrams. Especially when considering renovations or expansions, proceeding with design without accurately understanding the existing conditions can lead to misestimating the expected effectiveness of improvements. Verifying existing elevations, slopes, obstacles, shadow extents, inspection walkways, and equipment placement improves the accuracy of improvement proposals.

Improvements to increase power generation start with an accurate understanding of the existing equipment. By checking the discrepancies between the assumptions made at the design stage and the current conditions, it becomes easier to explain where the differences between predicted and actual performance originate. Also, for future retrofit plans, you can make decisions based on current site conditions rather than relying on past assumptions.

Improvements to Increase Power Generation Begin with Prioritizing the Causes

After identifying the causes of low power generation, it is important not just to list improvement measures but to prioritize them. Methods to increase power generation include cleaning, tree management, equipment inspection, settings verification, wiring repair, shading countermeasures, equipment upgrades, and layout review. However, it is not realistic to do everything at once. First, you should consider measures that have a large impact on generation, are easy to explain as the cause, and are easy to verify the effects after implementation.

When considering priorities, assess the combined impact on power generation, ease of response, safety, likelihood of recurrence, and operational constraints. For example, if a specific piece of equipment is offline, its impact on power generation is clear and improvements from restoration are easy to verify, so its priority will be high. If tree shading significantly affects winter mornings, you need to decide the timing of management measures with seasonality in mind. If soiling is localized and the affected area is limited, it is important to estimate the cleaning effectiveness in advance and narrow the necessary scope.

When considering improvement measures, record the pre-measure state. By keeping records of pre-measure power generation, solar irradiance conditions, the scope of the target, on-site photographs, anomaly history, and values for each system, you can compare the effects after the measures. If improvements are implemented without records, even if power generation increases, you cannot determine whether the increase is due to the measures or to differences in the weather. In power generation improvement, it is important not only to implement measures but also to be able to explain their effects.

Multiple causes can also coexist. If there is shading, dirt, and unstable output from some equipment, fixing just one of those issues may not increase power generation as much as expected. In such cases, rather than attributing the problem to a single cause, proceed by isolating and addressing causes in order of their impact. For example, first resolve any equipment outages, then reassess the effects of shading and dirt; performing checks in stages makes it easier to make a decision.

The verification period after improvements is also important. If you judge based only on the few days immediately after the measures, the results can be easily affected by differences in weather. If possible, select days with similar solar irradiance conditions for comparison, and check generation curves not only by day but also by time of day. For cleaning, focus on sunny days before and after cleaning; for shading countermeasures, focus on the time periods when shading occurred; and for equipment recovery, focus on the generation from the system that had been offline.

Improvements to increase power generation are not about implementing measures you happen to think of, but a process of identifying causes, setting priorities, and verifying effectiveness. By following this process, it becomes easier to explain the measures and to advance internal reporting and consensus-building with stakeholders.

Summary: Evaluate power generation improvements by measuring and comparing

The causes of low power generation are not necessarily singular. Some can be explained by natural conditions such as solar irradiance and seasonal variations, while others—such as shading, soiling, equipment shutdowns, output limitations, wiring faults, or deviations from the assumptions made during design—become apparent through on-site inspections and data review. When you want to increase generation, it is especially important not to rush into corrective measures but to first isolate the causes.

In practice, there are many situations where it is difficult to judge based on total generation alone. By breaking it down by month, day, time of day, equipment, and system, it becomes easier to explain why generation is low. Checking whether it is low only in the morning, only in the afternoon, doesn’t improve on sunny days, is low only in a particular system, or is uniformly low across the whole site will change which onsite locations you should inspect.

Also, to judge the effectiveness of improvements, comparing the before-and-after states of countermeasures is indispensable. When performing tasks such as cleaning, shadow countermeasures, equipment recovery, wiring checks, or configuration reviews, you need to record the state before implementation and ensure that comparisons can be made under the same conditions afterward. If records are retained, it becomes easier to explain the effects of the improvements to relevant parties and helps guide decisions on subsequent measures.

In improving power generation, accurately understanding the site conditions is the starting point. By organizing the panel layout, shadow coverage, tilt, surrounding obstructions, equipment locations, and system configuration, and verifying these against the generation data, the areas that need improvement become clear. Rather than relying on intuition, measuring, comparing, and narrowing down the causes is a reliable approach to increasing power output.

If you feel that a photovoltaic system's power generation is low, first check these six factors in order: solar irradiance, shading, dirt, equipment, wiring, and design assumptions. By accurately assessing the current situation and preparing to compare before-and-after conditions, you can improve the accuracy of decisions about power output improvements. If you want to efficiently record on-site conditions, organize the causes of generation decline, and perform pre-renovation checks, using surveying and inspection support tools that handle condition assessment and generation data organization is also an option.