What Causes Low Power Generation? 7 Factors to Check for Solar Power

When you feel that a solar power generation system's output is low, it is not appropriate to immediately assume equipment failure. Power output is affected by multiple factors such as solar irradiance, weather, shading, soiling, equipment condition, wiring, design conditions, and operational conditions. In practice, especially, the order in which you should check things changes depending on whether the output is lower than the same month of the previous year, lower than the design simulation, or lower compared to surrounding systems.

This article organizes seven factors that on-site personnel should check when solar power generation output is low, covering initial checks through isolating the cause from a practical perspective. Rather than relying on intuition alone, checking solar irradiance, generation performance, equipment condition, and site environment in that order makes it easier to narrow down the cause.

Table of Contents

• Standardize comparison conditions before concluding that power generation is low

• Factor 1 Solar irradiance and weather conditions differ from assumptions

• Factor 2 Shading is causing the output of some panels to drop

• Factor 3 Dirt or deposits on panel surfaces are reducing the amount of light received

• Factor 4 Panel degradation or damage is reducing output

• Factor 5 Faults in power conditioners or protective devices

• Factor 6 Wiring or connection faults are causing losses

• Factor 7 Design conditions or operational conditions are affecting generation performance

• Procedure for isolating causes of reduced generation on site

• Summary Verify each cause one by one and visualize the reasons for low generation

Align comparison conditions before concluding that power generation is low.

When you feel that the output of a solar power system is low, the first thing to check is what you are comparing it to. Whether it is lower than yesterday, lower than the same month last year, lower than the design simulation, or lower than nearby plants will change the possible causes. If you start investigating causes without aligning the comparison conditions, you may mistake natural weather-driven variation for equipment faults, or conversely overlook a persistent defect.

Solar power generation is greatly affected by the amount of solar irradiance. Even on days that look sunny, if there are many thin clouds, if the whole sky is hazy and white, or if yellow dust (Asian dust) or smoke haze is present, the power output may be lower than the apparent brightness would suggest. In addition, many photovoltaic modules tend to see their output decrease as temperature rises, so even with sufficient irradiance they may not increase as much as expected during the high temperatures of summer. It is important to check not only the power generation but also the solar irradiance, air temperature, rainfall, snowfall, and wind conditions for the same period.

In practice, separating monthly generation, daily generation, and generation by time of day makes it easier to organize causes. If monthly generation is low, possible causes include adverse weather, long-term soiling, changes in shading, prolonged equipment outages, and so on. If only specific days show low generation, consider bad weather, output curtailment, inspection or maintenance work, power outages, or communication loss. If generation is low only in the morning or only in the afternoon when viewed by time of day, check for shading, orientation, racking layout, and the effects of surrounding structures.

When comparing power plants, it is also important to look at generation per unit of installed capacity. If you compare plants with different installed capacities using only total generation, the difference in scale will be directly reflected in the results. Checking the value obtained by dividing by installed capacity evens out scale differences to some extent for comparison. However, if installation angle, orientation, region, surrounding environment, or maintenance conditions differ, generation per unit of capacity will not be under exactly the same conditions. Treat comparisons as clues to anomalies, and ultimately make a judgment by combining on-site conditions with equipment data.

Factor 1: Solar radiation and weather conditions differ from assumptions

As a cause of low power generation, the first things to check are solar irradiance and weather conditions. Photovoltaic power generation converts sunlight into electricity, so if solar irradiance is low, power output will also decrease. It is natural for output to be lower on cloudy or rainy days, but in practice you should be aware that differences can appear even in weather conditions that are intermediate between clear and cloudy. On days when thin clouds are spread across the sky or the entire sky is hazy and white, it may look bright to the eye, yet the power output can fall short of expectations.

When annual or monthly power generation is low, check how the weather during that period compared with the climatological average or the same month of the previous year. Prolonged rain during the rainy season, overcast skies associated with approaching typhoons, winter snowfall, and reduced sunshine hours all affect power generation. On a monthly basis, a few days to several weeks of unsettled weather can also depress monthly generation. Therefore, do not conclude equipment failure from a single month's results; it is important to check trends over multiple months and their relationship with solar irradiance.

Air temperature also affects power generation. Solar panels tend to produce more electricity the more sunlight they receive, but their output generally falls as panel temperature rises. In summer, although solar irradiance is higher, output reductions due to high temperatures are more likely to occur. Conversely, winter can be advantageous in terms of temperature, but shorter sunlight hours and a lower solar altitude mean total power generation may be harder to increase depending on the region and installation conditions. If you feel the generation is low, you need to consider not only the number of sunny days but also temperature and seasonal factors.

If a pyranometer or weather sensor is installed at the plant, compare its data with the power output. If power output is low on days with low solar irradiance, the cause is likely environmental conditions. Conversely, if solar irradiance is sufficient but only the power output is low, suspect other causes such as shading, soiling, equipment faults, wiring issues, or operational restrictions. Also verify the reliability of the measurement data, since the pyranometer itself may be soiled, relocated, or have missing data.

Factor 2: Some panels' output is reduced due to shading

One often overlooked cause of reduced solar power generation is the effect of shading. Depending on the circuit configuration and equipment arrangement, solar panels can have the output of the affected circuit reduced even if only part of a panel is shaded. A shadow that seems small across an entire plant can, if it occurs every day during specific time periods, lead to monthly or annual generation losses that are hard to ignore.

Causes of shading include nearby buildings, utility poles, overhead lines, fences, trees, weeds, adjacent equipment, slopes on developed land, distant mountains and structures, and so on. Even if there was no problem at the time of installation, shading conditions can change due to tree growth, construction of nearby structures, or addition of equipment. Also, because the sun’s elevation changes with the seasons, shadows that are not an issue in summer can extend in winter and reduce power generation. If power output drops in the morning or afternoon during winter, check for the presence of shadows.

When checking for shading, it is effective to look at power generation by time of day. If generation is low only in the morning, only in the evening, or the generation curve drops unnaturally at the same time every day, shading may be the cause. For on-site inspection, on a sunny day check the panel surface for shadows at separate times: morning, around noon, and afternoon. Because low-angle morning and evening light tends to create long shadows, judging that there are no shadows based only on observations around noon can lead to overlooking actual power losses.

In ground-mounted solar power plants, attention should also be paid to shadows caused by weeds. If grass grows in front of the panels, it casts shadows on parts of the lower edge. Even if the shadow appears small, if it continues for a certain period it can contribute to reduced power generation. Because weed growth varies by season, the impact of shading can suddenly appear from spring to summer. Check the weed-control area, any remaining cuttings after work, and whether any grass that casts shadows remains under the panels or around the mounting racks.

In the case of roof installations, chimneys, antennas, rooftop equipment, neighboring buildings, railings, and lightning protection equipment can cause shading. Renovations or the addition of equipment to a building can also create shading later on. Shading effects are a factor that can cause discrepancies between assumptions made during the design phase and the actual conditions after operation. When power generation is low, it is important not to rely solely on past design drawings but to verify the current on‑site conditions.

Factor 3: Reduced incident light due to dirt or deposits on the panel surface

Dirt on the panel surface is another factor to check when investigating low power output. Because solar panels generate electricity by receiving light on their surface, dirt, dust, bird droppings, fallen leaves, pollen, yellow sand (Asian dust), mud splashes, and deposits from exhaust can reduce the amount of light received and lower power generation. Some dirt will be washed away by rain, but not all dirt comes off naturally. In particular, localized, stuck-on dirt can remain for long periods.

The reduction in power generation caused by soiling varies depending on the installation environment. Near farmland or newly developed land, soil dust is easily stirred up, while along roads dust and exhaust-derived grime tend to adhere. In areas close to the sea, deposits containing salt can become a problem, and where trees are nearby, fallen leaves, sap, and bird droppings are likely to have an impact. Near factories or material storage yards, dust from nearby operations can also accumulate on panel surfaces.

When checking for soiling, determine whether the drop in power output is occurring across the entire plant or only in certain circuits. If the system is thinly soiled overall, the plant’s total output may gradually decrease. Conversely, localized soiling such as bird droppings or fallen leaves can concentrate the impact on specific panels or circuits. Because uneven soiling can also lead to localized heating, inspections are required not only for power output but also for safety.

When checking for soiling, do not only inspect the panel surface up close; also check for differences between columns and the lower edge of the tilt. After rainwater runs off, soiling can accumulate at the lower edge and remain in bands. Installations with a gentle slope tend to have poor drainage, making soiling more likely to remain. Confirming the drainage direction, wind direction, and the direction of dust inflow from the surroundings makes it easier to identify locations prone to soiling.

If cleaning is to be carried out, it is essential to follow the equipment specifications, the manufacturer's recommendations, and safety requirements. Cleaning by incorrect methods may damage the glass surfaces, frames, seals, and wiring. Work on roofs or at height carries a risk of falling. Even if soiling is believed to be the cause of reduced power generation, the necessity of cleaning should be determined by taking into account generation loss, work safety, equipment conditions, and the surrounding environment.

Factor 4: Output Is Reduced Due to Panel Degradation or Damage

Because solar panels are installed outdoors for long periods, they can experience performance degradation and physical damage over time. Some degree of aging is to be expected, but if power output is significantly lower than expected, or if only specific systems are showing reduced output, individual panel faults should also be checked. Some degradation or damage can be identified from appearance, while other issues are difficult to determine from appearance alone.

Visible anomalies that can be easily confirmed by appearance include broken glass, frame deformation, surface discoloration, burn marks, damage to the backsheet, abnormalities around the junction box, and cable damage. Panels can be damaged by typhoons, flying debris, snow load, animal damage, impacts during installation, and other causes. If the glass is cracked, not only can power generation decrease, but there is also a risk of insulation failure and safety issues, so prompt inspection is necessary.

Some faults that are difficult to detect by appearance include microscopic internal cracks, cell defects, localized overheating, and deterioration of connection points. Such anomalies can sometimes be identified by comparing power generation data or by using specialized inspection equipment. If only part of a circuit that should be under the same conditions shows reduced output, there may be a problem with the panel itself or with the surrounding connections. If an anomaly is suspected, have a professional carry out an inspection and consider replacement or repair as necessary.

Declines in output due to aging typically progress gradually over the long term rather than occurring suddenly and dramatically. For that reason, it is important not to judge performance solely on a single year’s generation but to examine trends over multiple years. If generation is decreasing year by year even after accounting for solar irradiance conditions, panel degradation, accumulation of dirt, reduced equipment efficiency, increased wiring resistance, and similar factors may be contributing in combination. In long-term operation, record changes in generation and regularly check for declines that exceed the normal range of variation.

When you suspect a panel malfunction, you must also pay attention to safety. Because solar panels generate electricity when exposed to light, they may be producing voltage even if they appear to be turned off. Avoid touching damaged panels, wet equipment, or submerged equipment carelessly, and carry out inspections in accordance with proper qualifications and procedures. When investigating reduced power output, it is essential not only to determine the cause but also to consider measures to prevent electric shock, falls, fires, and equipment damage.

Factor 5: Malfunctions in power conditioners or protective devices

When power generation is low, you need to check not only the solar panels but also the condition of the power conversion equipment and protective devices. Electricity generated by the solar panels is used or fed into the grid after passing through equipment for power conversion and protection. If these devices have faults, stoppages, output restrictions, temperature rises, or incorrect settings, the power output will be low even if there is no problem with the panels themselves.

First, check whether the equipment has stopped, whether any fault indications are displayed, or whether there is any alarm history. If power generation is close to zero throughout the day or if generation suddenly stops from a specific time, the equipment may have stopped. There are multiple possible reasons for a stop, such as a temporary grid anomaly, protection operation, internal fault, overheating, maintenance work, or communication failure. Check the on-site displays, monitoring screens, and operation history to determine when it started, which equipment is affected, and what condition it is in.

Next, check whether the output is being limited at the upper end. Depending on equipment capacity, grid conditions, settings, temperature conditions, and so on, output may not exceed a certain level. When looking at the generation curve on a sunny day, if the top appears flattened or cut off, the output may have reached some kind of upper limit. This is not necessarily a fault, but you need to verify whether it falls within the range assumed in the design or is an unexpected restriction caused by settings or equipment condition.

Output reduction caused by temperature rise is also a subject to check. Because power conversion equipment heats up during operation, if the installation environment has poor ventilation, the intake/exhaust vents are blocked, the ambient temperature is high, or cooling performance has degraded, the equipment may reduce output to protect itself. If power generation fails to increase only around midday on sunny summer days, temperature conditions may be involved. Check whether objects are placed around the equipment, whether ventilation is ensured, and whether dirt has accumulated on filters or ventilation openings.

Do not overlook the condition of protective devices and switchgear. If a circuit breaker has tripped, a fuse has blown, part of the circuitry inside a junction box has stopped, or an insulation-monitoring alarm has been triggered, the plant’s overall power output—or part of it—will be reduced. It is important to check whether generation is low across the whole plant or only in some systems, and to isolate the issue at the equipment and circuit levels.

Factor 6: Losses caused by faults in wiring and connection points

In photovoltaic systems, panels, junction boxes, power conversion equipment, and transformer and switchgear equipment are connected by wiring. If there are faults in the wiring or connection points, the generated electricity may not flow properly, which can lead to reduced power output and safety issues. When investigating the causes of low power generation, it is necessary to check not only the visible equipment but also the condition of the wiring routes and connection points.

Examples of wiring faults include poor connector contact, loose terminals, cable damage, chewing by animals, deterioration from ultraviolet exposure, wind and rain, stress from bending or pulling during installation, water ingress, and corrosion. These can appear as a sudden major outage, or the resistance may gradually increase and manifest as power generation loss or heat. Wiring exposed outdoors in particular is continuously subjected to wind, rain, sunlight, and temperature changes, so regular inspections are necessary.

Faults in wiring or connections are suspected when only certain circuits show reduced power generation. Compare multiple circuits that have the same orientation, the same tilt, and the same irradiance conditions; if only some show lower output, focus inspections on that circuit’s panels, wiring, junction boxes, and protective devices. Conversely, if the entire system shows similarly low output, prioritize considering factors such as irradiance, soiling, upstream equipment, and operating conditions. Comparing circuits is effective for narrowing down the cause.

Abnormalities at connection points may be accompanied by heating. Heating can lead not only to power generation losses but also to equipment damage and fire risk, so caution is required. During inspections, check for discoloration, burn marks, unusual odors, deformation of resin parts, loose terminals, and traces of rainwater ingress. However, inspections of energized equipment are hazardous and must be performed in accordance with appropriate qualifications and procedures. When investigating causes of decreased power output, it is important to consider both generation performance and safety simultaneously.

The condition of wiring route management also affects the risk of reduced power generation. If cables are in contact with the ground, located in drainage paths, have lost their fastenings and are swaying in the wind, or are positioned where they can be easily damaged by mowing or snow removal, they can cause future malfunctions. Rather than checking only after power output has declined, performing daily inspections to check the fastening and protective condition of the wiring makes it easier to prevent problems before they occur.

Factor 7: Design conditions and operational conditions are affecting power generation performance

The cause of perceiving low power output is not limited to equipment faults. The assumptions made during design and the operating conditions may also differ from the actual generation performance. When comparing simulated generation with actual results, you need to check what kind of solar irradiation data, loss assumptions, installation tilt, orientation, shading conditions, temperature conditions, and equipment specifications the simulation assumed. If those assumptions differ from the actual site conditions, the actual output may appear low.

Installation angle and orientation affect annual energy output and time-of-day energy output. In general, the direction and tilt of solar panels change the seasons and times of day when they produce power most effectively. It is important to verify that the angles and orientations on the design drawings match the actual installation. Small differences alone do not necessarily lead to large reductions, but when combined with other factors such as shading, dirt, or equipment conditions, they can contribute to reduced energy output.

The relationship between installed capacity and the capacity of the power conversion equipment is another point that should be checked. In designs where the capacity of the power conversion equipment is smaller relative to the panel capacity, output can reach its upper limit during periods of high solar irradiance. This may be adopted as a design approach, but if those reviewing actual performance do not understand that assumption, they may interpret the capped generation on clear days as an abnormality. When generation seems low, it is necessary to distinguish whether this is a design-intended output characteristic or an unexpected constraint.

Operational conditions to check include output curtailment, shutdowns for inspections, grid-side constraints, remote monitoring communication failures, and measurement equipment malfunctions. Even if the power generation equipment itself is operating normally, the amount of electricity sold or the recorded generation may appear low for grid or operational reasons. Especially when judgments are based on monitoring data, missing communications or abnormal measurements can cause the displayed data to differ from the actual generation status. Before concluding that generation is low, also verify that the measurement data itself is being correctly collected.

The simulation values are estimates based on certain assumptions. Actual weather, equipment condition, surrounding environment, and maintenance status vary from year to year. A result that is lower than the simulation does not immediately mean a failure; it is important to check solar irradiance and operating conditions and then review the measured performance. Conversely, if performance continues to decline over the long term even after accounting for solar irradiance and other factors, equipment degradation or environmental changes may be the cause. Separating and organizing design values, measured performance, and site conditions improves the accuracy of root-cause investigations.

Procedure for isolating the causes of reduced power generation on-site

When investigating the cause of low power output, it is important not to carry out ad hoc inspections based on whatever comes to mind, but to isolate causes in order from broad factors to more specific ones. First, confirm whether the drop in power output is temporary or persistent. The suspected causes differ between a drop that lasts only one day and one that continues for several weeks to several months. If it is temporary, consider weather, shutdowns, output curtailment, and work-related impacts; if it is persistent, you need to examine in detail soiling, shading, degradation, equipment faults, and wiring defects.

Next, check whether the drop in power generation is occurring across the entire plant or only in some systems. If it is low overall, prioritize checking solar irradiance, weather, overall soiling, upstream equipment, and operating conditions. If only part is low, focus on the corresponding panel strings, circuits, junction boxes, power conversion equipment, and wiring. Even just distinguishing between whole-plant and partial issues makes it easier to narrow the scope of investigation.

Power generation curves by time of day are also an important clue. If generation is low only in the morning, consider shadows on the east side; if it is low only in the afternoon, consider shadows on the west side; if output plateaus around midday, consider equipment capacity, output limits, or the effects of temperature rise. The generation curve on a clear day with few clouds is useful for assessing the condition of the equipment. Compare it with the normal curve and check for any unnatural dips, steps, sudden stops, or left-right asymmetry.

During on-site inspections, match the locations suspected from the power generation data with the actual equipment. If shading is suspected, confirm on site whether shading occurred during the relevant time period. If soiling is suspected, check the panel surface, bottom edges, and the surrounding environment. If equipment malfunction is suspected, check displays, alarms, operation history, and the temperature environment. If wiring faults are suspected, check connection points, cable securing, damage, water ingress, and signs of animal damage. By going back and forth between the data and the site, you can reduce the likelihood of overlooking causes.

Recording investigation results is also important. If you document when, on which system, what kind of drop in power output occurred, what was checked, and what the findings were, it will be useful for future comparisons. By linking and managing photos, generation data, inspection results, weather information, and response histories, it becomes easier to track trends in declining power output. Because solar power generation systems are long-term assets, it is important not to treat inspection as a one-time task but to continuously visualize their condition.

Summary: Check each cause of low power generation one by one and visualize them

The causes of low solar power generation output should not be attributed to a single factor; it is important to check, in order, solar irradiance, weather, shading, soiling, panel condition, power conversion equipment, wiring, design conditions, and operational conditions. Rather than judging solely by the generation figures, it is necessary to compare them with the solar irradiance conditions and on-site circumstances for the same period. Behind a result of low generation there can be temporary fluctuations due to natural conditions, as well as persistent losses caused by equipment malfunctions or environmental changes.

First, standardize the comparison conditions and verify whether the power output is actually low. Next, review monthly, daily, and hourly data to identify the scope and timing of the decline. Then check whether the decline can be explained by solar irradiance or weather, whether there is shading or soiling, whether the panels or equipment show any abnormalities, and whether there are problems with wiring or connections. Finally, by comparing these findings with the assumptions made during design and with operational constraints, you can narrow down the cause more precisely.

When responding to declines in power output, the accuracy of on-site inspections and the way records are kept make a big difference. By organizing photos, location information, generation data, and inspection histories so stakeholders can share what is happening and where, it becomes easier to carry out root-cause investigations and make improvement decisions. In the operation and maintenance of solar power installations, it is important not only to detect anomalies quickly but also to accumulate information in a form that can be compared with past conditions.

When you think the power output is low, it is important not to consider generation data and on-site conditions separately. Distinguish and organize whether the decline shown in the data is due to an actual equipment malfunction, operational conditions such as weather or curtailment, or measurement and communication issues. Rather than judging a decrease in power output by intuition, connecting and verifying the data with on-site conditions leads to stable operation of a solar power plant.