9 Cause-Specific Improvement Measures When Power Generation Drops

When operating a solar power system, you may at some point feel that power generation has decreased. There are various patterns of decreased generation, such as being lower than the same month in the previous year, not increasing despite clear skies, being weak only in the morning and evening, or being low on only a specific roof surface.

When aiming to increase generation, what's important is not to immediately consider replacing equipment or adding panels, but to systematically isolate what is causing the drop. In this article, aimed at practitioners searching for "how to increase power generation", we explain nine causes and countermeasures to check when solar power generation output has decreased.

Table of Contents

• When power generation drops, isolate by cause

• Cause 1: Temporary decline due to weather or seasonal variations

• Cause 2: Generation loss due to increased shading

• Cause 3: Dirt on panel surfaces and bird droppings

• Cause 4: Temporary obstruction by fallen leaves or snow

• Cause 5: Output decrease due to high temperatures

• Cause 6: Mismatch in orientation, tilt, and layout conditions

• Cause 7: Faults in wiring, connections, or power conversion equipment

• Cause 8: Equipment degradation and aging

• Cause 9: Mixing up self-consumption with generation

• Improvement measures to increase power generation should be implemented in order of priority

• Summary

When power generation drops, isolate causes by category

When you feel that the output of a solar power system has declined, the first thing to check is which benchmark you are comparing it to. Whether it is lower than last month, lower than the same month last year, lower than the simulation performed at installation, or lower than the typical generation for the same system capacity will determine which causes you should investigate. Because generation is affected by weather and seasonal variations, simply comparing it to the previous month alone does not allow you to make a correct judgment.

For example, a decrease in power generation from summer to autumn and from autumn to winter is a natural variation. Winter has shorter hours of sunlight and a lower solar altitude, so it is a season when annual power generation tends to decline. On the other hand, if generation is extremely low only in winter, you need to consider not only the hours of sunlight but also the effects of shading and snow accumulation. If generation does not increase in summer, check not only the solar irradiance but also rises in panel temperature, soiling, and the condition of the equipment.

When isolating a decline in power generation, examining data by month, time of day, installation surface, and system makes it easier to narrow down the cause. Viewing data by month reveals seasonality. By time of day you can identify patterns such as being weak only in the morning, dropping at midday, or falling early in the evening. Looking by installation surface or by system shows whether the entire installation is underperforming or only part of it. If the drop is system-wide, suspect weather, common equipment, temperature, or overall soiling. If only part is underperforming, check for shading, soiling, wiring, connections, or individual equipment faults.

To increase power generation, it is important not to start countermeasures before identifying the cause. If dirt is the cause, cleaning is effective; if shading is the cause, you need to review the source of the shade. If equipment downtime is the cause, cleaning will not improve the situation. If a lack of increase in self-consumption is mistakenly interpreted as a drop in power generation, you should check the facility’s electricity usage time periods rather than the generation equipment. By isolating causes, you can reduce unnecessary work and make it easier to choose effective improvement measures.

Cause 1: Temporary decrease due to weather and seasonal variations

The first thing to check when power output drops is whether it is a temporary decrease caused by weather or seasonal variations. Solar power generates electricity from sunlight, so output falls in months with few sunny days, in months with a lot of rain, or during periods of prolonged cloudiness. This is not an equipment malfunction but a fluctuation caused by natural conditions.

A common misconception among beginners is simply comparing power generation with the previous month. It is natural for generation to increase from spring to summer and to decrease from autumn to winter. In winter, because daylight hours are shorter and the sun’s altitude is lower, you may not generate as much even on clear days compared with summer. Also, months affected by the rainy season, prolonged rain, or typhoons have reduced solar irradiance and tend to see drops in generation.

As a corrective measure, first compare the power generation with the same month of the previous year or the same season. Furthermore, extracting and comparing only sunny days makes it easier to assess the condition of the equipment apart from the effects of weather. If simulation values are available, compare them with the expected monthly generation. By comparing not only the annual total but also the monthly expected values and actual results, it becomes easier to determine whether the variation is natural or an abnormal decline.

When the cause is weather or seasonal variation, there is often no major problem with the equipment. However, if generation is clearly lower than past performance for the same season, does not increase even on sunny days, or drops at the same time every day, other causes need to be checked. Separating declines caused by weather from those caused by equipment or site conditions is the first step to improving power generation.

Cause 2: Power generation loss due to increased shading

Shadows are a common cause of reduced power generation. When a solar panel is shaded, it cannot receive sufficient solar radiation, and its power output decreases. The impact of shading depends not only on the shaded area but also on the time of day the shading occurs, the season, panel layout, and connection configuration. Even a small shadow can have a significant effect on annual energy production if it falls during periods when generation would otherwise be high.

Causes of shading include surrounding buildings, rooftop equipment, penthouses, railings, piping, air-conditioning equipment, ventilation equipment, trees, utility poles, signs, slopes, and terrain elevation differences. Even if there was little shading at the time of installation, shadows may increase after several years as trees grow, rooftop equipment is added, or new structures are built nearby.

Shadows change significantly with the seasons. In summer, because the sun is higher in the sky, shadows are short, and an on-site inspection may appear to show no issues. However, in winter the sun’s altitude is lower and shadows lengthen. If power generation drops dramatically only in winter, you need to check winter shading as well as the shorter daylight hours.

As a countermeasure, check the power generation data by time of day. If generation is weak only in the morning, suspect shading on the east side; if generation drops early in the evening, suspect shading on the west side; if there is an unnatural dip around midday, suspect shading from equipment or a rooftop structure near the panels. If you have generation data by installation surface, check whether only a particular surface has lower output.

On site, record the sources of shade, the time periods when shade occurs, and the extent of the shading. If trees are the cause and they are within a manageable scope, consider pruning or trimming branches. If rooftop equipment or surrounding buildings are the cause, avoid heavily shaded areas when next reviewing the layout or expanding the installation. Rather than forcing additional panels into shaded areas, prioritizing areas with less shade can lead to improvements in effective power generation.

Cause 3: Dirt on the panel surface and bird droppings

Dirt on the panel surface and bird droppings are also a major cause of reduced power generation. Since solar panels generate electricity by receiving sunlight on their surface, when dirt adheres it makes it harder for light to reach the panels and their output decreases. Because dirt often accumulates gradually, it can be difficult to notice as the cause of the drop in power generation.

Causes of dirt include sand and dust, pollen, yellow sand, fallen leaves, bird droppings, exhaust-derived grime, particulate matter, and residues remaining after snowfall. In areas with many trees nearby, surfaces are more likely to be affected by fallen leaves and bird droppings. If there are unpaved areas, construction sites, farmland, or roads with heavy traffic nearby, soil dust and particulates are more likely to adhere. On rooftops, dirt can tend to accumulate near exhaust equipment and vents.

In particular, bird droppings tend to locally block sunlight and can repeatedly adhere to the same spots. If there are structures or trees nearby that attract birds, it is necessary to regularly check the condition of the panel surfaces. Some dirt can be washed away by rain, but bird droppings and adhered dust may not be easily removed by rain alone.

As a countermeasure, cross-check power generation data with on-site soiling. If power generation is gradually declining, is low only on a particular surface, or does not recover after rain, suspect soiling. When carrying out cleaning, prioritize safety and protection of the equipment. Because rooftop work is hazardous, it is important not to try to handle it in-house. Also avoid methods that scratch the panel surface or otherwise adversely affect the equipment.

After cleaning, check whether power generation has improved under similar weather conditions. By comparing power generation before and after cleaning, you can understand how much the dirt had affected it. Measures to address soiling are not a one-time task; it is important to schedule inspections according to seasonal causes such as pollen, fallen leaves, and dust.

Cause 4: Temporary Shading from Fallen Leaves and Snow

When fallen leaves or snow cover the surface of the panels, power output drops significantly. These are temporary factors, but because they can recur every year in the same season, they should not be overlooked as causes of reduced output. To increase power generation, you need to check not only fixed shading but also temporary obstructions that block sunlight, such as fallen leaves and snow.

Fallen leaves are a factor that tend to occur on roofs and land with trees nearby. They are not only more common in autumn but can also blow onto panels on windy days or after pruning. Dry leaves may be blown away, but if they become wet from rain and stick to the panel surface, they are more likely to remain. When they accumulate at the lower edge of panels or near the frames, they can cause a reduction in power output.

In snowy regions, there are periods when panels cannot generate power while snow is on them. Not only during snowfall, but also the period after snowfall when snow remains affects power generation. When the roof or mounting structure has a shallow tilt, snow may be less likely to slide off. Snow that has slid off can accumulate in front of or beneath the panels, potentially creating additional shading.

As a countermeasure, check power generation data during seasons when fallen leaves and snowfall are likely and verify it against on-site conditions. In areas with many fallen leaves, manage surrounding trees and inspect drainage outlets. In areas with heavy snowfall, check surfaces where snow tends to remain, areas where snow may fall, snow storage spaces, and inspection routes. Forced snow removal or work on roofs carries risks, so safety must be the top priority when making decisions.

Reductions caused by fallen leaves and snow appear as temporary drops in power output. They are often overlooked as seasonal declines, but if they recur every year they should be incorporated into maintenance planning. To increase power output, it is effective to anticipate these seasonal factors and decide in advance the timing of inspections and cleanings.

Cause 5: Reduced output due to high temperatures

Although solar power generation tends to increase with greater sunlight, output may decrease when panel temperatures rise. This temperature-related drop in generation requires particular attention in summer. If it is sunny but the system is not producing as much as expected, or if you find it generates more stably in spring and autumn, temperature-related losses may be involved.

Panels installed on roofs can be prone to higher temperatures depending on the heat of the roofing material and how air stagnates around them. If ventilation behind the panels is poor, or if they are surrounded by rooftop equipment that prevents airflow, heat can build up. Using low mounting racks on a flat roof can also make the panels susceptible to heat from the roof surface.

Even for ground-mounted installations, if grass grows and obstructs ventilation or surrounding structures cause air to stagnate, the temperature environment can be affected. Because temperature losses are hard to see, it is important to infer them from power generation data. If power output does not increase even on summer days with high solar irradiance, or if the output fails to reach its peak around midday, check for temperature effects.

As mitigation measures, ensure that airflow behind the panels is not obstructed, manage surrounding vegetation and obstacles, and prevent heat from accumulating around equipment. However, changes to rack height or tilt angle affect wind loads, constructability, inter-row shading, and maintainability, so decisions should be made comprehensively rather than prioritizing temperature measures alone.

It may be difficult to completely avoid output reductions caused by high temperatures. However, by understanding temperature-related losses, it becomes less likely that a reduction in summer power generation will be mistaken for an equipment malfunction. Also, when installing new systems, expanding capacity, or upgrading, considering installation conditions that facilitate heat dissipation can help stabilize annual power generation.

Cause 6: Orientation and tilt angle do not match power generation conditions

If the orientation or tilt angle does not match the power generation conditions, it can also cause lower power generation. Solar panels' power generation varies depending on the angle at which they receive sunlight. Even with the same system capacity, if the orientation or tilt differs, the annual generation, monthly generation, and generation by time of day will change.

Generally, surfaces that face more toward the south tend to produce more electricity over the year. However, east- or west-facing surfaces are not necessarily bad. East-facing surfaces tend to generate more in the morning, and west-facing surfaces tend to generate more in the afternoon. If a facility’s electricity demand is skewed toward the morning or the afternoon, generation from east- and west-facing surfaces can help with self-consumption. To increase generation, it is important not only to consider annual energy yield but also the alignment with the facility’s demand time periods.

Tilt angle affects seasonal power generation, soiling, and snow accumulation. A steeper tilt can increase exposure to winter sunlight, but attention must be paid to inter-row shading, wind effects, and installation spacing. A shallower tilt can make it easier to install more panels, but soiling and snow are more likely to remain.

With existing installations, it may be difficult to substantially change a roof’s orientation or pitch. In such cases, check the power output of each surface and see whether issues are concentrated on surfaces with poor generation conditions. When expanding or upgrading, prioritizing roof surfaces with better conditions and areas with less shading can be expected to improve power generation.

For land projects and flat roofs, it can be easier to compare orientations and tilt angles. Use simulations to compare multiple layouts and check annual generation, monthly generation, self-consumption, surplus electricity, inter-row shading, and maintainability. Rather than the theoretical maximum generation, it's important to choose conditions that can be constructed on site and managed over the long term.

Problems with orientation and tilt angle can sometimes be difficult to improve in the short term. However, understanding the causes allows you to make better designs during the next refurbishment, expansion, or upgrade. When power generation drops, you need to check not only the condition of the panels but also the installation conditions themselves.

Cause 7: There are losses or faults in wiring and power conversion equipment

Even if solar panels are receiving sunlight, losses or faults in the wiring or power conversion equipment can reduce the amount of electricity actually available for use. If generation suddenly drops or only certain circuits show low output, you need to check not only the panel surfaces and shading but also the electrical pathways.

Wiring losses occur in the process of transmitting power from the panels to the equipment and from the equipment to the facility-side equipment. When wiring distances are long, connection points are difficult to inspect, or wiring routes are complex, it becomes difficult to detect losses and faults. When installing new systems or expanding existing ones, it is important to consider wiring routes and equipment placement as factors that affect power generation.

The condition of power-conversion equipment is also important. If the equipment is offline or some functions are not operating properly, even if the panels are generating power they may not be able to deliver sufficient electricity to the facility. If generation suddenly drops, check for equipment shutdowns or faults as well as weather and soiling.

Also, depending on the balance between equipment capacity and panel capacity, the output may hit a ceiling during generation peaks. A capped output is not necessarily a bad thing in itself, but it can be a cause of the amount of power generated falling short of expectations. It is important to check which time periods and to what extent the output is being limited.

As a remedial measure, check the power generation data by system, by installation surface, and by time of day. Whether output is low overall or only in certain parts will change the scope of inspection. Because verifying equipment and wiring requires specialized knowledge and safety measures, it is important for operational staff to organize the data and on-site conditions and to clarify the necessary scope of inspection.

To increase power generation, check not only the panel surfaces and shading but also the route the generated electricity takes until it becomes usable within the facility. By reviewing the condition of wiring and equipment, you may be able to recover power generation that had been lost.

Cause 8: Equipment Deterioration and Aging

Solar power generation systems are intended for long-term use, and the condition immediately after installation does not remain unchanged. If equipment deterioration and aging are not taken into account, you may feel that "power generation has decreased" when comparing with the output at the time of installation or with simulated values.

Aging-related changes include decline in panel performance, degradation of power conversion equipment, changes in the condition of wiring and connections, deterioration of mounting structures and fastenings, accumulation of dirt, and changes in the surrounding environment. These do not necessarily appear abruptly or all at once; they may gradually affect power generation.

However, it is not appropriate to immediately conclude that a drop in power output is due to equipment deterioration. First, check the weather, shading, dirt, snow accumulation, equipment outages, and wiring faults. If an unexplained decline continues despite these checks, then consider equipment deterioration or aging. It becomes easier to judge if you determine whether only a specific section is underperforming or whether the entire system is gradually declining.

As a corrective measure, we compare the simulation conditions at the time of installation and the first-year power generation data with the current power generation data. We check how they have changed by month, by time of day, and by installation surface. We also review inspection records for equipment and connection points, cleaning histories, and changes in the surrounding environment.

While you cannot completely prevent equipment degradation, noticing it early and taking action makes it easier to limit declines in power output. It is important to continue regular inspections, recording generation data, cleaning, checking equipment condition, and managing the surrounding environment.

In long-term operation, it is more important to focus on maintaining the power generation that should be achieved than on increasing output. By putting in place a management system that assumes equipment deterioration and aging-related changes, you can reduce the risk of a decline in power generation.

Cause 9: Confusing self-consumption with power generation

Among the reasons you might feel that power generation has dropped are cases where it is not the generation itself but changes in self-consumption or the perceived effectiveness of the installation. The electricity generated by solar power is not all used within the facility. You need to consider power generation, self-consumption, and surplus electricity separately.

Generation refers to the amount of electricity produced by a solar power generation system. Self-consumption refers to the portion of that electricity that was used within the facility. Surplus electricity refers to the portion of generated electricity that could not be used during the same time period. Even if generation is sufficient, if the facility's demand is low, self-consumption will not increase.

For example, if a facility's operating hours change, holidays increase, daytime equipment operation decreases, or daytime demand falls due to energy-saving measures, self-consumption can decrease even if power generation remains unchanged. As a result, the apparent benefit of the installation may seem reduced. This is not a problem with the generation equipment but a change in electricity usage patterns.

As a countermeasure, compare power generation and electricity consumption by time of day by overlaying them. Determine whether the generation itself has dropped or whether generation exists but is not being fully used. If generation is low, check for shading, dirt, or equipment issues. If generation is sufficient but self-consumption is low, check facility operating hours, daytime load, surplus energy, and the potential for battery storage or load adjustment.

Confusing self-consumption with power generation can lead to unnecessary equipment inspections or the addition of panels. Clarifying whether you want to increase generation or boost self-consumption will change the direction of the improvement measures.

Prioritize measures to increase power generation

When power generation falls, the cause is not necessarily a single one. Shadows and soiling can overlap, and in some cases winter shading and snow, or summer temperature losses and equipment condition, can affect output simultaneously. Therefore, it is important to decide on priorities and proceed with improvement measures.

First, check the generation data. Look at generation output by month, by time of day, by installation surface, and by system to identify where it is declining. Next, inspect the site and look for shadows, dirt, fallen leaves, snow accumulation, trees, rooftop equipment, devices, wiring, and inspection access routes. Combining the data with the on-site inspection makes it easier to isolate the causes.

Improvement measures should be implemented starting with those that have the greatest impact on power generation and are easiest to address. If soiling is obvious and cleaning is feasible, verify the improvement from cleaning. If the cause of shading is trees that can be managed, consider pruning or other management. If equipment shutdown is suspected, prioritize inspection. Issues with orientation or tilt are often not possible to change immediately, but they can be used to inform design improvements when expanding or upgrading.

After improvements, we always verify the effects using power generation data. We check whether generation returned after cleaning, whether generation during specific time periods improved after shading countermeasures, and whether generation by each system recovered after equipment inspections. Recording the effects of improvements makes it easier to plan future maintenance.

Measures to increase power generation should not be implemented based on intuition. By isolating the causes, setting priorities, and verifying results after implementing measures, you can reduce unnecessary work and help ensure long-term maintenance of power output.

Summary

There are various causes for reduced power generation, including weather and seasonal variations, shading, dirt, fallen leaves, snow accumulation, high temperatures, orientation and tilt, wiring and equipment malfunctions, equipment degradation, and confusion with self-consumption. To increase power generation, it is important to isolate each of these causes and address them starting with the measures that offer the greatest improvement.

For Cause 1, check for temporary reductions due to weather and seasonal variations. For Cause 2, check for generation losses caused by increased shading. For Cause 3, check for dirt or bird droppings on the panel surface. For Cause 4, check for temporary obstructions from fallen leaves or snow accumulation. For Cause 5, check for output reductions due to high temperatures. For Cause 6, confirm that the orientation and tilt angle are appropriate for the generation conditions. For Cause 7, check for losses or faults in the wiring and power conversion equipment. For Cause 8, take equipment degradation and aging into account. For Cause 9, confirm that generation figures and self-consumption figures are not being confused.

Corrective measures will be implemented by combining power generation data and on-site inspections. We will check generation output by month, by time of day, by installation surface, and by system, and inspect for shading, dirt, equipment, wiring, and the surrounding environment. After measures are taken, we will record how much the generation output has improved and use that information to inform the next inspection and maintenance plan.

To accurately pinpoint the cause of a drop in power generation, precise on-site information is essential. If the installation area, rooftop equipment, obstacles, trees, site boundaries, orientation, tilt, inspection access routes, and potential connection points can be accurately identified, it becomes easier to sort out issues related to shading, soiling, wiring, and maintainability.

When you need to accurately record on-site installation ranges, obstacles, trees, rooftop equipment, site boundaries, orientation, slope, inspection routes, and so on, and identify the causes of reduced power generation to implement improvements, using LRTK—an iPhone-mounted GNSS high-precision positioning device—is effective. If you can acquire high-precision on-site position information, it becomes easier to sort out causes of shading, locations prone to soiling, feasible installation areas, cabling routes, and maintenance access, enabling a seamless process from on-site verification and simulation comparisons to post-installation performance management for improving power output. To select the appropriate corrective measures when output has dropped, it is important not only to rely on desk-based assumptions but to accurately grasp the site and address the causes reducing output in order.