10 checks to pinpoint the causes of insufficient power generation

When operating a solar power system, you may find yourself thinking, "It's sunny but generation isn't increasing," "generation is lower than last year," or "it didn't improve even after cleaning." The causes of low generation are not limited to panel performance. Multiple factors can overlap, such as weather, season, shading, dirt, temperature rise, orientation, tilt, wiring, equipment, snow accumulation, lack of maintenance, and differences in how self-consumption is viewed. This article explains the first 10 items you should check to identify the causes of low generation, aimed at practitioners searching for "how to increase power generation."

Table of Contents

• Don't assume there is only one cause for low power generation

• Check 1: Confirm seasonal variations using monthly power generation data

• Check 2: Check for shading and abnormalities using hourly power generation data

• Check 3: Compare with the same month of the previous year and with clear-sky days

• Check 4: Inspect buildings, trees, and equipment that cast shadows

• Check 5: Check for dirt, fallen leaves, and bird droppings on panel surfaces

• Check 6: Verify output reduction due to high temperatures

• Check 7: Check orientation, tilt, and layout conditions

• Check 8: Inspect wiring, connection points, and power conversion equipment

• Check 9: Check changes after snowfall, typhoons, and strong winds

• Check 10: Verify power generation and self-consumption separately

• How to proceed to increase power generation after identifying the cause

• Summary

Don't assume there is only one reason why power generation isn't increasing.

When you feel that the power output of your solar power system is not increasing, the first important thing is not to assume a single cause. When output is low, you may be inclined to suspect panel degradation or equipment failure, but in reality many factors are involved—weather and seasonal variations, shading, dirt, temperature, snowfall, and the facility’s own electricity usage. Cleaning or replacing equipment without confirming the cause may not lead to the expected improvement.

Solar power generation does not always produce the same output even when it is sunny. In spring and autumn, the balance between temperature and solar radiation is favorable, so generation can be relatively easy. In summer, although solar radiation is higher, output can drop because panel temperatures rise. In winter, daylight hours are shorter and the sun’s altitude is lower, so shadows tend to lengthen and generated power tends to decrease. In snowy regions, there can also be periods when no power is generated because of snow on the panels.

Also, the expression "power generation not increasing" can have several meanings. It may mean that the amount of power generated itself is small, or that although power is being generated, the amount available for use within the facility is small. You need to distinguish whether generation is lower than the previous year, lower than the simulation, or failing to reach the expected amount for self-consumption. To correctly determine how to increase power generation, the starting point is to first clarify "compared to what it is not increasing."

Check 1: Confirm seasonal variation in monthly power generation

The first thing to check to pinpoint why power generation isn’t increasing is the monthly power generation figures. Looking only at the annual total won’t reveal which periods are experiencing shortfalls. Reviewing the data month by month makes it easier to distinguish whether the decrease is a natural seasonal decline or a problem occurring during a specific season.

If power output is low only in winter, check the short daylight hours, the lower solar altitude, winter shading, and the effects of snow. If power output does not increase as expected in summer, check not only the solar irradiance but also output reduction due to rising panel temperature and high-temperature conditions of the equipment. If output stagnates in spring, pollen, yellow sand (Asian dust), and dust accumulation may be the cause. If power output drops in autumn, check for fallen leaves, dirt after typhoons, and debris blown by strong winds.

When examining monthly power generation, simply searching for months with low output is not enough. You should also review that month's weather, sunlight conditions, and the facility's operational status. It's natural for generation to decline in months with lots of rain or cloudy skies, but if output is low despite many sunny days, the cause may lie in local conditions or the state of the equipment. Monthly generation is an important indicator for investigating the onset of generation declines.

Check 2: Identify shading and abnormalities from time-of-day power generation

The second item to check is power generation by time of day. Monthly generation shows seasonal trends, but shading and equipment faults can appear at specific times of the day. By examining generation by time of day, you can narrow down the likely causes more specifically.

If the morning power output does not increase, shadows from eastern buildings, trees, utility poles, or rooftop equipment are suspected. If power output drops early in the evening, check for shadows on the west side. If there is an unusual dip around midday, shadows from nearby obstructions—such as rooftop structures (tower rooms), piping, railings, or air-conditioning equipment near the panels—or output limitations of power conversion equipment or abnormalities at connections may be involved.

When checking by time of day, it is important to look at the power generation curve on sunny days. Cloudy or rainy days are not suitable for isolating causes because overall output decreases. If a drop occurs at the same time every day even on sunny days, suspect shading or equipment-related causes. Distinguishing whether output is low throughout the entire day or only during specific time periods will clarify whether you should prioritize cleaning, checking for shading, or inspecting equipment.

Check 3: Compare with the same month in the previous year and with sunny days

The third item to check is the comparison benchmark. Even if you feel the power generation isn't increasing, if it is unclear what you are comparing it to, you cannot correctly determine the cause. Simply being lower than the previous month may be due to seasonal fluctuations. Because solar power generation varies with the seasons, judging an anomaly based solely on a month-to-month comparison is risky.

Comparing with the same month of the previous year is useful in practice. By comparing the same season, you can to some extent align differences in sunshine hours and solar elevation when making judgments. Furthermore, comparing sunny days with one another reduces the influence of weather and makes it easier to check the condition of the equipment and site. If a power generation simulation was created at the time of installation, comparing the monthly expected values with the actual results is also effective.

However, even when comparing with the same month of the previous year, differences in weather must be taken into account. If the previous year had many sunny days and this year has many cloudy days, a drop in power output is natural. Conversely, if sunny conditions are similar but output has decreased, you need to check for shading, dirt, equipment, wiring, aging, and so on. Having a proper baseline for comparison is essential to avoid taking the wrong measures to increase power output.

Check 4: Confirm buildings, trees, and equipment that cast shadows

The fourth item to check is shading. When solar panels are shaded, they cannot receive enough sunlight and their power output decreases. Shading is a very important cause of reduced power generation, but because it changes with the seasons and time of day, it is easily overlooked.

Sources of shadows include surrounding buildings, rooftop equipment, rooftop penthouses, railings, piping, air-conditioning equipment, ventilation equipment, utility poles, signs, trees, slopes, and differences in terrain elevation. Even if there were no problems at the time of installation, shadows can increase later if trees grow, rooftop equipment is added, or a building is erected on adjacent land. If power generation gradually stops increasing over the course of several years, you need to check for changes in the surrounding environment.

Winter shadows are also important. In winter, the sun’s altitude is lower, so shadows that didn’t reach the panels in summer can extend to them. Even if a site inspection during summer daytime shows few shadows, there may be shading in winter mornings and evenings. To increase power output, it is important to understand when, where, and to what extent shadows occur, and to avoid expanding or locating installations in areas with heavy shading.

Check 5: Inspect the panel surface for dirt, fallen leaves, and bird droppings

The fifth item to check is dirt on the panel surface. Since solar panels generate electricity by receiving sunlight on their surface, when dirt, fallen leaves, bird droppings, dust, and the like adhere to them, power generation decreases. Dirt often accumulates gradually and may appear as a slowdown in the increase of power output.

Causes of soiling include sand and dust, pollen, yellow sand, bird droppings, soiling from exhaust emissions, particulate matter, fallen leaves, and residual material after snowfall. Sites with many trees nearby are more susceptible to the effects of fallen leaves and birds. If unpaved land, farmland, construction sites, or roads with heavy traffic are nearby, soil dust and particulate matter are more likely to adhere. On rooftops, panels located near vents or exhaust equipment can become locally soiled.

When checking for soiling, examine its relationship with the power generation data. If only a specific surface shows reduced output, if generation does not recover after rain, or if output is slow to improve in spring or autumn, suspect soiling. When cleaning is necessary, prioritize safety and equipment protection. Avoid unsafe rooftop work and cleaning methods that could damage the panel surface. After cleaning, verify whether generation has improved on similar sunny days.

Check 6: Confirm output reduction due to high temperature

The sixth item to check is output reduction due to high temperatures. While solar power generation tends to increase with higher solar irradiance, panel output can decrease when panel temperatures rise. If generation doesn’t increase as much as expected on a sunny summer day, you need to check for temperature-related losses.

Panels installed on roofs tend to be affected by roofing materials that retain heat, and if ventilation behind the panels is poor, temperatures are likely to rise. On flat roofs, using low mounting structures or having many rooftop installations nearby that impede airflow also makes heat more likely to become trapped. For ground-mounted installations, overgrown grass that obstructs airflow or surrounding structures that cause air to stagnate can also affect temperature conditions.

Reduction in power generation due to temperature is a factor that is difficult to detect visually. When reviewing generation data, check whether power generation around late morning to noon on sunny summer days is not increasing as expected, or whether spring or autumn are generating more stably. As countermeasures, consider layouts that do not impede ventilation, vegetation management, and ensuring heat dissipation around equipment. However, changes to racking height or tilt also affect wind and constructability, so a comprehensive judgment is necessary.

Check 7: Confirm orientation, tilt, and placement conditions

The seventh item to check is orientation, tilt, and placement conditions. Solar panels’ power generation varies depending on which direction they face and at what angle they are installed. It can be difficult to significantly change the orientation or tilt after installation, but this is an important check for understanding why power generation isn’t increasing.

Surfaces that face close to south tend to produce more electricity over the year, but east- and west-facing orientations are not necessarily worse. East-facing surfaces tend to generate more in the morning, and west-facing surfaces in the afternoon. If a facility’s demand is high in the morning or afternoon, generation from east- or west-facing surfaces can help with self-consumption. When you feel the generation is underperforming, check not only the total generation but also how the generation timing matches the facility’s demand.

Tilt angle also affects power generation. If the angle is small, dirt and snow may remain more easily, while a large angle can cause inter-row shading and wind effects. For layout conditions, also check whether panels have been added unnecessarily in shaded areas or areas that are difficult to maintain. Even if installed capacity is increased, generation per unit capacity may be lower in poor locations. To raise energy output, it is important to choose a layout that is easy to generate from and easy to manage.

Check 8: Inspect wiring, connection points, and power conversion equipment

The eighth item to check is the wiring, connection points, and power-conversion equipment. Even if the panel surfaces and shading are fine, losses or faults in the electrical path will prevent power generation from increasing. The electricity generated by the solar panels is routed through wiring and equipment to be made usable by the facility. If problems occur during that process, the amount of energy actually available for use is reduced.

When power generation suddenly drops or only certain circuits are producing low output, it is necessary to inspect the equipment and connection points. If wiring distances are long, connection points are difficult to inspect, or wiring routes are complex, discovering faults can be delayed. During new installations or expansions, you need to check not only panel layout but also wiring routes and equipment installation locations as conditions that affect power generation.

The condition of the power conversion equipment is also important. If the equipment is stopped, the amount of electricity available to the facility will be reduced even if the panels are generating. If the output levels off around midday, check the equipment capacity and output conditions. If the equipment is located in places prone to high temperatures, difficult to inspect, or exposed to rain and snow, consider the long-term operational risks. To increase generation, you need to check not only the panels but also the paths that deliver the power.

Check 9: Confirm changes after snowfall, typhoons, and strong winds

The ninth item to check is changes after snowfall, typhoons, and strong winds. Since solar power generation equipment is installed outdoors, it is directly affected by the weather. If the panel surface or the surrounding environment has changed after severe weather conditions, it can be a cause of reduced power generation.

In snowy regions, there are periods when power generation is not possible because snow covers the panel surface. Not only the time during snowfall, but also the time snow remains after snowfall affects power output. If the panel tilt is small, snow may be less likely to slide off. Fallen snow can accumulate on the front or underside of the panels, creating shadows. If winter power generation is lower than expected, check for snowfall and remaining snow.

After typhoons or strong winds, fallen leaves, branches, airborne debris, sand, and dust can remain on panels and roofs. Even if the debris does not directly cover the panels, its accumulation in drainage outlets and inspection walkways can affect maintenance. If power generation suddenly drops after strong winds, inspect the panel surfaces, mounting structures, wiring, equipment, and nearby obstructions for any changes. Recording the condition after weather events will also help with future maintenance.

Check 10: Verify power generation and self-consumption separately

The tenth item to check is to view power generation and self-consumption separately. Even if you feel that power generation is not increasing, in reality it may not be the generation itself but the amount of electricity being used within the facility that is not increasing. If you do not separate power generation, self-consumption, and surplus electricity, you may choose the wrong direction for improvement.

Generation output refers to the amount of electrical energy 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 the generated electricity that could not be consumed during the same time period. Even if generation is sufficient, if generation occurs during periods when the facility's demand is low, self-consumption will not increase.

If a facility’s operating hours change, holidays increase, daytime equipment operation decreases, or daytime demand drops due to energy-saving measures, on-site consumption can decrease even if power generation remains the same. It is important to distinguish whether power generation is not increasing or whether on-site consumption is not increasing. Measures to increase power generation and measures to increase on-site consumption may differ.

How to Increase Power Output After Identifying the Cause

After identifying the cause, prioritize improvements starting with those that have the greatest impact on power generation and are easiest to address. If soiling is clearly present and can be cleaned safely, compare power generation after cleaning. If the cause of shading is trees that can be managed, consider pruning or managing branches. If equipment shutdowns or abnormalities in part of the system are suspected, prioritize inspection of wiring and power conversion equipment. If snow accumulation or fallen leaves occur every year, incorporate them into a seasonal inspection plan.

After implementing corrective measures, always verify their effects using power generation data. Check whether output returned after cleaning, whether generation during specific time periods improved after shading countermeasures, and whether system-specific generation recovered after equipment inspections. By reviewing not only generation but also self-consumption and surplus electricity, you can determine whether the improvements have translated into practical operational benefits.

Efforts to increase power generation are not completed in a single attempt. The site environment changes, and seasonal factors recur. If you record power generation data, on-site photos, inspection dates and times, actions taken, and generation before and after improvements, you can use them for future inspections, internal explanations, and consultations with contractors. Creating a flow to identify causes, implement countermeasures, verify them, and record the results leads to long-term improvements in power generation.

Summary

To identify the cause of underperforming power generation, it is important to check, in order, monthly generation, generation by time of day, comparisons with the same month in the previous year and with sunny days, shadows, dirt, high temperatures, orientation, tilt, wiring, equipment, snow, changes after weather events, and differences from self-consumption. Rather than attributing the cause to a single factor, combining data analysis with on-site inspections will clarify the points that need improvement.

In Check 1, confirm seasonal variations from monthly power generation. In Check 2, check for shading and abnormalities from time-of-day power generation. In Check 3, compare with the same month of the previous year and with sunny days to determine whether the variation is natural or an abnormal drop. In Check 4, inspect buildings, trees, and equipment that create shadows. In Check 5, check panel surfaces for dirt, fallen leaves, and bird droppings. In Check 6, check for output reduction due to high temperatures. In Check 7, confirm orientation, tilt, and layout conditions. In Check 8, check wiring, connection points, and power conversion equipment. In Check 9, check for changes after snow accumulation, typhoons, or strong winds. In Check 10, check power generation and self-consumption separately.

To increase power generation, it is important to identify the causes and then carry out shading countermeasures, cleaning, equipment inspections, layout revision, and maintenance planning according to priority. It is crucial not only to improve the numbers in simulations but also to recover the actual generation lost on site.

And to accurately identify the causes of stagnant power generation, accurate on-site information is indispensable. If you can accurately grasp the installation area, rooftop equipment, obstacles, trees, site boundaries, orientation, tilt, inspection access routes, and candidate connection points, it becomes easier to organize issues related to shading, soiling, temperature, wiring, and maintainability.

If you want to accurately record on-site installation area, obstacles, trees, rooftop equipment, site boundaries, orientation, tilt, inspection routes, etc., and identify the causes of underperforming power generation to develop improvements, using LRTK, an iPhone-mounted GNSS high-precision positioning device, is effective.

If you can acquire high-precision location information on site, it becomes easier to organize the causes of shading, areas prone to soiling, feasible installation areas, wiring routes, and maintenance flows, making it straightforward to carry out on-site verification, simulation comparisons, and post-installation performance management for improving power generation. To correctly identify the causes of underperforming power generation, it is important not only to rely on desk-based assumptions but also to accurately understand the site and isolate causes from both data and field observations.