Five Factors to Suspect on Days with Low Solar Power Generation

When managing a solar power plant, there are days when, looking at the daily reports or monitoring screens, you feel "generation is lower than yesterday," "it isn't as high as past performance for the same period," or "it looks lower compared to nearby similar facilities." The cause of low power generation may be explained by natural variations such as the weather, or it may indicate equipment faults or changes in the site environment.

What is important is not to dismiss a decline in generation as a temporary phenomenon, but to systematically isolate and check the factors that should be verified. In this article, aimed at practitioners searching for "low generation," I explain five factors to suspect on days when solar generation is low, presented in a format that is easy to use for on-site checks and management decisions.

Table of Contents

• On days with low power generation, first standardize the comparison conditions before checking.

• Factor 1 Check the impact of solar irradiance and weather

• Factor 2: Suspected soiling or shadowing on the panel surface

• Factor 3: Check for equipment shutdowns or reduced output

• Factor 4: Do not overlook abnormalities in wiring and connection points

• Factor 5: Check for discrepancies in monitoring data and measurement conditions

• Recording Methods to Quickly Detect Declines in Power Generation

• On-site verification procedure to isolate causes of degradation

• Summary: On days when power generation is low, break down the causes one by one and assess them.

On days with low power generation, first standardize the comparison conditions

When you feel that solar power output is low, the first thing to do is not to immediately suspect a fault. First, make the comparison conditions consistent and clarify what you are comparing it to. The causes you should suspect will differ depending on whether it is lower than yesterday, lower than the same period in the same month last year, lower than another section within the plant, or lower than similar facilities in the surrounding area.

Solar power generation is affected by solar irradiance, ambient temperature, cloud movement, season, panel orientation, tilt angle, installation environment, and so on. Therefore, simply looking at the generation figures alone cannot determine whether the system is operating normally or abnormally. Even on a day that appears sunny, if thin clouds persist for a long time the output will decrease. Conversely, on days with low temperatures and strong sunlight, generation can increase more than you might expect from how it feels.

When making comparisons, also check whether the system capacity, panel orientation, tilt angle, surrounding environment, and operational status are similar. Even when comparing with neighboring installations, you cannot make a simple comparison if capacity or installation conditions differ. When comparing with past performance, you also need to confirm whether conditions have changed due to expansions, shutdowns, equipment replacements, output control, or maintenance and inspection work.

In practice, it is important to look not only at daily power generation totals but also at the generation curve by time of day. Whether the output is low overall from morning to evening, drops only during specific time periods, fails to show a peak around noon, or suddenly falls to near zero will change the direction of the investigation. If it is low overall, the influence of weather or solar irradiance conditions can be considered, but if it drops only during certain time periods, you need to check for shading, temporary shutdowns, communication anomalies, and so on.

Also, at solar power plants it is important to look not only at the plant’s total output but also at the output by system. By checking whether the whole plant is uniformly low or only specific sections, specific pieces of equipment, or specific circuits are underperforming, it becomes easier to distinguish between natural causes and equipment-related causes. When a low-output phenomenon is detected, clarifying whether it is a plant-wide issue or a localized problem in advance will make subsequent on-site inspections more efficient.

A decline in power generation is not always caused by a single factor. On thinly cloudy days, panel soiling can compound the issue, and shadows from weeds and output drops in some equipment can occur simultaneously. Therefore, on days with low generation, you should avoid assuming a single cause and instead take a systematic approach to check multiple factors in sequence.

Factor 1 Check the effects of solar radiation and weather

The first factors you should suspect on days when solar power output is low are solar irradiance and the weather. Because solar power generation converts sunlight into electricity, if solar irradiance is low the output will also decrease. This is a natural variation that can occur even when the system is operating normally, not a malfunction. Even if you feel the output is low, checking the day's weather conditions may show that the decrease was within a reasonable range.

One point to pay particular attention to is that what looks like clear weather and what is favorable for power generation are not the same. Even if the sky appears bright, thin clouds can spread and weaken the solar irradiance reaching the ground. On days when clouds move and solar irradiance fluctuates, the power-generation curve tends to show small ups and downs. Even if it feels clear on site, monitoring data may show that the peak does not extend and power output does not increase as much as expected.

While it’s easy to see that power generation drops on rainy or overcast days, days that alternate between sunny and cloudy are harder to judge. On such days, even if high generation occurs for short periods, the total cumulative generation for the day may not increase. Looking only at the daily report total makes it appear low, but if you look at an hourly graph you may see generation fluctuating in line with passing clouds.

Seasonal effects cannot be ignored. The sun's altitude and hours of sunlight vary with the seasons. In summer, days are longer, but high temperatures can make it harder for output to increase. In winter, lower temperatures can, under certain conditions, be advantageous for conversion efficiency, but shorter daylight hours and a lower solar altitude make systems more susceptible to shading. When evaluating power generation, it is important not to apply the same standard year-round, but to take seasonal trends into account.

Ambient temperature also affects power generation. Solar panels produce electricity when solar irradiance is strong, but if the panel temperature becomes too high the output tends to drop. On a clear midsummer day, if generation doesn’t increase as much as expected, the cause may be heat rather than equipment failure. In particular, in environments with little wind where heat on the back of the panels cannot dissipate easily, they are more susceptible to the effects of temperature rise.

When checking these factors, it is important to examine not only power generation but also solar irradiance, weather, temperature, and time-of-day variations. If the day's meteorological conditions are consistent with the shape of the generation curve, it is easier to judge that the decline is due to natural causes. Conversely, if a particular section is underperforming despite sufficient irradiance, if a clear peak does not appear even on sunny days, or if output suddenly drops regardless of weather changes, you should suspect the following causes.

Factor 2: Suspect dirt or shadowing on the panel surface

If a drop in power generation cannot be explained by weather alone, the next thing to check is dirt or shading on the panel surface. Solar panels generate electricity when sunlight reaches their surface. Therefore, dirt, fallen leaves, bird droppings, sand and dust, pollen, snow, weeds, shadows from nearby objects, and the like can reduce power output.

The impact of soiling tends to progress little by little each day, so it is hard to notice. Rather than a sudden large drop in power output on a single day, you may find that output gradually underperforms compared with days that have similar weather. In particular, power plants located near unpaved roads, development sites, farmland, or forested areas are more likely to have dust and plant-derived deposits carried by the wind. Rain can wash some of it away, but the soiling may remain unevenly distributed.

Even partial soiling, such as bird droppings or fallen leaves, can create shading-like conditions. Because solar panels are made up of multiple cells, even partial obstruction can affect output depending on the extent of the shadow and the circuit configuration. Rather than assuming small dirt is not a problem, it is important to check whether it has been stuck in the same spot for a long time, whether it spans multiple panels, or whether it is concentrated in a particular string.

Shadows cast by weeds are also a factor that is easily overlooked as a cause of reduced power generation. At ground-mounted solar power plants, the way grass grows varies greatly with the seasons. Even if there was no problem when the site was last inspected, within a few weeks they can grow to a height that casts shadows on the lower edge of the panels. Especially in the morning and evening, when the sun angle is low, shadows from weeds that are not noticeable during the day can affect power generation.

Shadows from buildings, utility poles, fences, trees, adjacent equipment, and the like also need to be checked. Trees that posed no problem at the time of installation can grow and begin to cast shadows after several years. If new structures are installed nearby, they may also cast shadows depending on the time of day. If days with low power generation are concentrated in particular seasons or times of day, the relationship between the sun’s altitude and the position of shadows should be suspected.

When checking the impact of shadows, inspecting the site only once may be insufficient. Even if there are no shadows around midday, shadows may occur in the morning or evening. If the power generation curve shows a drop only during specific time periods, inspecting the site during those times makes it easier to identify the cause. Keeping site photos and fixed-point records makes it easier to compare with the previous inspection and helps prioritize mowing and cleaning.

Dirt and shadows on panel surfaces are not internal equipment failures, but if left unchecked they can lead to a continued reduction in power output. In maintaining a power plant, it is important not only to check whether it is generating power, but also to continuously verify that the panels are receiving sufficient sunlight.

Factor 3: Check for equipment shutdowns or output reductions

Shutdowns or output declines of equipment are also important causes of low power generation. If solar irradiance conditions are good and panel soiling or shading are not significant yet power generation is low, there may be abnormalities in conversion equipment, collection equipment, protective devices, communication equipment, and so on. In particular, if low generation affects only some sections of the plant rather than the entire plant, checks at the equipment level are necessary.

First, what you should check is whether any equipment has stopped. If the monitoring screen displays statuses such as "Stopped", "Standby", "Fault", or "Alarm", these may be a direct cause of the decrease in power generation. If equipment has stopped, the power generation from the panels connected to that system will not be reflected, so the total output of the plant will also decrease.

However, even if there is no clear stop indication, output may still be reduced. For example, at a power plant where multiple units operate in parallel, if only one unit has a lower power output, the anomaly may not be apparent from the aggregate value for the whole facility. In such cases, it is necessary to check each unit's power output, voltage, current, operating hours, and alarm history.

Output reduction can sometimes be caused by the equipment’s protective actions. High temperatures, overvoltage, voltage fluctuations, grid conditions, degradation of internal components, and similar factors can cause the equipment to limit its output. These may occur temporarily or may recur at specific times of day. If days with low power generation continue to occur at similar times, you should check not only solar irradiance and shading but also the equipment’s operating condition.

During on-site inspections, we check the equipment displays, alarm indicator lights, operating noise, ventilation conditions, ambient temperature, and the condition inside the panel, among other things. If vents are blocked or there is grass or other objects around that hinder heat dissipation, the equipment temperature can rise easily. Because low power output may be due to the installation environment rather than the equipment itself, it is important to inspect the area around the equipment as well.

Also, one cause of delays in detecting drops in power generation is that the monitoring granularity is too coarse. If you only look at the total generation for the entire plant, it becomes difficult to notice malfunctions in specific equipment. If possible, make the system capable of comparisons at the equipment level, system level, and section level, as this helps with early detection of anomalies. By understanding normal generation trends, you can more easily narrow down which equipment is the likely cause on days with low generation.

Equipment abnormalities, if left unattended, can not only cause ongoing power generation losses but may also require time-consuming inspection and restoration. Rather than assuming there is no problem just because no alarm has been issued, it is important to check whether the output is lower than that of other equipment under the same conditions and whether the power generation curve appears abnormal.

Factor 4 Do Not Overlook Abnormalities in Wiring and Connections

When solar power generation is low, attention often goes to equipment shutdowns or dirty panels, but abnormalities in wiring and connections must not be overlooked. Problems with wiring, terminals, junction boxes, circuit breakers, connectors, and the like can prevent generated electricity from flowing properly, causing reduced output or the shutdown of part of the system.

Abnormalities in wiring and connections can be difficult to detect just by looking from the outside. Some faults are obvious, such as a broken wire, while others—like poor contact, loose terminals, corrosion, water ingress, insulation degradation, or overheating—are hard to notice without inspection. If low power generation persists and cannot be explained solely by weather, shading, or equipment shutdowns, it is necessary to check the electrical pathways.

Especially at outdoor power plants, there are many factors that affect wiring, such as rain, humidity, temperature changes, wind, animals, and contact with vegetation. If cables are detached from their mountings, protective conduits are damaged, connections are susceptible to water ingress, or wiring is subjected to strain during grass cutting or maintenance, these conditions can lead to long-term failures.

Reductions in power generation caused by wiring abnormalities can manifest as lower readings in specific circuits, parts of the system producing no output regardless of the time of day, or failing to reach expected levels even on sunny days. When comparing multiple systems, if one section is weaker despite apparently the same solar irradiance conditions, you should check not only the panels but also the wiring routes.

Loosening or deterioration of connections can be accompanied by heating. Because heating can lead to safety risks, investigations into drops in power generation require not only evaluation of the loss of generated power but also consideration from a safety-management perspective. However, checking electrical equipment requires specialized knowledge and appropriate procedures. Rather than having on-site personnel forcefully inspect inside panels or energized parts, it is important to decide to hand the task over to specialized personnel according to the scope of management and qualification requirements.

When investigating the cause of low power generation, it's important not to rely only on what is visible. Even if the panels are clean and the equipment appears to be operating at a glance, if there is a problem in the electrical pathway, power generation will not increase. You should combine recorded figures, equipment-by-equipment comparisons, site appearance, and inspection history, and adopt an approach that looks for abnormalities in wiring and connection points.

Factor 5 Check for discrepancies in monitoring data and measurement conditions

Even when solar power output is displayed as low, the actual output is not necessarily low. Monitoring data errors or deviations in measurement conditions can make the output appear low. In practice, you need to check not only for faults in the generation equipment itself but also the status of data acquisition, aggregation, display, and communications.

Anomalies in monitoring data include communication outages, missing data, aggregation delays, instrument malfunctions, clock drift, and differences in display settings. For example, a power generation facility may be operating normally, but if communication is temporarily interrupted, the power output may not be recorded correctly. In such a case, the monitoring screen may show low power output even though it was actually generating.

When reviewing daily or monthly reports, it is important to check that all data are complete. If data are missing for specific time periods, the daily cumulative power generation will be lower. If the generation curve is unnaturally interrupted, suddenly drops to zero and then immediately returns, or multiple devices are missing data simultaneously, you should suspect not only equipment shutdowns but also communication or measurement problems.

Time discrepancies also affect power generation evaluations. If the clocks of measuring devices or monitoring systems are off, daily aggregations and time-of-day comparisons may not be performed correctly. Even if the generation itself is normal, shifts in date boundaries or in graph displays can make the output for a particular day appear lower. When comparing multiple installations in particular, it is important to confirm that they are being viewed on the same time axis.

Pay attention to differences in measurement units and display conditions. If you confuse whether you are looking at instantaneous values or cumulative values, AC-side or DC-side values, or values for the entire plant or only part of the equipment, you may mistakenly conclude that generation is low. When multiple people review reports or monitoring screens, you need to share which values are being used as the reference.

Also, when comparing with past data, check whether equipment capacity or operational conditions have changed. If there have been expansions, sectional shutdowns, output curtailment, inspection work, or equipment replacements, a simple past comparison can lead to incorrect conclusions. When generation is low, before assuming the problem lies with the equipment, it is important to confirm that the data’s underlying assumptions are consistent.

Monitoring data is indispensable for power plant management, but the data itself also needs to be verified. When you see an indication that power generation is low, checking the actual on-site conditions, equipment operational status, communication status, and aggregation conditions together can reduce unnecessary on-site responses and misidentification of the cause.

Recording Methods for Early Detection of Power Generation Declines

To quickly identify the cause of low solar power output, it is important to keep records that allow for regular comparison. If you rush to check only after an anomaly occurs, and you do not know the usual condition, it will be difficult to determine how low the output is, when it started to decline, and over what range it is occurring.

First and foremost, it is important not just to record daily power generation as mere numbers, but to record it together with the weather and site conditions. By linking power generation, impressions of solar radiation, weather, temperature, whether any on-site work was performed, whether any alarms occurred, and whether grass cutting or cleaning was carried out, it becomes easier to trace causes later. If there is a day with low power generation, you can check whether it was continuously cloudy that day, whether part of the equipment was shut down for inspection, or whether there was a communication outage.

Photographic records are also useful. Regularly photographing the panel surface, around the mounting structures, weed height, how shadows fall, around equipment, around junction boxes, drainage conditions, and so on makes it easier to see changes from the previous inspection. In particular, shadows from weeds and trees gradually have a larger impact over time, so having photos taken from the same fixed point for comparison makes assessment easier.

Preserving generation curves is also important. Relying only on daily cumulative generation can make it difficult to identify the cause of a decline in output. If you maintain the ability to check output changes by time of day, you can determine whether output is low only in the morning and evening, whether the midday peak is not appearing, or whether there is a sudden drop at a specific time. The shape of the generation curve provides clues to distinguish among weather, shading, shutdowns, communication anomalies, and so on.

When there are multiple sections or multiple pieces of equipment, records that allow side-by-side comparisons under the same conditions are useful. In addition to the overall totals, keeping trends by section, by equipment, and by circuit makes it easier to quickly detect cases where only part of the system is producing less power. Because weather conditions are generally common within the same power plant, it becomes easier to identify locations showing differences.

Records should ideally be presented in a way that not only on-site personnel but also managers, maintenance staff, and those receiving reports can view with the same understanding. A report that only says the power generation is low makes it difficult to decide what to check next. If you can organize and convey when, where, to what extent, during which time period, and which equipment experienced the decline, the initial response can be faster.

Addressing reductions in power generation is not work to be done only when an anomaly occurs. By routinely standardizing the granularity of records and linking site conditions with the data, it becomes easier to identify the factors causing the decline.

On-site Procedure for Isolating Causes of Degradation

When investigating the cause on-site on a day when solar power generation is low, it’s important not to inspect places at random but to decide on an order and isolate the issue systematically. To check efficiently, organizing your assessment in the order of natural factors, site environment, equipment, electrical paths, and data conditions makes it easier to reach a judgment.

The first things to check are the day's weather and the power generation curve. If it was cloudy, rainy, covered by thin clouds, or there were sudden weather changes, low power output itself may be a natural result. If the generation curve rises and falls in line with cloud movement, first verify consistency with the meteorological conditions. On the other hand, if it is sunny yet the peak does not increase, only specific equipment shows low output, or the output suddenly drops and does not recover, suspect on-site factors.

Next, determine whether the decrease is across the entire power plant or only in certain sections. If the whole plant is similarly low, broad factors such as weather, output control, or measurement conditions are likely candidates. If only some areas are low, localized factors such as shading, soiling, equipment shutdowns, or wiring abnormalities are likely candidates. Walking the site without making this distinction expands the scope of checks too much and makes it take longer to pinpoint the cause.

On-site, we first check visible factors. Inspect panel dirt, fallen leaves, bird droppings, shadows from weeds, shadows from trees, shadows from nearby structures, accumulated snow, and puddles. If the time period when a dip appears on the power generation curve coincides with the time when shadows occur, that potential cause is given high priority.

After that, check the operating status of the equipment. Look at alarm displays, shutdown history, operating status, and the power output of each piece of equipment. If only some of the multiple pieces of equipment show low output, focus your inspection on the area connected to those pieces. Checking the ventilation and appearance around the equipment can also provide clues about temperature rises and the installation environment.

Furthermore, if abnormalities in wiring or connections are suspected, have a professional inspection carried out. Check for visible damage, sagging cables, damage to protective components, and any effects from animals or grass‑cutting operations, and request electrical measurements or detailed inspections as necessary. For safety‑related matters, it is important that on‑site personnel do not try to make judgments on their own.

Finally, verify the consistency of the monitoring data. Review whether there are any missing records for data judged to show low power generation, any communication abnormalities, whether the date or time boundaries are shifted, or whether the displayed target equipment is incorrect. If the site is generating power but the data show low output, it may be a recording or communication issue rather than an equipment fault.

Thus, when confirming a decline in power output, it is important to break the phenomenon down into detailed elements and examine them in sequence. Relying solely on experience can cause you to be swayed by obvious causes and overlook the real factors. Standardizing the verification procedures will reduce variability in judgment even when personnel change.

Summary: On days with low power generation, break down each possible cause and evaluate them individually

On days when solar power generation is low, it's important not to immediately assume a fault; first standardize the comparison conditions and assess the situation. Generation output is affected by a variety of factors, such as solar irradiance, weather, season, temperature, shading, dirt, equipment, wiring, and monitoring data. Even when it looks sunny, thin clouds or high temperatures can limit generation, and conversely, declines that cannot be explained by the weather may be caused by shading, equipment malfunctions, or communication failures.

For operations personnel, the important thing is not just to note that generation is low, but to check over what area, at what times, and to what extent it is low. Whether the entire plant is underperforming, only certain sections are, the output is low throughout the daytime, or it drops only at specific times will determine where you need to inspect. Combining daily generation figures, generation curves, site photographs, and inspection records makes it easier to narrow down the cause.

When power generation declines, the sooner you notice it and identify the cause, the easier it is to minimize losses. To do that, it is essential to routinely record on-site conditions and manage them by linking data with the field. Especially at large solar power plants, it is difficult to inspect every location in the same detail and frequency, so a management system that makes it easy to detect signs of abnormalities is important.

Rather than dismissing a day of low power generation as a transient occurrence, if you check in the order of weather, soiling, shading, equipment, wiring, and data, you can calmly determine the cause. Continuously accumulating on-site records and generation data, and establishing operations that can detect early signs of decline, leads to the stable operation and maintenance of a solar power plant.