5 Checks to Identify Causes of Low Power Generation Based on the Generation Start Time

When power generation is low, many sites suspect, in order, solar irradiance, panel soiling, equipment failures, shading, and power conditioner abnormalities. However, an often overlooked starting point for narrowing down the cause is the generation start time. Looking only at the total daily generation tells you nothing more than that it is "low," but checking when generation actually started and whether it is delayed compared with the previous day or sunny days in the same season makes it easier to distinguish causes such as shading, orientation, circuit faults, startup conditions, or missing measurement data.

Table of Contents

• Basics to Keep in Mind Before Checking the Power Generation Start Time

• Check 1: Look at the difference between the sunrise time and the start time of power generation

• Check 2 Compare the ramp-up curve on a clear-sky day

• Check 3: Look for discrepancies in start times across circuits and per power conditioner

• Check 4: If power generation is delayed only in the morning, suspect shadows and obstructions

• Check 5: Verify start-time recording offset and communication loss

• Precautions to avoid making judgments based solely on the power generation start time

• Summary: Narrow down the causes of low power generation by focusing on morning activity

Basics to Keep in Mind Before Checking the Power Generation Start Time

The generation start time refers to the time when the power output begins to rise on the monitoring screen or in the recorded data. Strictly speaking, the time when sunlight first begins to hit the panels, the time when the panels begin to generate voltage, the time when the power conditioner (inverter) starts operating, and the time when the monitoring device records the power output are not the same. Therefore, when looking at the generation start time, it is important to first align which moment is being treated as the "start".

For example, if you treat the time when output first appears on a monitoring system's graph as the start of power generation, that time includes the effect of the measurement interval. With recordings every 5 min, the rise can appear several minutes later than the actual event. With recordings every 15 min or 30 min, the timing is even coarser. Therefore, rather than trying to trace causes with 1 min accuracy, a practical approach is to compare with the previous day, the previous week, a day with the same conditions, or another installation on the same site to see whether there is an obvious delay or an unnatural difference.

The reason the generation start time is useful when investigating low power output is that features of the equipment and surrounding environment often become apparent during the morning ramp-up. When the sun's elevation is low, shadows from buildings, trees, mounting racks, fences, utility poles, and adjacent equipment tend to lengthen, and depending on the position of obstacles they can affect some panels or strings. Also, because power conditioners require certain voltages and input conditions to start operating, a fault in part of a circuit may manifest as a delayed start in the morning.

However, it is dangerous to look only at the generation start time and immediately conclude there is a fault. Cloudy skies, rain, fog, snowfall, yellow sand, morning dew, temporary shadows around the site, data transmission delays, and so on can also change the start time. What is important is not a single day's delay but to check whether the same trend continues compared with days with similar weather, whether only specific equipment is delayed, and whether it is correlated with a decrease in total power generation.

When consulted about low power generation, if you only look at monthly generation at first, seasonal and weather variations can bury the causes and make them hard to see. On the other hand, by looking at the daily generation curve and separately checking the morning start time, midday peak, and evening decline, you can see which time periods are experiencing losses. The generation start time is a straightforward indicator for confirming the start of the morning period.

Check 1: Examine the difference between the sunrise time and the power generation start time

The first thing to check is the difference between the sunrise time and the time power generation begins. Solar power systems do not produce maximum output at the exact moment of sunrise. Just after sunrise the sun’s altitude is low and sunlight is weak, so the amount of power generated is small. In addition, installation orientation and tilt, surrounding terrain, and shadows from buildings affect when sufficient light actually begins to reach the panel surface. For these reasons, power generation starting later than sunrise is not, by itself, abnormal.

The problem arises when, under the same season, the same weather, and the same equipment conditions, the start of power generation is clearly later than before. For example, if sunrise becomes earlier from spring to summer but the generation start time in the monitoring data remains late or becomes even later, there may be some issue preventing the system from receiving morning light. Conversely, in winter sunrise is later and the sun’s altitude is lower, so the start of generation can appear delayed. If you make a simple comparison without accounting for these seasonal differences, you may mistake normal changes for anomalies.

In practice, it is clearer to look at the difference between the power generation start time and past normal start times than to use the sunrise time itself. For example, compare data from a sunny day in the same month last year, a recent sunny day, and another section on the same site installed with a similar orientation. By aligning the comparison targets, you can even out the natural differences caused by weather and season to some extent. Rather than extracting only days with low generation, placing them alongside days with normal generation makes it easier to determine whether a delayed start time is related to the cause of the low generation.

If there is a large gap between sunrise and the start of power generation, the first thing to consider is morning shading. For installations with buildings or trees on the east side, the panel surface can be shaded for a certain period in the morning, making the start of generation appear delayed. In particular, tree growth that was not a problem at the time of installation, new structures on neighboring land, changes to material storage areas, and the erection of temporary structures can later cause a reduction in power output. When you look only at the data without knowing about local changes, this is something that is easily mistaken for an internal equipment fault.

Also, it is necessary to check the power conditioner’s startup conditions. During early morning periods when solar irradiance is weak, the DC-side voltage and input conditions can be unstable, causing repeated starts and standby. If power generation begins later than usual, it may be that some panels or strings are not contributing sufficiently, there are variations in connection conditions, or there is an abnormality in the input circuit. However, because electrical checks require safety management, on-site personnel should not forcibly inspect the inside of the equipment; instead, prioritize organizing records and sharing them with the specialist in charge.

The important thing in this check is not to get carried away by the difference from sunrise alone. It's not realistic to simply decide that a power generation start time a certain number of minutes after sunrise is normal and another number of minutes is abnormal. The appropriate appearance varies with the orientation of the equipment, the surrounding environment, the season, and the granularity of the monitoring data. That's precisely why the standard should be not general rules but the ramp-up behavior when that equipment was operating normally. If you feel the power output is low, the starting point is to first compare the morning behavior during normal operation and confirm whether the time at the system's input has changed.

Check 2 Compare ramp-up curves on clear-sky days

The next thing to look at is not just the power generation start time but the ramp-up curve after the start. Even if generation begins at the same time, if the subsequent rise in output is sluggish, the total generated energy will be lower. Conversely, if the start time is slightly later but the output quickly ramps up to the normal level thereafter, the impact may be limited. To pinpoint the cause of low generation, it is important to look at the whole morning curve rather than the start time as a single point.

On sunny days, the power generation curve generally rises gently in the morning, peaks around midday, and then declines toward the evening. If there are few clouds and little shading, the curve will be relatively smooth. If, after generation begins, output fails to increase much and then suddenly rises at a certain time, it is possible that shadows had not cleared until that time. If output increases in a stair-step pattern only in the morning, it may indicate that multiple circuits or sections are beginning to receive sunlight in sequence.

When making comparisons, choose clear-sky days whenever possible. On cloudy days, output tends to fluctuate as clouds pass, and both the start time and ramp-up can be unstable. On rainy or foggy days, the start of generation can appear delayed even if the equipment is functioning normally. Therefore, rather than judging based only on days with low generation, line up several days with similar weather and check whether the morning curve shows repeatability. If the ramp-up is consistently delayed on similar clear days, environmental or equipment-related causes become more likely.

When examining the ramp-up curve, monthly or daily bar charts alone are not sufficient. The daily total generation aggregates morning losses, midday curtailment, evening shading, and communication outages. If possible, check output by time of day—at least hourly, and preferably at finer intervals. If output is low only during the 7–9 AM period and then close to normal afterward, the suspected causes differ from when output is low throughout the day.

If the morning start-up is sluggish, it cannot always be explained by dirt alone. Dirt across the entire panel surface tends to depress output throughout the day, but if performance is extremely poor only in the morning, the effects of shading, orientation, or localized obstruction may stand out. Of course, morning dew or damp soiling can temporarily block light, but in that case they tend to change in step with weather conditions. If it is low in the same way at the same time every morning, it is worth checking for fixed obstacles or layout-related shading.

Also, if the output fluctuates in small increments immediately after power generation begins, you need to check the operating status of the power conditioner and the input conditions. If it appears that the system is repeatedly starting and standing by during periods of weak solar irradiance, the morning data may show a jagged pattern. However, because a similar appearance can result from the monitoring data’s display interval or rounding, do not immediately conclude it is an anomaly; verify by checking the operation history, alarm history, and differences with other equipment in the same installation.

When using the power generation start time to detect the causes of low output, it's easy to focus only on how many minutes the start was delayed. However, in actual assessments, what matters is how output increased after the start. Whether the morning ramp-up was gentler than usual, whether output suddenly recovered at a certain time, or whether it fluctuated intermittently will change whether you should prioritize checking shading, start-up conditions, communications, or weather. The generation start time is the entry point, and the ramp-up curve is a clue indicating the likely cause.

Check 3: View deviations in start times by circuit and power conditioner

If a power generation system is made up of multiple circuits, multiple power conditioners, and multiple compartments, check not only the overall generation start time but also the start time for each piece of equipment. Even when the total generated power is low, the diagnosis changes significantly depending on whether all equipment is equally low or only some are delayed. If only some equipment starts generating later, you should prioritize checking the panel surfaces, strings, input circuits, and installation locations connected to those units rather than attributing the issue to the weather.

For example, if only the east-side section on the same site begins generating power later, shadows on the east side or nearby obstructions may be the cause. On the other hand, if among several units with the same orientation and similar sunlight conditions only a particular unit starts late, you should check that unit's input side and operating state. Furthermore, if only a specific string shows a different voltage or current ramp-up, this leads to more detailed inspections for issues such as poor connections, open circuits, degradation, shading, or abnormalities on the panel side.

At sites with low power generation, people may look only at the total figure and conclude that "everything is bad." However, in reality, sometimes only certain circuits are not contributing sufficiently to generation, and that is dragging down the overall output. In such cases, rather than broadly inspecting the entire facility, identifying first the equipment or circuits whose start times are delayed makes it easier to prioritize inspections.

When comparing circuits, bear in mind that installation conditions may not be exactly the same. Even on the same site, orientation, tilt, racking height, surrounding shading, and the position of panel rows affect how morning irradiance arrives. Therefore, a different start time does not immediately indicate an abnormality. Suitable comparison targets are units with similar installation conditions that previously showed similar power generation curves. Checking whether the gap has widened compared to past normal conditions improves the accuracy of your assessment.

Also, if only one of several power conditioners starts generating later than the others, check that unit’s alarm history and operation history as well. If records show it remained in standby for a long time in the morning, stopped immediately after startup, or repeated a specific error, the cause may lie with the equipment’s control or input conditions rather than simply insufficient solar radiation. However, the absence of alarms does not necessarily mean it is operating normally. Minor connection faults or the effects of shading may not be recorded as clear alarms and can appear as differences in the power generation curve.

During on-site verification, we confirm on the drawings the panel location corresponding to the circuit whose start time is delayed. By cross-referencing which column, which face, which orientation, and what surrounding environment it is located in, you can associate the data with the on-site conditions. If the drawings and the current conditions do not match, relying only on the circuit name in the data to make a judgment may lead you to inspect the wrong location. For equipment that has been modified, expanded, or had connections changed, it is important to check the latest system diagrams and connection information.

The purpose of this check is to pinpoint where low power generation is originating, rather than vaguely treating it as a system-wide issue. If the entire system's generation start is delayed, suspect weather, widespread shading, or conditions of the monitoring data. If only part is delayed, suspect factors specific to that section or to individual equipment. By breaking down generation start times by equipment, you can narrow the inspection scope and more easily reduce unnecessary checks.

Check 4: If power generation is slow only in the morning, suspect shadows and obstructions

If the power generation start time is late, and generation is particularly low only in the morning, prioritize checking for shading effects. On mornings when the sun’s elevation is low, shadows extend long. Buildings or trees that shadows do not reach at noon can still fall on the panel surface during the morning hours. If the generation start time is delayed to the same time every day and output then recovers, generation may be suppressed until the sun rises above an obstruction.

One troublesome aspect of shadows is that it can be difficult to determine the cause by inspecting the site at midday. At the time of inspection the shadows may have disappeared and the panel surfaces may look normal. Therefore, if morning power generation starts late, it is useful to check on-site photos or records from the time periods when the delay actually occurs. Recording whether the same shadow appears at the same time every day, whether it changes with the seasons, or whether it only falls on specific rows will make it easier to correlate with the generation data.

Shadows are not caused only by buildings and trees. Fences, signs, utility poles, power lines, antennas, lightning protection equipment, air-conditioning equipment, adjacent mounting structures, temporary scaffolding, materials, weeds, and so on can also cause shadows. Especially for ground-mounted equipment, there are many factors that can change after operations begin, such as the growth of vegetation, accumulation of fallen leaves, the placement of bird protection materials, and nearby stored items. Even if there are no problems immediately after installation, shadow conditions may change after several months or years.

Morning shadows are reflected not only in the start time of power generation but also in the shape of the power output curve. If output hardly increases until a certain time and then suddenly rises, the shadow may have cleared. If output increases sequentially across multiple circuits, the shadow may be moving along the rows of panels. These movements are hard to see from total generation alone, but they become relatively clear when you look at graphs broken down by time of day.

When assessing the impact of shading, it is important not to simply assume that even a small amount of shade will cause large losses. The actual impact depends on the extent of the shading, its duration, the connection configuration, the arrangement of the panel surfaces, and electrical controls. A small shadow may more readily affect certain circuits in some cases, while if it is brief the overall effect on total generation may be limited. Therefore, when you find shading, verify whether it corresponds to a drop in output during that time period.

When checking shadows, the way you take on-site photos is also important. During the period when the start of power generation is delayed, record the entire panel surface, surrounding obstacles, the sun's direction, and how shadows fall. If you photograph only a part from up close, it's difficult to grasp the overall relationships. Conversely, if you take only wide shots, you can't tell which panels or circuits are shaded. To allow cross-checking with power generation data, record the time the photo was taken, the photo location, and the target section; this makes it easier to sort out the causes later.

When shadows are suspected as the cause of low power generation, countermeasures depend on the cause. If the shading is due to weeds or fallen leaves, cleaning or revising maintenance practices should be considered. If it is caused by trees, you need to check whether pruning is possible and confirm the scope of management. If it is caused by buildings or structures, since they cannot be easily removed, you should identify the periods of shading and the amount of loss and then perform an operational assessment. In any case, rather than subjectively saying “shading is bad,” it is important to show with records how much a delay in the start of generation is connected to a decrease in generation output.

Check 5: Verify start time recording discrepancies and communication loss

When investigating causes using the power generation start time, what you must always check is the reliability of the records themselves. Even if monitoring data makes the start of generation look late, it may actually have been generating and simply not displayed because of communication or recording problems. When you feel the generated power is low, if you do not check not only equipment faults but also missing measurement data, time offsets, communication delays, and differences in aggregation methods, you may reach an incorrect judgment.

For example, if a monitoring device sends data at regular intervals, missing data during periods of unstable communication can make it look on a graph as if power generation started later. If the first data point in the morning is missing, generation that actually ramped up earlier won’t be recorded and it will appear as if generation began at a later time. In this case, the problem may be with how the monitoring data appears, not with a lower actual power output.

Misaligned time settings also require attention. If the clocks of monitoring devices, measuring instruments, power conditioners, and aggregation systems are not synchronized, comparisons of generation start times will be inaccurate. Especially when comparing multiple devices, if only one device’s clock is off, it can make that particular device appear to start earlier or later. It is worth checking whether time settings are misaligned after a date change, recovery from a power outage, changes to communication settings, or after replacing equipment.

Also, the judgment changes depending on whether the data used to view the power generation start time are instantaneous values, time-averaged values, or cumulative values. If they are instantaneous values, changes in the ramp-up are relatively easy to see, but with averaged or cumulative values there can be a discrepancy between the displayed time and the actual generation timing. When looking only at data aggregated in daily or monthly reports, the fine details of the morning ramp-up may have been rounded.

When suspecting communication loss, check not only the power generation start time but also for data gaps, consecutive identical values, unnatural spikes, and missing daytime data. If gaps occur not only in the morning but also at other times of day, it becomes more likely that the issue lies with communication or recording rather than the equipment. Conversely, if the communication data are continuous and normal and only a specific circuit starts late on days with the same conditions, that provides grounds for investigating the equipment or the surrounding environment.

When a report says power generation is low, decisions are sometimes made based solely on the appearance of the monitoring screen. However, the power generation start time shown on the screen is the result of multiple stages: actual generation, measurement, communication, aggregation, and display. If there is any loss or delay at any of these stages, the screen can present a different view from the actual on-site generation status. Distinguishing whether the drop in generation is due to an actual generation problem or a recording issue is important to avoid unnecessary site visits and incorrect repair decisions.

When checking for recording discrepancies, compare other data from the same day. Cross-referencing the power conditioner’s history, the monitoring device’s communication logs, the daily report’s cumulative values, and records from separate systems relating to power sales and consumption makes it easier to see which data are missing. However, detailed inspections of electrical equipment should not be performed by unqualified personnel attempting to access the inside of devices. The person responsible for operations should organize the anomalous data points, the time of occurrence, the affected equipment, and the issue’s reproducibility, and, when necessary, hand the information over to a specialist.

Precautions to Avoid Judging Solely by Power Generation Start Time

The generation start time is a useful clue when investigating low power output, but it alone is not enough to determine the cause. A delayed start in the morning does not necessarily indicate equipment failure. Conversely, even if the start time appears normal, total generation can still be low due to factors such as midday voltage-rise suppression, output reduction from temperature increases, soiling, aging, poor connections, or communication loss.

Pay particular attention to the effects of weather. Thin cloud cover, morning fog, post-rain conditions, low clouds, snow cover, and morning dew can affect the start time of power generation and its ramp-up. These are not equipment abnormalities but variations caused by natural conditions. Even if there is a day with low power output, do not judge based on that day’s start time alone; compare multiple days. Ignoring differences in weather conditions and lining up sunny and cloudy days together can lead to an incorrect diagnosis of the cause.

Seasonal changes in the sun’s position also have a large impact. In winter, the solar altitude is low, shadows are longer, and the start of morning power generation can appear delayed. In summer, sunrise is earlier and morning solar irradiance is more likely to reach the site, but output reductions due to high temperatures can occur at other times of the day. Even for the same installation, the normal power generation curve varies by season. Therefore, comparisons should, as much as possible, be made using the same season or days with similar solar conditions.

Consideration must also be given to the orientation of the installation. East-facing surfaces tend to come online earlier in the morning, while west-facing surfaces may appear to come online later in the morning. Even south-facing surfaces can receive different morning light depending on the surrounding environment. In installations where panels of multiple orientations coexist, looking only at the overall generation start time can make it difficult to tell which surface is causing the effect. If possible, check separately per face, per circuit, and per device.

Verifying the power generation start time becomes more meaningful when combined with other inspection items. On days when generation starts late, cross-check whether morning shadows can be observed, whether specific equipment is starting up late, whether alarm histories are recorded, and whether there are communication losses. If multiple clues point in the same direction, it becomes easier to prioritize candidate causes. Conversely, if the clues do not match, it is safer not to rush to a conclusion and to collect additional records.

When power output is low, there is a greater urge to decide the cause quickly. However, power generation equipment operates through a combination of weather, environment, equipment, wiring, and measurement factors. The generation start time is an entry point for finding the cause, not a final judgment. By checking, in order, the start time as the entry, the ramp-up curve, differences between individual devices, on-site shading, and the reliability of the records, inspections can proceed based on evidence rather than intuition.

Summary: Narrow down the causes of low power generation by examining morning behavior

When investigating the causes of low power generation, looking only at daily or monthly total generation can make it hard to understand what’s happening. The generation start time is useful for that. By checking whether the morning ramp-up is delayed, the output after startup is sluggish, or only specific circuits or power conditioners are late, you can more easily organize candidate causes such as shading, obstacles, input conditions, equipment differences, and communication failures.

First, check the difference between sunrise time and the power generation start time, and, taking season and weather into account, determine whether it falls within a natural range. Next, compare the ramp-up curves on clear days to distinguish whether the output fails to increase only in the morning or remains low throughout the day. Furthermore, by comparing start times for each circuit and device, narrow down whether the issue is system-wide or confined to certain sections or equipment. If the delay occurs only in the morning, suspect shading or obstructions, and also check for any record misalignment or communication loss.

When verifying the power generation start time, it is important not to judge based on a single day's numbers. Comparing with normal operation, with other days under the same conditions, and by comparing equipment within the same installation helps narrow down the causes of reduced generation. Also, linking site photos, drawings, monitoring data, and operational history makes it easier to prioritize inspections.

If low power output is left unaddressed, it can lead to continued lost opportunities while the cause remains unknown, or to unnecessary replacement decisions. Conversely, rushing to conclude the cause can result in overlooking basic factors such as shading or communication loss. The time when generation starts in the morning is a practical checkpoint to reduce these kinds of oversights.

By recording changes in the power generation start time through daily monitoring and understanding the normal ramp-up behavior, you can detect the early signs of anomalies more quickly when generation seems low. It is important to check the overall plant generation curve, differences between equipment, on-site shading, and data gaps together, and to connect these findings to the necessary inspections. To efficiently organize the causes of generation decline and carry out on-site verification, it is essential to organize generation data, on-site photos, drawings, and inspection histories to the same standard so they can be compared later.