6 Numbers to Check in Maintenance Inspection Reports When Power Generation Is Low

When you feel that a solar power generation system’s output is low, one of the first documents you should check on site is the maintenance inspection report. The report often contains not only visual inspection findings but also numbers that can be used to isolate causes—generated power, solar irradiance, voltage, current, resistance values, downtime, and so on. If you judge based only on impressions—“the weather was bad,” “the equipment has degraded,” or “it might be a fault”—you may check in the wrong order and delay your response. What’s important is to read the numbers listed in the report one by one and organize where abnormalities may exist.

Table of Contents

• Why You Should Pay Attention to the Reported Figures Especially When Power Generation Is Low

• Number 1: Examine the magnitude of the decrease in generated electrical energy

• Number 2 Separate weather factors by solar radiation

• Number 3: Assess the equipment-side potential by the difference from the expected power generation

• Value 4: Observe the differences between circuits using string current and voltage

• Number 5: Observe changes in safety with insulation resistance and grounding resistance

• Number 6: View operating loss by downtime and number of anomalies

• Connect the figures in the report to the next inspections and improvements

Why You Should Look at the Numbers in the Report Especially When Power Generation Is Low

When customers report low generation, they tend to focus only on monthly or annual energy generation figures. However, generation is affected by multiple factors such as solar irradiance, ambient temperature, shading, soiling, system capacity, equipment outages, voltage rise, output curtailment, and shutdowns during inspections. Therefore, looking at the generation numbers alone makes it difficult to determine whether the decline is due to natural conditions, equipment-side issues, or operational losses.

The role of a maintenance inspection report is that it allows the numbers measured on site and the observations made during the inspection to be confirmed within the same document. For example, even if the generated energy is lower than in the same month of the previous year, it does not necessarily indicate an equipment fault if the solar irradiance for the same period was also low. Conversely, if the solar irradiance has not changed significantly but only the generated energy has decreased, there is reason to suspect equipment- or management-related causes such as dirty panels, shading, circuit malfunctions, power conditioner shutdowns, or missing communication records.

Inspection reports become more valuable when read not just for a single instance but in comparison with past reports. Monthly figures can fluctuate due to weather or the inspection date, but comparing several months, the same month in the previous year, adjacent sections within the same facility, or strings under the same conditions makes the direction of change easier to discern. The purpose of reviewing maintenance inspection reports is not to memorize the numbers. When generation is low, organize which figures to check first and which figures need to be aligned before moving on to the next decision.

In this article, we highlight six representative numbers you should look at in maintenance and inspection reports when power generation is low. Each of these is a basic indicator frequently handled in on-site management of solar power systems. However, the listed items and their names vary depending on the system's scale, installation conditions, contract terms, measurement instruments, and the report format. Here, without relying on any specific equipment or service, we organize how practitioners should read such reports.

Figure 1 Magnitude of decrease in generated electrical energy

The first number to check is the electrical energy generated. In maintenance inspection reports, the generated electrical energy may be recorded on a daily, monthly, or period-total basis. The unit is generally shown as kWh, and it indicates how much electricity the equipment produced over a given period. Since the issue at hand is low generation, this figure is the starting point.

However, when looking at generated energy, you should not simply judge it as "low"; you need to clarify what it is being compared to. Whether it is lower than the same month of the previous year, lower than the previous month, lower than the expected generation, or lower than another section within the same site will change how you interpret it. If it is only lower than the previous month, seasonal factors may be the cause. If it is significantly lower than the same month of the previous year, you need to check for differences in weather, downtime, or changes in equipment condition.

When reading generation figures, it's also important to make sure the reporting periods match. When comparing monthly generation, even a difference in the number of days in the month being compared can cause differences in the totals. You should confirm whether an inspection report's aggregation period is from the first to the last day of the month, from one meter-reading date to the next, or the recording period of the monitoring device; otherwise the basis for comparison can be misaligned. In particular, if the sales statement for electricity sold, monitoring data, and inspection reports use different periods, figures that appear to be for the same month may not exactly match.

It is also useful to convert to generated energy per unit of installed capacity. When comparing sections or power plants of different sizes, you cannot judge based only on the total kWh. Converting to generated energy per 1 kW of installed capacity evens out scale differences and makes comparison easier. For example, if multiple plots within the same site have similar orientation and tilt, and a clear difference in generation per capacity persists for a particular plot, that provides a clue to check for shading, soiling, circuit faults, or equipment outages.

On the other hand, the amount of electrical energy generated is a result and not a figure that directly indicates the cause. It can show the fact that output is low, but it cannot reveal the reason. Therefore, we next verify by combining solar irradiance, expected power generation, voltages and currents for each circuit, and records of equipment shutdowns. It is realistic to treat the amount of electrical energy generated not as a number for concluding an anomaly, but as an entry point for narrowing down what to investigate.

Number 2: Separate weather factors by solar irradiance

The next metric to look at is solar radiation. Because solar power generation is greatly affected by solar irradiance conditions, if you suspect an equipment fault when output is low without checking the solar radiation, you may misidentify the cause. If maintenance inspection reports or monthly reports record solar radiation, it is important to check it alongside the generated energy.

Solar irradiance is an indicator of how much sunlight reached a panel surface or the horizontal plane. In reports it is presented in various forms, such as measured values from pyranometers, nearby meteorological data, and values obtained from monitoring systems. What is important here is to confirm where, in which orientation, and over what period that irradiance was measured. Horizontal-plane irradiance and panel-surface irradiance have different meanings, and measured values within a plant and regional reference values can differ due to cloud cover and local weather variations.

Even if the generated electricity for a given month is lower than the same month of the previous year, if solar irradiance has also decreased similarly, the impact of weather may be significant. Conversely, if solar irradiance is about the same as the previous year but only the generation is lower, checks on the equipment or operation are necessary. In this way, solar irradiance is a metric used to determine whether the low generation can be explained solely by weather.

However, you should avoid drawing conclusions based solely on irradiance. Even if irradiance is sufficient, power generation can decrease due to dirt on the panel surface, shading from weeds or structures, increases in panel temperature, shutdown of the power conditioner, output curtailment due to voltage rise, and other factors. Also, if the pyranometer is dirty, its installation angle is misaligned, or the measuring instruments are malfunctioning, the reliability of the irradiance data itself requires attention. If the report includes the pyranometer’s cleaning status or inspection results, it is advisable to check those as well.

When looking at solar irradiance, also pay attention to the ratio between irradiance and power output. If there are many sunny days but power output does not increase, if output responds sluggishly to increases or decreases in irradiance, or if a particular installation produces less power than adjacent installations under the same irradiance conditions, a more detailed inspection is necessary. Solar irradiance should be used as a benchmark to isolate equipment-related causes, not as a figure to dismiss a drop in power output as a natural-condition issue.

Number 3: Evaluating equipment potential based on the difference from expected power generation

The third figure you should check is the difference from the expected or assumed power generation. Some reports include an expected generation value based on design conditions and solar radiation alongside the actual generation. This figure helps make the assessment closer to one that accounts for current weather conditions, rather than a simple historical comparison.

Expected power generation refers to an estimate of the amount of electricity expected to be produced over a given period, based on installed capacity, solar irradiance, panel orientation and tilt, loss coefficients, and other factors. The actual calculation methods vary depending on the report preparer and management approach. Therefore, when looking at expected generation figures, it is important to check which assumptions were used to calculate them. The accuracy of the assessment changes depending on whether irradiance is based on measured values or regional reference values, and on how thoroughly effects such as panel temperature and soiling are taken into account.

If actual power generation is substantially below expected power generation, there may be equipment-side losses. For example, possible causes include insufficient generation in a specific circuit, stoppage of the power conditioner, soiling of panels, shading from weeds or surrounding structures, faults in connection points, and missing communication records. What is important here is not to determine the cause solely from the difference with expected generation, but to check the period over which the discrepancy occurred, the magnitude of the discrepancy, and whether the discrepancy is continuing.

If the difference is temporary, sudden weather changes, temporary shutdowns during inspections, grid-side voltage fluctuations, or missing communications can be considered. On the other hand, if the output consistently falls short over several months or the discrepancy is larger only on sunny days, more carefully examine the possibility that part of the equipment is not delivering its intended capacity. In particular, if the gap widens during sunny periods with high solar irradiance, it is worth checking for output curtailment, equipment capacity, voltage rise, temperature effects, and circuit imbalance.

The difference from expected power generation is a figure that is easy to use when explaining things to managers. The expression "I feel the power generation is low" does not convey the priority for action well, but if you can explain that "even taking solar irradiation conditions into account, the output has remained lower than expected," it becomes easier to lead to on-site checks and detailed inspections. However, avoid making major decisions while the calculation conditions for the expected power generation are unknown. Check the report's notes and calculation conditions, and, if necessary, make judgments together with measured data and on-site photos.

Number 4: Observe differences between circuits using string current and voltage

The fourth numbers to look at are the current and voltage for each string. If the cause of low power generation lies in part of the equipment, it is difficult to pinpoint the problem area from total generated energy alone. Therefore, check the current and voltage for each string, the circuit unit of the solar panels. If the report includes string measurement values, they are useful for isolating the cause of the power shortfall.

String current should basically be compared between circuits that are subjected to similar solar irradiance conditions. If strings are connected to the same power conditioner and have the same orientation, the same tilt, and similar shading conditions, their current values tend not to deviate significantly. If only a particular string shows a low current, that is a clue to suspect causes such as panel soiling, shading, open circuits, poor connections, panel defects, or problems around fuses and junction boxes.

On the other hand, string voltage is a figure used to check the number of modules connected in a circuit and their condition. For strings with the same configuration, open-circuit voltage and operating voltage tend to fall within a certain range. If only a particular string shows low voltage, or if its recorded data are unnaturally missing, check for differences in the number of connected modules, broken wires, short circuits, faults at connection points, differences in measurement conditions, and so on. However, because voltage is also affected by ambient temperature and operating conditions, it is necessary to consider the measurement time, weather, and the operating state of the power conditioner together.

When reading the numbers for strings, you should pay attention not only to individual values but also to side-by-side comparisons. When measurements for multiple strings are listed in a report, check whether any circuit deviates significantly from the average, whether any circuit has decreased compared to past inspections, and whether circuits near the same location tend to be lower. If current is lower only in certain rows or at the edges, site environmental factors such as shadows, dirt, grass growth, remaining snow, or splashed water may be involved.

Also, when comparing measurements, it is essential to align the conditions. If measurements are taken on a day when clouds and sunshine alternate over short periods, differences between strings may result from solar irradiance fluctuations rather than equipment faults. If the inspection report records the measurement time, weather, and irradiance, check them together with the string currents and voltages. When generation is low, start from the overall figures and then narrow down locations using string-level values; this workflow makes on-site responses more efficient.

Number 5: Observe changes in safety with insulation resistance and grounding resistance

The fifth values I want to check are insulation resistance and grounding resistance. When you are concerned about low power output, you tend to focus only on figures related to generation performance, but safety-related figures are also important in maintenance inspection reports. Problems with insulation or grounding can lead to equipment shutdowns, operation of protective devices, leakage current risks, and corrective measures during inspections. As a result, they can be related to reduced power output or decreased operating rates.

Insulation resistance is a numerical measure used to determine whether an electrical circuit is properly insulated from earth or other parts. When insulation resistance decreases, ingress of moisture, deterioration of cable sheathing, faults around junction boxes or connectors, damage to the back of panels or wiring routes, and similar issues may be suspected. If readings tend to fall after rain or during periods of high humidity, the relationship with the weather should also be checked.

Ground resistance is a numerical measure that indicates whether the grounding, which safely dissipates electricity during abnormal conditions, is functioning properly. If ground resistance falls outside management standards, safety verification of equipment and corrective actions may be required. Although this value may not seem directly linked to reduced power generation, it cannot be ignored because it can lead to the activation of safety devices or equipment shutdowns.

When examining these resistance values, check not only individual readings but also changes over time. If values have dropped significantly compared with the previous inspection, worsen only after rain, or repeatedly show abnormal trends in specific junction boxes or circuits, focused on-site verification is required. Even if the report includes assessments such as "Good" or "Needs attention," it is important to review the actual numerical trends rather than relying on the assessment alone. Even when the assessment shows no problem, if values are gradually deteriorating, investigating the cause early may help prevent a major outage.

Insulation resistance and ground resistance are treated differently depending on the measurement method and equipment conditions, so when reading the figures in a report you should also check the standards applied, the measurement locations, the weather at the time of measurement, the measurement range, and so on. In power generation equipment in particular, the points to be checked may be separated between the DC side and the AC side. If you do not know which circuit and which value is being referred to, you cannot correctly correlate it with a decrease in power generation.

When power output is low, the purpose of checking safety-related figures is to detect signs of faults early. Even at an early stage when the decline in power output is still small, a decrease in insulation resistance or a change in grounding condition may be an early sign that could lead to future shutdowns or other problems. Safety metrics should be read not only to improve power output but also as fundamental information for the continued operation of the equipment.

No. 6 Viewing operational loss by downtime and number of abnormalities

The sixth figures to check are downtime and the number of faults. An often-overlooked cause of low power generation is operational loss — that the equipment was actually stopped during hours when it could have been generating. If maintenance inspection reports or monthly reports record items such as power conditioner downtime, number of abnormal occurrences, time to recovery, and communication outage duration, these are items you should prioritize checking.

Downtime is the figure that indicates the time during which generation opportunities were lost. Nighttime downtime is treated as normal operation, but if the system is down during daytime hours with solar irradiance, it will affect power generation. Even if downtime is short, if it occurs around noon on a clear day, it can have a large impact. Conversely, even if downtime appears long, if it occurs during periods with little solar irradiance, its impact on power generation may be limited. Therefore, downtime needs to be considered together with solar irradiance and the time of occurrence.

The number of abnormal occurrences is also important. If anomalies occur frequently, it may indicate a recurring operational issue rather than an isolated failure. For example, possible causes include rises in system voltage, increases in the internal temperature of equipment, abnormalities in the DC-side input, unstable communications, or actuation of protective devices. However, avoid determining the cause based only on the abnormality names in the report; confirm the time of occurrence, weather, load conditions, recovery method, and whether the same equipment has experienced recurrence.

When looking at downtime, it’s also important to know which equipment stopped. Whether the entire facility stopped, only some power conditioners stopped, or only a specific input circuit wasn’t generating power, the scope of the impact changes significantly. If it’s a complete shutdown, check the grid side, the incoming service equipment, the main breaker side, and any planned outages. If it’s a partial shutdown, check the affected equipment, connection boxes, strings, and local shading and temperature conditions.

Planned outages for inspections or construction can also be a factor in reduced power generation. If the report lists work hours or downtime, check whether those periods affected generation. Shutdowns for inspections are necessary work, but when evaluating generation you need to organize the data without omitting the fact that production was stopped. Especially when monthly generation is low, examine not only abnormal shutdowns but also planned outages, communication outages, and periods of missing records.

Downtime and the number of faults are essential figures when considering equipment availability. Even if generation capacity is sufficient, frequent stoppages will reduce power output. Even if there are no major problems with panels or strings, if the power conditioner stops frequently, it will manifest as a decrease in generation. When generation is low, it is important not only to investigate why generation failed, but also to confirm whether there was sufficient time during which generation actually occurred.

Use the numbers in the report to guide subsequent inspections and improvements

The six figures to check in a maintenance inspection report are: generated energy, irradiance, the difference from expected generation, string current and voltage, insulation resistance and grounding resistance, and downtime and number of anomalies. These should not be read in isolation but used in combination to narrow down causes. When the result shows low generation, checking in order what the irradiance conditions were, how far they deviate from assumptions, whether there are differences between circuits, whether safety-related values have changed, and whether equipment stoppages occurred will improve the accuracy of your assessment.

In practice, it is important not to assume a single cause from the outset. A drop in power generation can be caused by a combination of factors such as weather, shading, soiling, equipment shutdowns, wiring faults, measurement errors, curtailment, and maintenance outages. The figures in reports are material for separating and considering those multiple factors. If an anomaly is found in the numbers, cross-check them with on-site photos, monitoring data, past reports, work history, and equipment drawings, and, if necessary, follow up with an on-site inspection.

Also, the figures in reports help explain the situation to stakeholders. The phrase "generation is low" by itself does not convey the urgency or point toward likely causes. However, if you can explain that "generation is low relative to solar irradiance," "only the current of a particular string is low," "downtime is concentrated during daytime," or "insulation resistance has decreased compared with the previous measurement," it becomes clear what to check next. Having the maintenance company, construction company, asset owner, and operations manager look at the same numbers while discussing things is highly meaningful for reducing the likelihood of oversights in the response.

When using reports, it is important not only to store each inspection result but also to organize them in a format that makes comparison easy. If you can review power generation, solar irradiance, downtime, number of anomalies, and key measured values in a time series, you will be more likely to notice signs of change. Especially since solar power generation equipment is intended for long-term operation, continuous trends in the numbers are more important than isolated inspection results. By catching small changes early, you can more easily prevent major generation losses and prolonged outages.

When power generation is low, what’s needed is not guesswork-based root-cause hunting but calm, methodical troubleshooting that starts from the figures in the reports. First, confirm the magnitude of the drop using generated energy, and check weather factors with solar irradiance. Next, consider equipment-side possibilities from the difference between expected and actual generation, and narrow down the location with string current and voltage. Then verify safety with insulation resistance and grounding resistance, and quantify operational losses using downtime and the number of fault occurrences. Following this sequence as a basic procedure makes it easier to organize the causes of a generation decline.

The ability to read maintenance inspection reports is directly linked to improving power plant operations. If it is difficult to judge from numbers alone, establishing a system that links and manages site location information, photos, inspection records, and power generation data will make verification work easier. Organizing the condition of low-output equipment on a per-site basis and connecting the report figures with on-site information to guide the next actions helps identify and reduce generation losses.