5 checks to make the most of monthly reports for facilities with low power output

When you suspect low power generation, one useful document for sorting out the causes is the monthly report. A monthly report may include clues for isolating factors behind the decline, such as power generation, solar irradiance, equipment operating status, downtime, anomaly history, and inspection records. However, simply looking at the numbers can make it difficult to determine whether the cause is weather, equipment malfunction, or managerial oversight.

To make use of monthly reports, it is important not to judge based solely on a single month's generation output, but to sequentially check comparisons with the past, comparisons with adjacent facilities, sectional differences within the installation, outage history, and reconciliation with on-site conditions. This article explains five checks for practitioners searching for "low generation" to help infer causes from monthly reports and link them to subsequent inspections and improvements.

Table of Contents

• For installations with low power generation, use the monthly report as the starting point for identifying the cause.

• Check 1: Determine whether the decrease in power generation is temporary or persistent by looking at monthly trends

• Check 2 Verify the validity of the decrease in light of solar irradiance and weather conditions

• Check 3: Narrow down the abnormal range by examining differences among systems and among devices.

• Check 4: Interpreting the timing of losses from downtime and anomaly history

• Check 5: Cross-check with the current on-site inspection and incorporate into next month's improvement items.

• Use monthly reports for continuous improvement to avoid overlooking declines in power output

For facilities with low power generation, use the monthly report as the starting point for investigating the cause

When the power generation of a solar PV system is low, it is important to inspect the site. However, if you rely only on on-site inspections, it can become unclear which areas should be prioritized, leading to missed checks and duplicated work. This is where monthly reports are useful. Monthly reports are not just documents for summarizing generation results; they can be used as management materials to interpret the condition of the equipment over time.

Monthly reports may include monthly power generation, daily power generation, year-on-year comparisons for the same month, deviations from expected values, the relationship with solar irradiance, downtime, anomaly history, and inspection comments. The level of detail varies depending on the equipment and management structure, but what is universally important is not to leave a low generation result as a single figure; instead, break it down to see under which conditions, when, and over what range the decline occurred.

For example, even if the monthly power generation appears lower than the previous year, if the weather that month was poor and the solar irradiance also decreased, it does not necessarily indicate an equipment fault. On the other hand, if the solar irradiance is average for the year but only the power generation is low, or if only a portion of the system is producing less than surrounding areas, you need to check for soiling, shading, equipment shutdowns, wiring or system faults, incorrect settings, or missing monitoring data.

What should be avoided when making use of monthly reports is judging low power generation solely by intuition. Impressions like "it feels low recently" or "it looks lower than last year" can prompt an inspection, but they are not sufficient evidence for determining the cause. In practice, it is important to link the figures recorded in the monthly report with the conditions on site and proceed with checks while setting priorities.

Also, a monthly report is not something you create once and finish. To enable comparisons in subsequent months, keep a record of what was checked, the reasons for your judgments, items put on hold, conditions found on site, and changes after improvements, as this helps with early detection of drops in power generation. Because a facility's power output is affected by the weather, you should avoid judging solely by simple increases or decreases; however, by reviewing records from the same perspective each month, it becomes easier to notice signs of abnormalities.

Check 1: Determine whether the drop in power generation is temporary or persistent by examining monthly trends

For a facility with low power generation, the first thing to confirm is whether the decline is temporary or persistent. In monthly reports, check not only the generation for the current month but also the trend over the past several months, a comparison with the same month last year, and the year-to-date cumulative trend. Judging based on a single month alone can lead you to mistake the effects of bad weather or a temporary shutdown for equipment failure.

In the case of a temporary decline, power generation may drop on specific days or during specific periods and then return to near-normal levels. In such cases, poor weather, temporary output control, short power outages, shutdowns for inspection or maintenance work, or missing data due to communication failures may be involved. Of course, these are not necessarily the causes, but if the decline is transient you can first narrow down the date and time range when it occurred.

On the other hand, if low power generation persists for several months, suspect persistent causes. Possibilities include accumulation of dirt on panel surfaces, increased shading from nearby vegetation or structures, performance degradation in some equipment, abnormalities in part of a string or circuit, or changes in the conditions under which monitoring data are acquired. Such factors can appear as a sudden, large drop or as a gradual decrease in generation output.

When examining monthly trends, it is important not to rely solely on year‑over‑year comparisons for the same month. Even if a month shows lower output than the same month of the previous year, the previous year may have experienced particularly favorable solar radiation conditions. Likewise, if the same month this year had many rainy or cloudy days, lower power generation is to be expected. Conversely, even when there is little difference in the year‑over‑year comparison, a lower value compared with surrounding equipment or expected figures may be a sign that warrants investigation.

In the monthly report, in addition to monthly generation totals, we pay attention to day-to-day variability. Checking whether output fails to increase on sunny days, appears low only on cloudy days, or drops on specific weekdays or workdays will change the direction of the suspected cause. If it doesn’t rise on sunny days, equipment-related factors are more likely; if it’s only low on poor-weather days, meteorological conditions are likely to be the main influence.

Also, be careful about the aggregation period used in monthly reports. If the meter-reading period, the monitoring system’s aggregation period, and the internal reporting period do not match, comparisons can be skewed. What you think is the generation from the first to the last day of the month may actually have been aggregated using a different closing period, making year-on-year comparisons or comparisons with neighboring facilities inaccurate. Before concluding that generation is low, it is important to confirm that the periods being compared are aligned.

Check 2: Confirm the plausibility of the decline against solar irradiance and weather conditions

Solar power generation output is largely affected by solar irradiance and weather conditions. Therefore, when a monthly report shows low generation, we compare it against the solar irradiance and weather conditions. If you do not check whether only the generation is low or whether the solar irradiance is similarly low, it becomes difficult to distinguish between equipment problems and the effects of natural conditions.

Even if the monthly power generation is lower than the previous year, the decrease can often be explained to some extent if the solar irradiance for the same month was lower. In particular, during the rainy season, periods with frequent typhoons, or prolonged snowfall or extended rainy periods, power generation tends to fluctuate more. Conversely, if solar irradiance is around the long-term average or not significantly different from the previous year but power generation is still low, it is necessary to examine equipment-related factors in detail.

The important point here is not to assume that solar irradiance and power generation change by the same proportion. The output of a photovoltaic system is affected not only by solar irradiance but also by panel temperature, shading, dirt, snow, ventilation, equipment conversion efficiency, downtime, and other factors. Therefore, a slight decrease in solar irradiance does not necessarily cause an equivalent decrease in power generation. Monthly reports should include a check to see whether the drop in power generation relative to solar irradiance is unnaturally large.

When assessing weather conditions, check not only the monthly averages but also the behavior on days favorable for power generation. For example, if there were many sunny days yet generation did not increase, constraints on the equipment side may be suspected. Conversely, even if a month appears to have sunny days overall, if clouds or rain were concentrated during the hours favorable for generation, the monthly generation can be low. If the monthly report includes a daily graph, it becomes easier to judge by checking whether a mountain-shaped generation curve typical of sunny days appears.

The influence of ambient temperature cannot be ignored. Solar panels generally tend to produce less output when they become hot. Therefore, even if solar irradiance is high on a clear midsummer day, power output may not increase as much as expected. However, one should avoid attributing everything to high temperature alone. If output is clearly lower than in typical years or lower than neighboring installations under the same conditions, other causes such as soiling, shading, or equipment downtime should also be checked.

Pay attention to the location where solar irradiance is measured. When using the facility’s pyranometer or nearby meteorological data, confirm how well that data represents the actual conditions at the target facility. In mountainous areas, along the coast, on reclaimed land, or where there are nearby elevation differences, cloud cover and the appearance of shadows can change over short distances. Even if the solar irradiance data looks acceptable, some portions of the site may still experience shading.

In practical work using monthly reports, it's important not to quickly conclude that "there's nothing to be done because solar insolation was low." By checking the relationship between insolation and power generation and separating declines that can be explained from those that cannot, the next items to examine become clear. If unexplained declines remain, proceed to check differences among equipment, shutdown histories, and on-site conditions.

Check 3: Narrow down the abnormal range by examining differences across systems and equipment

When investigating the causes of low power generation, looking only at the total for the entire facility makes it difficult to pinpoint the scope of the anomaly. If the monthly report records generation by grid, section, device, circuit, etc., break the overall figures down one level finer and check them. Whether the drop in generation is widespread across the entire facility or concentrated in certain areas will change which causes you should suspect.

If the entire installation shows similarly low values, factors such as weather, output control, outages on the supply/receiving side, overall settings, malfunctions of shared equipment, or the conditions used to aggregate measurement data may be involved. Conversely, if only certain systems or pieces of equipment are low, focus on that scope and check for panel soiling, shading, wiring, connection points, equipment shutdowns, abnormal alerts, protective operations, and so on. When differences are visible in the monthly report, it becomes easier to determine the priority for on-site inspections.

When comparing by system or by equipment, it is important not to judge solely by raw power output. If capacity, orientation, tilt, installation conditions, and shading differ between sections, it is natural that generation will differ. When making comparisons, either compare ranges with similar conditions or look at generation per unit of installed capacity. Because plots with larger capacity naturally produce more and smaller plots produce less, conditions must be aligned when judging whether there has been a decline.

Also, when monthly reports include rankings or shares of power generation, it is useful to look at changes in rank compared with the previous month or the same month of the previous year. If a section that has been stable suddenly falls to a lower rank, something may have changed at that time. Conversely, if the same section is consistently low at the same time each year, seasonal shading, leaf fall, snowfall, surrounding vegetation, or changes in solar altitude may be involved.

When looking at differences between equipment, also check them in combination with downtime. If a piece of equipment has low power generation, the response differs depending on whether it was simply offline for a long time or whether it is online but its output is not increasing. If downtime is long, verify the reason for the outage and the sequence of events until recovery. If it is not stopped but generation is low, suspect input-side anomalies, shading, soiling, measurement anomalies, or the effects of output curtailment.

When narrowing down areas of low power generation, comparing adjacent systems can also be helpful. If neighboring sections have similar solar irradiance conditions but only one shows lower output, a local cause is likely. For example, dirt may tend to accumulate only on certain rows of panels, slopes or trees may cast shadows at specific times, poor drainage may cause mud splashing, or there may be problems with cables or connections.

However, you should avoid concluding the cause based solely on the monthly report. The monthly report is a document for narrowing down the range of anomalies, and final judgment requires on-site verification and review of detailed data. If you find a low range on the monthly report, treat that range as a priority area for the next inspection and correlate it with photos, measurements, and work records.

Check 4 Read the timing of loss occurrence from downtime and anomaly history

In months with low power generation, check the downtime and history of abnormalities. The reason for low monthly generation may be due not to equipment performance degradation but to periods of downtime. The longer the downtime, the more generation opportunities are lost. In particular, downtime during sunny hours can have a large impact on generation even if it is brief.

If downtime is recorded in the monthly report, check not only the total downtime but also when the outages occurred. An outage at night or during periods of low solar irradiance has a very different impact on generation than an outage during daytime on a sunny day. Even with the same total downtime, losses tend to be larger if the outage occurs during peak generation hours, while an outage in the early morning or late evening may have a relatively smaller impact.

When reviewing anomaly history, check not only the number of occurrences but also the duration, recovery status, and whether it has recurred. An anomaly that occurred only once and was quickly recovered and an anomaly with the same issue occurring repeatedly differ in importance. If it is recurring, there may be a persistent problem related to equipment, settings, connections, or protective operations rather than a temporary environmental factor.

It is also important to cross-check abnormality logs with the days when power generation declined. If there is a day on the monthly report when generation dropped significantly, check whether an abnormality occurred on that day, whether a shutdown happened, whether output curtailment was implemented, or whether inspection work was carried out. If the timing of the generation drop matches the timestamps in the abnormality logs, it becomes easier to identify the cause. Conversely, if generation is low but no abnormality is recorded, you should consider factors that do not readily trigger alerts, such as shading or soiling, measurement issues, or data gaps.

When evaluating downtime, also review the sequence of events leading to recovery. Depending on whether time was taken to notify others after detecting the stoppage, to perform on-site verification, or to arrange parts and work, the corrective measures for next time will differ. Keeping a record of the response history in the monthly report not only helps analyze the causes of reduced power generation but also contributes to improvements in the management system.

At power generation facilities, monitoring data can be missing due to communication failures. It is necessary to verify separately whether generation actually stopped while communication was interrupted or whether generation continued but the data could not be acquired. Even if the monthly report shows low generation, measurement or communication problems may be involved. Checking the consistency among electricity sales records, on-site meters, and monitoring data makes it easier to distinguish between a drop in the data and an actual drop.

Anomaly histories may list technical names and codes, but in practice we do not judge by the name alone; we consider the occurrence time, affected equipment, duration, and recovery method together. Even if the anomaly name is the same, the conditions of occurrence and the scope of impact may differ. Adding a brief observation to the monthly report will speed decision-making when the same anomaly occurs again.

Check 5: Compare with current on-site inspection and incorporate into improvement items for next month

When you identify trends of decreased power generation or abnormal ranges in the monthly report, cross-check them against the results of on-site inspections. For areas shown as low on the report, check for actual soiling, shading, vegetation overgrowth, poor drainage, traces of snow, bird damage, cable damage, abnormal equipment displays, and the like. By linking the monthly report with on-site conditions, numerical changes become concrete improvement items.

In on-site inspections, prioritize checking the systems and equipment that showed declines in the monthly report. Rather than inspecting the entire facility aimlessly, focusing on the areas narrowed down from the report makes verification more effective within the limited time available. This is especially important at large solar power facilities, where simply moving between sections can be time-consuming, so decide the inspection points in advance.

One of the causes of reduced power generation that is often found on site is dirt on the panel surface. Accumulation of dust, pollen, bird droppings, fallen leaves, mud splashes, etc. can, in some cases, affect power output. However, the presence of dirt does not necessarily lead to a significant drop in power generation. Check the extent of the dirt, its severity, the location where it occurs, its duration, and whether it improves after rainfall, and see if this matches the downward trend in the monthly report.

Checking shadows is also important. Because the solar altitude changes with the seasons, the way shadows fall can vary by time of year even at the same location. If surrounding trees have grown, a structure has been built nearby, vegetation within the site has become taller, or shadows from mounting racks or fences fall during specific time periods, such changes can cause a reduction in power generation. If monthly reports show declines at the same time each year, you should consider seasonal shading as a possible factor.

The results of the current on-site inspection should be recorded in a format that allows comparison in subsequent months. Recording the date of inspection, weather, scope of inspection, conditions found, actions taken, and reasons for any unaddressed items makes it easier to determine whether power output has recovered after improvements. By observing changes not only immediately after taking action but also in the monthly reports for the next month and the following month, you can verify the effectiveness of the measures.

When breaking improvement items down, it becomes easier to manage if you divide them into those that require immediate action, those to be addressed on a planned basis, and those to be monitored over time. For example, abnormalities related to safety or shutdowns require prompt verification. Mowing, cleaning, checking for shading, and improving drainage should be planned taking into account site conditions and work arrangements. If the cause is unclear, keep it as an item for focused monitoring next month.

The important thing when using monthly reports for on-site improvements is not to stop at simply creating the report. By establishing a process of estimating the causes of low power generation, verifying them on site, taking corrective actions, and then validating the results in the next report, the report becomes a practical management tool. Especially when managing multiple facilities, keeping records in the same format makes it easier to compare equipment and set priorities.

Use Monthly Reports for Continuous Improvement to Avoid Overlooking Drops in Power Generation

To make monthly reports useful for facilities with low power generation, it's important not to look only at that month's generation but to link and review trends, solar irradiance, differences within the facility, downtime history, and on-site inspection results. A decline in generation can sometimes be explained by weather, while other times it indicates faults in part of the equipment or issues in management. By using monthly reports, you can organize the scope and priorities for checks without relying on subjective judgment.

First, determine whether the decline in power generation is temporary or sustained. Next, compare it with solar irradiance and weather conditions to confirm whether the decline can be explained by natural factors. Then, examine differences by system and by equipment to narrow down whether abnormalities are present across the entire facility or concentrated in specific parts. Furthermore, check downtime and anomaly histories to understand when generation opportunities were lost and whether issues are recurring. Finally, reconcile findings with on-site inspections and convert them into improvement items for the coming month and beyond.

Continuing this process each month enables early detection of declines in power generation. In particular, not only sudden stoppages but also gradual issues—such as soiling, shading, vegetation overgrowth, poor drainage, and equipment malfunctions—can be difficult to notice with only daily monitoring. Consistently reviewing monthly reports from the same perspective makes it less likely that small changes will be overlooked.

Monthly reports also serve to share information among stakeholders. If on-site staff, managers, inspection personnel, and equipment owners can review the same materials together, it becomes easier to align understanding of the causes of low power generation. Recording not only the generation figures but also the reasons for decisions and the next actions improves the quality of reporting.

If low power generation is left unaddressed, it can lead not only to lost opportunities to sell electricity but also to overlooked equipment faults and delayed responses. That is why it is important to use the monthly report not just as a record of results but as a document for root-cause analysis and improvement. By combining and checking generation output, solar irradiance, shutdown history, and on-site conditions and linking them to next actions, the precision of equipment management is improved.

If you want to make monthly reports more useful for practical operations, it is also effective to establish a system that centrally organizes on-site confirmation records, photos, inspection results, and comparative information for each facility. By continuously monitoring the condition of facilities with low power generation and creating a framework that links on-site checks to improvement management, it becomes easier to apply monthly reports to subsequent actions.