5 ways to interpret the causes of low power generation from the electricity sales statement

When you feel that your solar power generation is low, one of the easiest documents to check first is the electricity sales statement. The statement often lists the amount of electricity sold to the utility, the meter-reading period, the expected payment amount, and so on, making it a good starting point to review your generation performance. However, it is risky to conclude from the electricity sales statement alone that the equipment is faulty or the panels have degraded. The amount of electricity sold may not match the actual generation, since it is affected by weather, season, the number of meter-reading days, self-consumption, output control, measurement conditions, and other factors.

This article is aimed at operations personnel who search "low power generation" to organize causes. It explains the information that can be read from electricity sales statements and, from that, the next points to check divided into five perspectives. By using the electricity sales statement not merely as a document for confirming payments but as primary information for understanding the condition of the power plant, you can more efficiently proceed with narrowing down the causes.

Table of Contents

• Separate the information to check on the electricity sales statement from the power generation figures.

• Perspective 1 Do not judge the amount of electricity sold solely by the month-to-month difference

• Perspective 2 Read seasonal factors by comparing with the same month of the previous year

• Point 3 Check the difference in days of the meter-reading period

• Perspective 4: Look at changes in energy rather than payment amounts

• Interpretation 5: Reconcile the amount of electricity sold with on-site data

• What to check after finding the entry point of an anomaly in the electricity sales statement

• Summary: Use the electricity sales statement as a starting point for isolating the problem, not to definitively determine the cause

Separate the information you should check on the electricity sales statement from the amount of power generated

When checking an electricity sales statement, the first thing to be aware of is that the figures shown on the statement do not necessarily represent the total output of the generation equipment. In particular, for systems with self-consumption, the portion of solar-generated power used on-site may not be reflected in the amount sold. In such cases, even if actual generation has not decreased significantly, increased consumption by the building or equipment can make the amount sold appear lower.

Even in a configuration that sells all generated electricity, it is premature to determine the cause based solely on the electricity sales statement. The amount of electricity sold is the result of multiple factors—solar irradiance, temperature, snowfall, rainy weather, cloud cover, shading, dirt on the panel surface, power conditioner shutdowns, grid-side constraints, the length of the meter-reading period, and so on. The electricity sales statement is a document that shows the results and does not directly indicate the underlying causes.

Therefore, when looking at an electricity sales statement, you should first distinguish what is included and what is not. Many electricity sales statements show the amount of electricity sold, the applicable period, the amount based on the unit price, the meter-reading date, and contract information. On the other hand, it is common that they do not reveal day-by-day generation trends, stoppages by time of day, variations at the string level, abnormalities at the panel level, on-site shading occurrences, or the exact times equipment errors occurred.

In practice, when you see on the electricity sales statement that the amount is “less than usual,” it is important not to immediately arrange an on-site inspection but first to align the comparison conditions. Check in order whether it is lower compared with the previous month, lower compared with the same month of the previous year, still lower when the meter-reading days are matched, and whether the power generation monitoring data shows the same trend. By following this procedure, you can more easily separate natural variations caused by weather or differences in the period from declines due to equipment abnormalities.

Perspective 1: Don't judge solely by the month-to-month change in electricity sold

When you feel your power generation is low, the most noticeable figure is the change in electricity sold compared with the previous month. Lining up the electricity sales statements by month makes it easy to spot months where the amount sold has fallen from the prior month. However, because solar power generation has seasons when it is easier or harder to generate electricity, judging an anomaly based only on the month-to-month difference is not appropriate.

For example, during periods with shorter sunlight hours or prolonged rainy weather, the amount of electricity sold can decrease even if the equipment has no problems. Conversely, during periods when temperatures become too high, generation efficiency can be affected even if solar irradiance is strong. In solar power generation, more sunny days do not necessarily guarantee maximum output. Panel temperature, cloud patterns, humidity, wind, installation angle, and the surrounding environment also influence generation results.

When comparing with the previous month, it's important to pay attention not to whether it decreased but to whether the decrease is within a natural range. If a similar decline is seen around the same time every year, it may be seasonal variation. On the other hand, if it has fallen significantly even compared with the same month in past years, if a lower level has persisted since a particular month, or if the amount of electricity sold has not increased despite many sunny days, there is reason to suspect equipment or operational problems.

Also, when looking at month-over-month differences, plotting them on a graph makes it easier to grasp trends. Organize the electricity sales statements by month and arrange the amount of electricity sold in chronological order; this will show whether the drop is a single occurrence or a sustained decline. If it is a single drop, prioritize checking temporary causes such as bad weather, power outages, construction, inspections, output control, and short meter‑reading periods. If it is a sustained decline, suspect accumulated dirt, increased shading from trees or structures, partial equipment shutdowns, or configuration and communication faults.

One important point is not to immediately attribute a drop in electricity sold to age-related degradation. Although long-term performance declines can occur in solar photovoltaic (PV) systems, sudden decreases on electricity sales statements are often caused by factors other than degradation. When there is a sudden, significant drop, you are more likely to identify the cause by first checking for shutdowns, shading, soiling, grid-side constraints, or measurement and recording issues, rather than assuming the decline is due to aging.

Viewpoint 2: Interpreting Seasonal Factors by Comparing with the Same Month of the Previous Year

When reading the causes of reduced generation from feed-in statements, comparison with the same month of the previous year is more important than month-to-month differences. Because solar power generation has strong seasonality, it's easier to judge by comparing the same season rather than simply comparing months from different seasons. Comparing with the same month of the previous year, and if possible with the multi-year average for that month, makes it easier to determine whether the amount of electricity sold is low for that month.

When comparing with the same month of the previous year, first check the amount of electricity sold. If there is no large difference compared with the same month of the previous year, the possibility of a serious abnormality in the power generation equipment appears relatively low. However, being similar does not necessarily mean everything is normal. This is because it is possible that output had decreased for the same reason in the previous year as well. For example, if the site is affected by nearby trees or mountain shadows at the same time every year, even if the amount of electricity sold is similar to that of the same month of the previous year, it may still be that generation opportunities are not being fully utilized.

If the figure is clearly lower than the same month of the previous year, first check for weather differences. Months with prolonged rain, typhoons, snowfall, yellow sand, volcanic ash, dust from nearby construction, or extended cloudy periods can see a decrease in electricity sold. If weather factors are strong, comparing with nearby power plants or generation records from the same region makes it easier to distinguish between an individual equipment anomaly and a trend affecting the entire area.

Next to check is any change in the environment around the power plant. If buildings, temporary structures, material storage areas, overgrown trees, weeds, slope changes, or increased shadows around fences that weren’t there the previous year have appeared, the amount of electricity sold can decrease even in the same season. Especially during times of low solar altitude in the morning and evening, even small obstacles tend to cast much larger shadows. Sales statements alone do not show the time shadows occurred, but if there is a difference from the same month of the previous year, it can prompt cross-checking with on-site photos and daily generation data.

When comparing the same month year‑over‑year, it is useful to look not only at that single month but also at the months before and after. If only one month is low, it may be due to temporary weather or an outage. If the same month falls below the previous year’s level for several consecutive months, persistent causes are suspected. Persistent causes can include soiling of panel surfaces, shading from weeds or trees, partial shutdowns of power conditioners, string disconnections or poor connections, malfunctions of metering equipment, and record loss due to communication failures.

Also, pay attention not only to whether the value is lower than the same month of the previous year, but also to the magnitude of the decline. If the difference is slight, it may fall within the range of natural variation, but if a decline beyond a certain level continues, it is worth verifying with materials other than the statement. In practice, using the power sales statement as an entry point and combining solar irradiance, operational status, equipment logs, on-site checks, and inspection records is the key to preventing excessive worry and oversights.

Point 3: Check the difference in days of the meter-reading period

One thing that is easy to overlook when looking at a feed-in statement is the difference in the number of days in the meter-reading period. If you only look at monthly feed-in amounts, a particular month may appear to have low generation. However, if that month's meter-reading period is shorter than other months, it is natural for the amount of electricity sold to look smaller. Conversely, months with longer meter-reading periods tend to appear to have larger feed-in amounts.

An electricity sales statement may list the applicable period and the meter-reading date. For months when you feel the generation output was low, first check how many days the applicable period covers. Rather than relying on the simple monthly total, converting to and comparing the electricity sold per day can reduce the impact of differences in the period. If the daily electricity sold is still low, further check weather and equipment-related factors. If there is no large difference on a per-day basis, a short meter-reading period may have been the main reason.

Differences in the meter-reading period can also cause problems when the period covered by internal month-end reports does not match the period covered by the electricity sales statement. Internally, generation is managed from the 1st to the end of the month, while the electricity sales statement may be aggregated using a different date cutoff. In such cases, even if you compare the company's generation monitoring data and the electricity sales statement as the same month, discrepancies will arise because the covered periods are misaligned. Rather than immediately concluding there is a measurement error or sales leakage because of a discrepancy, you should first check for a period misalignment.

Also, if there are inspections, construction, interconnection shutdowns, or grid-side work, we check which meter-reading period those impacts fall into. Even if it feels on site like the stoppage occurred in the previous month, on the power sales statement it may appear to affect the following month’s figures. In particular, when the meter-reading date does not coincide with the end of the month, the date the cause occurred and the month shown on the statement are more likely to be misaligned.

When operational staff manage electricity sales statements, it’s useful to record not only the monthly electricity sold but also the period start date, period end date, number of days, and electricity sold per day together. This allows you to quickly determine, when you find a month with low electricity sales, whether it was simply due to a short meter-reading period or still low even after adjusting for the period.

If you begin investigating causes without first checking the meter-reading period, it can lead to unnecessary site inspections or incorrect conclusions. Power sales statements are documents that are easy to check, but you cannot compare them correctly unless you align the assumptions used for aggregation. When reading the statements to determine why generation is low, it is important to confirm which period the figures cover before looking at the numbers themselves.

Perspective 4: Focus on changes in energy consumption rather than payment amounts

Because power sale statements often include information about payment amounts, in practice attention tends to be drawn to increases or decreases in those amounts. However, when reviewing them to investigate the cause of low power generation, you need to focus on the amount of electricity generated rather than the payment amount. The payment amount is affected not only by the volume of electricity sold but also by contract terms, calculation adjustments, and the applicable period, so it is difficult to use it as a direct indicator of generation performance.

When evaluating the condition of a power generation facility, what you should check is how much electricity was sold. Even if revenue has fallen, if the amount of electricity sold has not changed significantly, the cause may be something other than a decline in the facility’s output. Conversely, if the monetary figure appears largely unchanged but the amount of electricity sold has decreased, there may be some change occurring on the generation side.

Especially when managing multiple power plants, arranging power sales statements by monetary amount makes comparisons difficult due to differences in installed capacity and contract terms. If you want to compare generation performance, it is important to normalize the sold electricity (energy) by installed capacity and the number of days covered, and view them under comparable conditions. Rather than simply comparing total sold volumes, check whether the generation results are reasonable relative to the facility’s size.

Also, if you focus too much on the amounts shown on the electricity sales statement, you may overlook early signs of declining generation. For example, at stages where the numerical differences are small, changes may not be noticeable in monetary terms, yet the trend in energy output can show a gradual decline. Such a gradual decline can be related to the accumulation of panel dirt, the growth of weeds or trees, aging of equipment, deterioration of connections, measurement errors, and so on.

When reviewing energy output, check not only the monthly amount of electricity sold but, if possible, also daily and hourly generation data. Since the sales statement often only shows a monthly total, monitoring data and on-site measurements are needed to investigate the causes. A low monthly total alone does not reveal whether output is slightly low every day or whether it dropped sharply on particular days. If only specific days are low, weather or shutdown work may be a factor. If it is similarly low every day, persistent causes such as shading, soiling, or partial equipment shutdowns are suspected.

Because electricity sales statements are also used for accounting and payment confirmation, they tend to be treated as documents for confirming amounts. However, when reading the causes of low generation, you need to switch your focus. By using the electricity sales statement not as a document for looking at payment amounts but as a record for tracking changes in energy output, you will more easily notice changes in equipment condition.

View 5: Cross-check Electricity Sold with On-site Data

When a potential decrease in generation is detected from the electricity sales statement, the next step is to cross-check it with on-site data. The sales statement alone can show the drop in output, but it cannot identify the cause. Therefore, verify by combining generation monitoring data, the operational status of the power conditioner, on-site meters, inspection records, weather information, and on-site photographs.

The first thing to check is whether the trend in the electricity sales on the sales statement matches the trend in the generation monitoring data. If both are declining, it is more likely that the cause lies with the generation equipment or environmental factors. On the other hand, if the monitoring data show strong generation but the sales statement shows low electricity sales, you need to check for increased self-consumption, differences in measurement ranges, mismatches in aggregation periods, differences in how meters or records are verified, and so on.

Next, examine generation at the power conditioner unit level. Even if the overall amount of electricity sold is low, the diagnosis differs depending on whether all units are similarly low or only some units are low. If all units are similarly low, consider factors such as weather, snowfall, overall soiling, widespread shading, or grid-side constraints. If only some units are low, proceed to check the circuits, strings, breakers, wiring, communications, settings connected to that unit, and any local shading or soiling around it.

On-site photos are also important. Dirt on panel surfaces, bird droppings, fallen leaves, dust, shadows from vegetation, nearby temporary structures, remaining snow, mud splatter from poor drainage—these are factors that cannot be seen in power sales statements but can be easily confirmed with photographs. In particular, if the amount of electricity sold has been gradually declining, the effects of dirt and vegetation shading may be accumulating. Keeping fixed-point photos makes it easier to compare with the same month in the previous year.

Also, cross-checking with inspection records is indispensable. For months with low power sales, check whether equipment shutdowns, part replacements, maintenance work, grid-side work, communication failures, on-site construction, periods before grass cutting, etc., overlap. Aligning the sales statement's reporting period with the dates on the inspection records makes it easier to understand the relationship between declines on the statement and on-site events.

The important point here is not to assume the cause based solely on the numbers in the power sales statement. For a result of low power sales, checking the weather, the period, the local site conditions, equipment operation, and measurement conditions in that order will reveal the investigation priorities. Rather than immediately performing detailed inspections of all equipment, it is more efficient to narrow down the timing of the anomaly from the statement, narrow the scope with monitoring data, and confirm candidate causes through on-site verification.

What to check after finding an initial indication of an anomaly in your electricity sales statement

If you notice from the power sales statement that generation may be low, the next step is to classify and check the causes. Broadly speaking, it’s easier to think about them if you organize them into: reductions due to natural conditions; equipment or wiring problems; changes in the on-site environment; measurement or aggregation issues; and operational constraints.

Natural conditions include insufficient solar radiation, prolonged rainfall, snowfall, cloudy weather, rising temperatures, and the effects of yellow sand and dust. These factors can reduce the amount of electricity sold even when the equipment is operating normally. For months in which a decline is observed on the electricity sales statement, checking how the weather conditions compared with average years can provide clues to distinguish between equipment faults and natural variation.

Examples of equipment and wiring issues include power conditioner shutdowns, circuit breaker operation or tripping, poor connections, string faults, terminal abnormalities, and faults in communication or metering equipment. These cannot be determined directly from the power sales statement alone, but they are items to check when the amount of electricity sold suddenly decreases or when only part of the equipment has a drop in generation. If error logs or shutdown histories remain in the monitoring data, cross-check them with the period covered by the statement.

Examples of changes in the on-site environment include panel soiling, weeds, trees, nearby structures, changes to slopes or the ground, poor drainage, bird damage, and fallen leaves. These can gradually increase their impact and may be difficult to notice from a single month alone. When you review power sales statements over several months, this can appear as a gradual decline compared with the same month in the previous year.

Measurement and aggregation issues should not be overlooked. The aggregation range and period may differ between the power generation monitoring system, on-site meters, and electricity sales statements. If figures do not match, confirm which meter and which period you are looking at before concluding there is an equipment anomaly. In particular, when monthly documents are prepared by multiple people, transcription errors, unit mix-ups, and shifts in the applicable period can occur.

Operational constraints include output control, planned shutdowns, maintenance inspections, grid-side work, and temporary stoppages due to on-site construction. These may not indicate abnormalities, but they are reflected on the statements as reductions in the amount of electricity sold. In practice, it is important to keep daily operational records so that, for months showing decreased sales on the electricity sales statements, the history of stoppages and control actions can be verified.

As an order for confirming the cause, it is practical to first organize the relevant period and the electricity volume on the statement, then compare it with the same month of the previous year and historical averages, and finally cross-check monitoring data with on-site records. Because electricity sales statements are compiled monthly, there can be a time lag before anomalies are noticed. If early detection is a priority, it is necessary to use not only the electricity sales statements but also daily generation data and on-site inspection records.

Summary: Use the feed-in statement as a starting point for isolating the cause, not to determine the cause definitively

When you feel the power generation is low, the electricity sales statement is a useful reference. By tracking trends in sold electricity, comparing with the same month of the previous year, noting the meter-reading period, and following changes in energy output, it becomes easier to notice potential abnormalities. However, the sales statement only shows the results and does not directly indicate the cause. Rather than immediately concluding that low sold electricity is due to equipment failure or aging, it is important to check step by step while keeping comparison conditions consistent.

What's particularly important is not to judge solely by month-to-month differences, to compare with the same month of the previous year, to check the difference in the number of days in the meter-reading period, to look at the amount of electricity rather than the payment amount, and to cross-check with on-site data. Just keeping these five perspectives in mind will significantly change the accuracy of the information you can read from the feed-in statement.

In practice, in addition to keeping electricity sales statements on a monthly basis, organizing the relevant period, the amount of electricity sold, daily sales volume, year‑on‑year comparisons for the same month, and records of inspections and shutdowns will speed up the initial response when you notice a drop in generation. If you can correctly isolate the cause, you can reduce unnecessary investigations and more easily focus on the necessary on‑site checks.

To identify the causes of low power generation more quickly, in addition to checking electricity sales statements, a system that can aggregate on-site generation status, photos, inspection records, and equipment location information is helpful. By establishing a system that can link and manage any anomalies found in the sales statements with on-site data, you can reduce overlooked causes and make it easier to proceed to the next inspection or improvement decisions.