5 checks to determine if dirt is the cause of low power generation

Table of Contents

• What to check first when you notice low power generation

• 5 checks to determine whether dirt is the cause

• Check 1: Confirm how many percent lower it is compared with the same month of the previous year on sunny days.

• Check 2: Verify whether power output returns after rain

• Check 3: Inspect the panel surface for bird droppings, dust, pollen, and yellow sand.

• Check 4: Confirm changes in shadows, weeds, and the surrounding environment

• Check 5: Inspect the power conditioner and monitor for abnormalities

• How to distinguish power output drops caused by soiling from those caused by faults

• What you can safely do yourself and what is dangerous when power generation is low

• Cases When You Should Request Cleaning and the Criteria for Deciding

• Three measures to prevent a decline in power generation

• Frequently Asked Questions

• Summary

What to check first if you think your power generation is low

When people check the monitor for their solar power system and feel that "generation has been low recently," the first thing many suspect is dirt on the panels. Indeed, if debris, dust, bird droppings, and the like adhere to the surface of solar panels, power generation can decrease. The Solar Power Generation Association also explains that when dirt and dust accumulate on module surfaces, generation can drop by a few percent, and that in an average urban area the output reduction due to soiling is generally around 5% or less.

However, it is risky to immediately conclude that cleaning is necessary just because power generation is low. Solar power output varies due to multiple factors such as weather, season, hours of sunlight, temperature, installation orientation, installation angle, shading, and the condition of the power conditioner. Kansai Electric Power also explains that output tends to decrease during winter or prolonged periods of bad weather, and that excessively high temperatures tend to reduce efficiency.

In other words, when power output is low, the important thing is not to start cleaning immediately, but to determine, in order, whether the cause is dirt, the effects of weather or the season, or an equipment malfunction.

This article explains five checks to determine whether low power generation is caused by dirt, in a way that's easy to understand for residential solar power users. It focuses on methods you can perform without climbing onto the roof, so if you want to identify the cause safely, check in the following order.

5 checks to determine whether dirt is the cause

When determining the causes of low power generation, check the following five items.

By going through these five items in order, it becomes easier to determine whether a drop in power generation will be resolved by cleaning or will persist even after cleaning.

Particularly important is the rate of decline in power generation. According to the Solar Power Generation Association, in typical residential areas dirt such as garbage and dust is often washed away by rain, and there is rarely a need to climb onto the roof for routine cleaning. Therefore, if power generation has fallen sharply by 20% or 30%, you should suspect not only mere dirt but also shading, snow accumulation, equipment malfunctions, or the power conditioner stopping.

Check 1: Verify what percentage lower it is compared to the same month last year and a sunny day

The first thing to check is "how low the power output is."

Even if you feel the power output is low, it is often simply because a run of bad weather continued, because winter had shorter hours of sunlight, or because high summer temperatures reduced efficiency. Solar power generation is strongly affected by solar irradiance: it is lower in the morning and evening and higher during the daytime, and on cloudy or rainy days the output falls compared with clear days. In its explanation, Tokyo Electric Power Company Energy Partner also states that solar power output varies almost proportionally with solar irradiance and is greatly influenced by the weather.

When making comparisons, it's important not to simply look at "lower than yesterday" or "lower than last month," but to ensure the following three conditions are met.

For example, if a household that used to generate 20 kWh in a single sunny day in April now produces about 19 kWh on a sunny day this April, the degradation rate is about 5%. At that level, light soiling, aging, or differences in solar irradiation conditions can be considered.

On the other hand, if, on a similarly sunny day, the output falls from 20 kWh to 14 kWh, the drop rate is 30%. In this case it is difficult to explain the decrease by dirt alone, and you should also suspect causes such as part of the panels being shaded, the power conditioner not operating properly, a tripped breaker, or part of the circuit being shut down.

Guidelines for Reduction Rates

If you feel your power generation is low, it's important to look at the "numbers" first, rather than relying on "intuition." Use a power generation monitor or app to separately compare the same month of the previous year, the previous month, sunny days, and rainy days.

Check 2: Confirm whether power output returns after rain

Next, what we want to check is whether the power output recovers after rain.

In typical residential areas, dirt and dust on solar panels are to some extent washed away by rain and wind. The Japan Photovoltaic Energy Association also explains that in Japan, regular rainfall and wind wash away the dirt, restoring the panels to almost their original performance.

Therefore, if the cause of low power output is light dust or pollen, you may see a slight recovery in output after a substantial rainfall. In particular, during periods with heavy spring pollen, yellow sand, and dust, it becomes easier to determine this by comparing power output trends before and after rain.

The method for checking is simple.

Look at the daily power generation and compare a sunny day before the rain with a sunny day after the rain. If the power generation clearly recovers after the rain, light surface dirt may have been affecting it. Conversely, if the power generation remains low after the rain, consider causes that rain would not remove, such as oily stains, bird droppings, stubborn dirt, shading, or equipment malfunction.

However, there are some points to be careful about when judging power generation after rain. Even the day after rain, conditions such as heavy cloud cover, high humidity, high temperatures, or low solar irradiance can make it difficult for generation to increase. Therefore, use a day that is clearly sunny after the rain as the comparison, rather than a cloudy day immediately following the rain.

What Can Be Learned from Changes After Rain

If the soiling can be removed by rain, you may be able to wait and monitor the situation instead of rushing to request cleaning. On the other hand, if there is soiling across a wide area that rain will not remove, or if bird droppings are attached to multiple panels, the reduction in power generation may continue.

Check 3: Inspect the panel surface for bird droppings, sand and dust, pollen, and yellow dust

The third check is whether there are any noticeable deposits on the panel surface.

There are two types of dirt on solar panels: those that are easily washed away by rain and those that are not easily removed by rain alone. Dirt that is easily washed away by rain includes light dust, sand dust, pollen, etc. On the other hand, dirt that is not easily removed by rain includes bird droppings, oil-containing grime, sap, stains from fallen leaves, and mud that has adhered for long periods. The Japan Photovoltaic Energy Association also explains that in areas adjacent to roads with heavy traffic, oily airborne particles can adhere and may not be washed away by rainfall alone.

Check for dirt from the ground or other safe locations such as a balcony, rather than by climbing onto the roof. Using binoculars, a smartphone’s zoom, drone surveys, or photos from inspection professionals lets you inspect while avoiding danger.

Particular attention should be paid to "localized, heavy soiling," such as bird droppings. Compared with a panel that is lightly soiled across its entire surface, when part of a panel is heavily soiled it can more easily affect the balance of power generation. Because solar panels generate power in coordination with multiple panels, if only a part is heavily shaded it can affect the power generation efficiency of the entire circuit.

Types of Stains and Guidelines for Assessment

Even if dirt is visible to the naked eye, you may not need to incur cleaning costs right away if the reduction in power output is small. Conversely, if even a single spot is heavily soiled and the power output is greatly reduced, it is worth consulting a specialized contractor.

Check 4: Verify changes in shadows, weeds, and the surrounding environment

The fourth check is to see whether anything other than dirt is blocking the sunlight.

A cause of low power generation that is as easily overlooked as dirt is "shading." Because solar power generation is strongly affected by the duration of sunlight and the amount of solar radiation, power output decreases when light is blocked by buildings, trees, utility poles, antennas, renovations or extensions to neighboring houses, storage sheds, weeds, and so on. Kansai Electric Power also explains that installation orientation and tilt affect generation efficiency, and that sunshine duration and the seasons influence power output.

The troublesome thing about shadows is that their occurrence changes with the season and time of day. Something that wasn’t a problem in summer can become one in winter, when the sun’s altitude is lower and shadows from nearby buildings or trees reach the panels. Also, if power output is low in the morning but fine at midday, or it drops sharply only in the evening, shading is more likely than dirt.

When checking, please look at the following times of day.

Shadows from weeds are rare on rooftop residential solar installations, but with carport-type systems, ground-mounted systems, low roofs, or warehouse roofs, shadows from weeds and nearby equipment can have an impact. In particular, for commercial or self-consumption systems, grass around fences, equipment boxes, signboards, and material storage areas can also cast shadows.

If shadows are the cause, cleaning the panels will do little to improve power output. If the time periods when generation is low are almost the same every day, suspect shadows rather than dirt.

Check 5: Inspect the power conditioner and monitor for abnormalities

The fifth check is for abnormalities in the power conditioner and monitor.

Low power output is sometimes caused by problems on the system side rather than by dirt. In particular, if the output suddenly drops significantly, if the output is close to zero, or if the monitor display is clearly abnormal despite sunny conditions, prioritize checking for equipment faults over dirt on the panel surface.

The points to check are as follows.

One thing to be aware of here is that "low amount of electricity sold" and "low amount of electricity generated" are not the same. If you use a lot of electricity inside your home during the daytime, the amount sold will decrease even if generation is occurring. If devices such as EcoCute, storage batteries, EV charging, air conditioners, IH, or dryers are being used during the day, the reduction may be only in the amount sold while the generation itself remains normal.

Be sure to check the "total electricity generation," not the amount of electricity sold. If the total electricity generation is normal but only the electricity sold is low, it may not be due to dirt or a malfunction but rather an increase in household consumption.

However, if total power generation is extremely low even during daytime on sunny days, there may be an equipment malfunction. If generation suddenly falls to less than half, an error appears on the monitor, the power conditioner has stopped, or there are unusual noises or a burning smell, do not attempt to operate it yourself; consult your retailer, installer, manufacturer, or maintenance provider.

How to distinguish reductions in power generation caused by soiling from those caused by faults

When power output is low, looking at the "pattern of decline" is useful to distinguish between dirt and faults.

If soiling is the cause, power output often declines gradually, and even on sunny days it will appear as a "slightly lower than before" pattern. In particular, if you observe a drop of around 5% during periods when pollen, yellow sand, dust, or bird droppings increase, it may be due to soiling.

On the other hand, when a malfunction or shutdown is the cause, power generation often drops sharply. For example, if it had been producing 20 kWh until yesterday but suddenly produces 5 kWh on a similarly sunny day, it would be difficult to explain that by mere soiling.

Power Generation Decline Patterns by Cause

When determining the causes of low power generation, it is recommended to look at trends over at least 1-2 weeks rather than relying on data from a single day. In particular, recording sunny days, cloudy days, and rainy days separately makes it easier to distinguish between soiling, weather, shading, and equipment faults.

What You Can Do Yourself and What Is Dangerous When Power Generation Is Low

There are things you can do yourself when the power output is low. However, climbing onto the roof or disassembling electrical equipment is dangerous.

The things you are allowed to do yourself are basically "ground-based checks" and "recording".

Conversely, the following actions should be avoided.

The Solar Power Generation Association also explains that, in typical residential areas, there is seldom any need to climb onto the roof to perform routine cleaning. Don’t try to climb onto your roof yourself just because your power output is low.

Cases When You Should Request Cleaning and Decision Criteria

So, in what situations should you request cleaning of solar panels?

The decision should not be based solely on whether dirt is visible. Whether it is worth incurring cleaning costs should be considered comprehensively, taking into account the rate of power generation loss, the type of soiling, recovery after rain, the installation location, and the risks involved in the work.

Cases When You Should Consider Cleaning

However, in the following cases, you should prioritize inspection over cleaning.

When choosing a cleaning contractor, check whether they have experience cleaning solar panels, whether they take manufacturer-recommended methods into account, whether they have safety measures for roof work, and whether they will provide before-and-after photos and comparisons of power generation. Choose a contractor who can perform the work in a way that won’t damage the panels, rather than one who simply sprays water.

Three Preventive Measures to Avoid a Decline in Power Generation

To prevent low power output, continuing the three practices of record-keeping, observation, and regular inspections is more effective than performing large-scale cleaning on a daily basis.

1. Record power generation once a month

To detect a decline in power generation early, it is important to keep a record of monthly power generation.

The items to record are monthly power generation, year-on-year comparison for the same month, maximum generation on sunny days, weather trends, and whether any error messages are displayed. If you can check these with a smartphone app, simply saving screenshots can also be useful.

In residential solar power systems, a guideline is about 2.7 kWh per day per 1 kW of system capacity, and about 1,000 kWh per year. However, because this varies depending on region, orientation, tilt angle, and weather, it should be used only as a starting point for comparison.

2. Understanding seasonal variations

You are more likely to perceive low power generation during the rainy season, the autumn rains, winter, periods with snowfall, and stretches of consecutive extremely hot days.

In winter, sunlight hours are shorter and the sun’s altitude is lower, so power generation tends to decrease. During the rainy season and autumn rains, cloudy and rainy weather persists, making it difficult to expect the same level of generation as on sunny days. Also, in midsummer, although solar radiation is strong, efficiency can decline if temperatures are too high. Kansai Electric Power also explains that there is a tendency for efficiency to drop when temperatures are too high, and in some cases power generation can be more efficient around April to May.

If you are unaware of this seasonal variation, you may attribute even a decrease within the normal range to "dirt." By making a habit of comparing with the same month in the previous year, you can more easily avoid unnecessary cleaning and inspection costs.

3. Aim to check the surrounding environment once a year

Even if the solar panels themselves don't change, the surrounding environment will change.

Nearby trees grow, extensions are added to neighboring houses, antennas and equipment are installed, carports and storage sheds are set up, and birds begin to gather on power lines—changes that affect power generation occur gradually.

At least once a year, check how shadows fall on a sunny day in the morning, at midday, and in the afternoon. Especially in winter shadows tend to be longer, so it's important not to judge based only on summer.

Frequently Asked Questions

Q1. Is low power generation mostly caused by dirt?

No. While dirt can be a cause, common reasons also include weather, season, hours of sunlight, temperature, shade, equipment malfunctions, and increased self-consumption. In particular, during prolonged cloudy or rainy periods, in winter, and in midsummer, power generation can be lower even without dirt.

Q2. Should solar panels be cleaned regularly?

In typical residential areas, light debris and dust are often washed away by rain, so there is generally little need to climb onto the roof and clean it on a daily basis. However, if bird droppings, oily stains, yellow sand, pollen, or fallen leaves are conspicuous, or if power generation does not recover after rain, it is worth considering cleaning or inspection.

Q3. How much of a decrease in power output should prompt suspicion of soiling?

If the decrease is around 5% compared with sunny days in the same month of the previous year, it may be due to light soiling or differences in weather. If a reduction of 10% or more persists, check not only for dirt but also for shadows and equipment abnormalities. A decrease of 20% or more, or a sudden drop, should prompt consideration of inspection before cleaning.

Q4. Will all the dirt be washed away when it rains?

Light dust and pollen can be washed away by rain, but bird droppings, oily stains, tree sap, and caked-on mud can be difficult to remove with rain alone. Near roads with heavy traffic, oily airborne deposits can adhere to surfaces and may not be washed away by rain alone.

Q5. If the amount of electricity sold is low, can I assume that the power generation is also low?

No. A low amount of electricity sold alone does not necessarily mean that power generation is low. If you use a lot of electricity at home during the daytime, the electricity generated will be consumed within the household, and only the amount sold will decrease. What you should check is the total electricity generated, not the amount sold.

Q6. Is it okay to clean it yourself by pouring water on it?

Avoid climbing onto the roof, as there is a risk of falling. Also, pressure washers, stiff brushes, and detergents chosen at your own discretion can damage the panels and surrounding components. If you determine that cleaning is necessary, check the manufacturer's or installer's instructions and consult a contractor experienced in cleaning solar panels.

Q7. What if the panels look clean but the power output is low?

Even if the panel surface is clean, factors such as shading, power conditioner malfunctions, breakers, communication errors, aging deterioration, and increased self-consumption can make the power generation appear low. If an error is displayed, or if output suddenly drops sharply even on a sunny day, prioritize checking the equipment rather than assuming dirt.

Summary

If you feel the power output is low, it is important not to immediately conclude that "the panels are dirty." The power output of a solar power system varies depending on the weather, season, hours of sunlight, temperature, shading, installation conditions, and the condition of the equipment.

To determine whether dirt is the cause, please check the following five items in order.

If it's only light dust, rain will often wash it away, and if the drop in power generation is only a few percent, it may be fine to wait and see. Conversely, consider cleaning or an inspection if production doesn't recover after rain, bird droppings or oil stains are noticeable, output is more than 10% lower than the same month last year, generation suddenly falls to less than half, or there are error messages.

For safety, avoid climbing onto the roof to clean it yourself. Accurately identifying the cause of reduced power generation and consulting a specialist only when necessary is the quickest way to keep solar power generation operating reliably for a long time while minimizing unnecessary costs.