The following guide provides an overview of solar panels cleaning features: why should solar panels be cleaned? Which substances cause the most harm to the panels? Which are the main safety risks of dirty PV modules? How is the cost-effectiveness of a system cleaning determined and what should be considered when choosing the right partner for your PV system?

1. The Solar Panels Cleaning Guide

The atmospheric conditions and other environmental influences constantly affect the state of PV plants. Dirt and other kinds of contamination agents, which are deposited on the surfaces of the modules, prevents or reduces the direct incidence of sunlight, with negative effects on the yield of the cells of the system.

The contamination of solar panels occurs in different ways and forms, not always visible to human eyes. However, even the thinnest layers of dust and soiling, such as soot and pollen, together with birds’ excrements and leaves, can strongly affect PV systems performances.

A significant risk is represented by mosses and lichens, which attack the frame and the module components, causing permanent damage to the plant.

All these impurities represent a substantial risk for the PV system efficiency.

This guide provides a brief overview of the basic aspects of solar panels cleaning. What are the factors that play a role in the soiling of solar panels? Which cleaning techniques are used by professional companies? What are the requirements for cleaning? When is it worthwhile to contact a specialist company? In brief: how to clean solar panels effectively?

2. Types And Causes of PV Systems Soiling

a. Pictures of contamination

Although the type and degree of contamination may differ from place to place and from region to region, recurrent types of contamination can still be observed. Professional cleaning companies are prepared to recognize these images and to clean the PV system with the right technology. The most common types of contamination are the following:

Dirt on solar panels

Bird droppings on a solar panel

Pollen traces on solar panels

Dirt accumulating on a solar panel

b. Causes and contamination factors

Not only do images of contamination have a recurring character. Structural, territorial and meteorological factors affecting the deterioration of PV installations can also be identified. Among the most important risk factors for increased contamination are:

Roof or panels inclination

As the inclination of the modules decreases, the risk that dirt and dust particles will resist on the surfaces, despite the rain, increases. As a result, dirt on edges and frames accumulates quicker as the inclination angle gets smaller, with the long-term risk of accumulation on the inner surface of the module. Increasing the width of the edges can make the absorption of additional dirt particles faster.

Solar panels frames

Very often dirt and particles accumulate on the frames of PV modules, as explained above. Precipitations carry this dirt and debris to the frames, where it settles and can contribute to the formation of moss and soot. In this sense, frameless modules can be an advantage (e.g. thin film), although they are considered somewhat more unstable.

Transverse mounting of the solar modules

An alternative method for the installation of solar modules is the so-called transverse mounting: the longer side of the solar panel is mounted downwards/upwards. Transverse mounting increases the surface area exposed to dirt, as the longer side of the module is exposed to rain.

In most solar panels, the distance between the frame and the solar cell on the longer side of the module is also smaller. Dirt and mosses thus accumulate more quickly, reducing the yield of the modules.

Agricultural and farming environment

In agriculture, especially when heavy machinery is used, dust and dirt are created and transported through air and deposited on the solar panels. At the same time, the use of chemicals and the emission of fumes on livestock farms can cause massive and stubborn contamination of PV modules.

Industrial environment

The industrial environment usually has a lower negative impact on PV plants than the agricultural. However, even in this case, contamination by fine dust and soot can occur, which significantly reduces the yield in absence of adequate maintenance measures.

Dry regions and periods of low rainfall

Rain and snow help to remove dirt from the solar panels cleaning. In regions with low rainfall or in periods of drought, this automatic cleaning effect inevitably decreases and dirt accumulates faster on the modules. However, it is important to point out that rainfall alone is not sufficient to ensure the long-term cleanliness of PV systems.

3. Degrees of Contamination

In June 2018, Milk the Sun published its first study on the economic convenience of ground-mounted solar panels cleaning. In this study, five degrees of contamination were defined, which can serve as a benchmark for determining the washing requirements.

The five levels of contamination are defined as follows:

Degree of contamination 1
Very low contamination: light layer of dust on the solar panels, barely visible to human eye.

Degree of contamination 2
Low contamination: slight layer of dust, clearly visible on the modules.

Degree of contamination 3
Medium contamination: more consistent dust deposits, traces of bird excrements, early signs of moss and lichens on the edges of the modules.

Degree of contamination 4
Heavy contamination: significant layer of dirt with incrustations, bird excrements and a high percentage of mosses and lichens on the edges.

Degree of contamination 5
Very heavy contamination: consistent and stubborn incrustations on the modules, diffusion of mosses and lichens on the edges, (possible) module surface no longer recognizable and (possible) irreversible contamination of the modules.

4. Safety Risks of Dirty PV Systems

If individual cells of the module are partially soiled or covered, they may overheat. In the worst-case scenario, if the cells overheat too much and the module is damaged, combustible material that lays under the module may catch fire. In most cases of overheating, however, long-term damage to the modules, with a consequent reduction in performance, occurs without a fire.

If rainwater accumulates on the lower edge of the modules, which can happen especially with a low inclination of the modules, and remains there for a long period of time, moisture can enter the laminate and cause delamination problems.

5. Overview of Solar Panels Cleaning Techniques

In general, the various cleaning techniques commonly used on the market can be divided into three macro areas: manual cleaning, machine cleaning and robot cleaning. Within these areas, the washing process also differs considerably in the use of detergents, water filters, instrumentation and equipment.

The most common method of solar cleaning is the use of resin filters, which desalinate and demineralise the water used to avoid residues. Some suppliers also use special cleaning methods. When using detergents, you should ensure that they are approved by the module manufacturers and environment friendly.

a. Manual cleaning

Manual cleaning includes all washing techniques where the cleaning is carried out by manpower using special manual brushes. In case of very small systems, the cleaning is usually carried out according to the principles and standards of the cleaning of normal glass surfaces.

For the cleaning of larger systems (from 7-10 kWp upwards), Brushes are placed on telescopic rods, which are rotated electrically or mechanically by water pressure. Due to the higher water flow rate, high-pressure water-operated systems usually achieve slightly better surface performance than comparable electric brushes.

b. Machine cleaning

When the cleaning is carried out with a machine, the brush systems are moved using vehicles (e.g. tractors). This considerably accelerates the cleaning process. Large module surfaces can thus be cleaned very efficiently in a short period of time.

This type usually requires that the distance between the module rows is large enough to allow the vehicle to pass through.

c. Robot cleaning

This technique involves the use of specific robots. These robots are mounted on the systems by the service provider and guided through the modules with sensor technology or by remote control.  Although the technology here is constantly improving, solar panels cleaning with robots is still limited by the inclination or by mounting features of the modules.

6. When Should A Cleaning Take Place?

The necessity of keeping the PV system clean is based on two essential factors (apart from security): an efficient solar yield and the preservation of the investment.

In the event of a clear loss of performance of the PV system due to contamination, washing is more than welcome. The economic aspect of the matter is dealt with in point 6a below.

If the contamination is found to be affecting the long-term preservation of the system, the safety and operation of the modules and the construction, the system must be washed as soon as possible.

a. When is the solar panels cleaning worthwhile?

When it is worthwhile to clean a photovoltaic system is ultimately an economic question. The cleaning costs have to be covered by the additional efficiency generated by the cleaning.

To calculate the economic viability of a washing service, two factors must be taken into account: the number of kilowatt hours that a PV plant must generate as additional yield to recover the cost of washing, and the percentage of additional yield for the total production of the plant.

The result is ultimately a percentage value that indicates how much the system efficiency must increase in a year (!) thanks to the cleaning, so that the costs are amortised within this period.

b. The cleaning study by Milk the Sun

In 2018, Milk the Sun conducted the first scientific study on ground-mounted PV systems cleaning to find out what influence professional cleaning has on yields.

During the study, 17 different ground-based systems were partially cleaned. The yields of the clean surfaces were then compared for a period of six months with the yields of the uncleaned sections of the plant.

The factors that most influenced the success of the washing were: the degree of inclination of the modules, the degree of contamination and the plant grid connection date.

The study can be requested in German following this link:

Cleaning Study

7. Solar Panels Cleaning Requirements

Usually, the requirements for washing a PV system are: accessibility to the surface of the modules, connection to running water and connection to the power grid. The latter, if necessary, can be provided at an additional cost by the company providing the service. All other necessary equipment and tools, on the other hand, are usually provided by the service supplier.

Depending on the type of washing, there are special requirements regarding the accessibility of the system, which must be examined on a case-by-case basis.

Safety precautions for rooftop installations

In case of rooftop cleaning, the employees of the service companies are obliged to observe all the special measures needed to grant their safety and the compliance with the safety regulations. For this reason, it is very important to inform the service provider in advance of the condition of the roof and the system in preparation for the washing operations. An adequate notice enables the personnel to be optimally prepared for the work and to carry out the cleaning with the mandatory safety precautions and to use the necessary lifting or scaffolding equipment.

8. How To Find The Right Partner

There are many service companies for your photovoltaic cleaning. Therefore, it is necessary to consider the offer, presentation and professionalism of the potential partner.

In order to facilitate the owners of PV systems in the assessment of service providers, several certificates are issued, for example by TÜV Rheinland or OQS. The relevant certification criteria concern: the quality of training; the knowledge of the structure and functioning of solar technology and the special cleaning techniques for the respective module types; the use of professional equipment and a structured procedure for carrying out the washing; the availability of documentation on recognisable damage and potential sources of danger for the PV system.

Find the right cleaning partner with Milk the Sun

Milk the Sun relies on a network of specialized, certified and experienced partners to respond to the needs of its customers. Do not hesitate to contact us when you are considering the cleaning of your PV system:

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