Solar-panel output can fall when dust, pollen and bird droppings reduce the light reaching photovoltaic cells.
The scale is site-dependent: the IEA estimates soiling accounts for 4–7% of global solar-energy losses, while an NREL study found pollen cut output by up to 15% at several North Carolina sites.
These figures show why performance claims need operating context; they do not establish a universal cleaning gain.
The IEA’s photovoltaics program estimates that dirt is responsible for about 4% to 7% of global solar power losses. In a study of several facilities in North Carolina, NREL found that pollen reduced output by up to 15%. At the same facilities, cleaning with water and cleaning tools increased output by 5% to 11%. Rain alone did not remove all the dirt.
Results vary from installation to installation. In an informal experiment on YouTube, three 100-watt panels, with varying degrees of dirtiness, increased their output by about 2,4%, 10,8%, and 13,4% after cleaning. A Tesla installation owner reported a larger increase, but the calculation was based partly on a prediction and not on a comparison to a similar installation that had not been cleaned.
Therefore, the promise that a wash will automatically increase production by 20% does not apply to every installation. Bird droppings can unevenly shade individual elements of the panels and contribute to the creation of hot spots. In more challenging environments, salt air and certain pollutants can also increase the risk of corrosion.
When is cleaning worth it?
Cleaning makes economic sense when the value of the extra electricity generated covers its cost. Researchers at the University of California, San Diego, found that after 145 days without rain or cleaning, efficiency had dropped by 7,4%. If you cleaned a typical 5-kilowatt residential system for half that time, you would earn about $20 in extra electricity by the time the drought ended.
For example, if you are losing $20 worth of electricity per month due to dirt and the cleaning costs $200, it will take ten months to recover the cost. This is only true if the loss of production continues for that entire period. The production recorded by your system is a better way to judge whether cleaning is needed than the appearance of the panels. Prolonged drought or heavy pollen may make cleaning worthwhile.
How to clean panels safely
Before you begin, check the manufacturer's instructions, as not all cleaners are approved for the same use. Clean water is the safest place to start. Clean the panels when they are cool and check for cracks or other damage first. If you find any, notify the installer.
Use clean water with a little salt. If necessary, you can use a soft sponge, cloth or non-abrasive brush with an extension. If you use a cleaner that is approved by the manufacturer, rinse it thoroughly. Water with a lot of salt can leave metal residues, while abrasive sponges, metal scrapers and high-pressure washing can cause damage.
Qcells states that improper cleaning can damage the glass or its anti-reflective coating, cause layers to peel off, and void the warranty. The company does not set a fixed cleaning interval for all installations.
If you need to climb onto your roof and you don't have the proper training, hire a professional to clean it. According to Thumbtack, professional cleaning costs an average of about $209 in the U.S., with prices ranging from $142 to $310.
There are also automatic cleaning systems, such as timer-controlled sprinklers and robots that clean without water. Smaller systems are sold for residential use, while autonomous robots are mainly aimed at large facilities. Ecoppia, for example, targets ground-based solar projects of 50 MWp and above.
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