How to Clean Your Solar Panels Properly (And Why It’s Important)

Solar panels stay outdoors for their entire lifespan, so it’s optimistic to think they’ll stay pristine. Dust, pollen, bird droppings, and sometimes a layer of mysterious dirt build up on the glass – and if enough builds up, the amount of sunlight reaching the cells below is affected.

Not all roofs that are a little dusty require emergency cleaning. The amount of cleaning needed is highly dependent on the location, amount of rain, angle of the panels and what is inside them. The biggest problem is cleaning them poorly. Scratches in the coating, hard water deposits, and an overzealous pressure washer can lead to a problem even more expensive than the dirt you started with.

Power loss from dirty panels is not a fixed number. According to the IEA’s Photovoltaic Energy Systems Program, dirt is responsible for about four to seven percent of global photovoltaic power losses, and an NREL study found that pollen reduced energy production by up to 15 percent at several solar power sites in North Carolina. Rain was also not enough to completely repair the problem, with wet mechanical cleaning improving performance by 5-11%.

Concrete results can be everywhere. In an informal test on YouTube, three progressively soiled 100-watt panels experienced a gain of approximately 2.4%, 10.8%, and 13.4% after cleaning. Another Tesla Rooftop Grid owner reported a much larger improvement, but this was partly based on projected production rather than a comparable control setup.

And that’s the point. If a cleaner guarantees that washing your panels will automatically unlock an extra 20%, your roof might not want to comply. Uniform dust means above all a loss of production. Localized damage can be more problematic. Bird droppings can unevenly shade individual cells and contribute to the formation of hot spots, while salt-laden air and certain pollutants can increase the risk of corrosion in more severe environments.

There is also money. UC San Diego researchers found that panels that lasted 145 days without rain or cleaning lost 7.4 percent of their efficiency, but cleaning a typical 5 kW residential system in the middle of that period would have produced only about $20 in additional electricity before the drought ended.

If the grime costs you $20 a month in extra electricity on the grid and the cleaner costs $200, then you have a 10-month refund. This only works if the loss persists for 10 months.

Always start with the manual for your panels. No, seriously. Not all manufacturers recommend the same cleaning products. Some allow mild soap while others allow certain window cleaners or limited isopropyl alcohol for stubborn marks. The safest place to start is clean water.

Work during cool parts of the day, preferably early in the morning or late in the evening, as the temperature of the solar panels can also affect performance. First check the glass for cracks or damage and contact your installer if you spot any. Otherwise, rinse gently with clean, low-mineral water. Use a soft sponge, cloth or non-abrasive extension brush for dirt that won’t come off. Rinse any approved cleaner thoroughly.

Hard water can leave behind another layer of light-blocking mineral deposits as it dries, which rather defeats the purpose. Avoid abrasive pads, metal scrapers and pressure washing. Qcells states that improper cleaning may damage the glass or anti-reflective coating, cause delamination and leave the resulting damage outside of its warranty. It also does not prescribe a fixed cleaning interval for each installation.

Your production data is a better clue. Cleaning may be helpful if yield drops after a prolonged dry period or high pollen concentration. If production appears normal, a dusty appearance alone does not justify paying someone to go on your roof. If you have to climb onto the roof to reach the panels and you are not trained to do so, hire someone who is. Thumbtack says most people pay around $209 for professional solar panel cleaning, with a national average range of $142 to $310.

Automatic cleaning systems are another option, from timer-controlled roof sprinklers to waterless robots. Smaller sprinkler systems are sold for homes, while fully autonomous robots are largely intended for large-scale sites. Ecoppia, for example, targets ground-mounted solar projects of 50 MWp and above.