As a solar home owner, I’ve often wondered if it’s necessary to clean my solar panels. Dust collects during dry summer months, pollen and tree flowers accumulate in the spring, tree leaves and seeds gather in the fall, and snow piles up in the winter. The type and amount of material that accumulates on solar panels varies from locale to locale and season to season.
Any type of debris on photovoltaic (PV) panels can reduce electric generation as much as 10%, according to our local expert, Mike Hewitt at E2 Solar in Bend, Oregon. Researchers at University of California at San Diego found that dirty panels cut energy production by about 7.4%. Because of the high electric rate in San Diego, this reduction may cost the homeowner almost $100 per year in lost energy production. At my house in Central Oregon, I noted a 6.8% improvement in output after cleaning the accumulated dust off of my solar panels. My electric rate is half that in San Diego, so the dollar savings were much less. Clearly the economic benefit of cleaning panels will be higher if you pay more for electricity or if you’re living off the grid than if you are grid connected in a low rate area. This is good to keep in mind before you consider hiring someone to clean your panels or try doing it yourself.
Under most circumstances cleaning panels is just not worth the time. Most solar professionals advise owners to let nature do the work. Eventually, rain will wash away the dust, wind will remove leaves and other debris, and the sun will melt the snow. The amount of additional energy you get from a totally clean residential PV system seldom significantly exceeds the cost or the effort to clean them yourself, especially with the risk of falling off a ladder or roof. So for PV panels, cleaner is not necessarily much greener.
When Cleaning Might be Justified
Panels mounted at a lower angle are more likely to accumulate dust, leaves, pollen, snow, and other debris, and are more likely to need cleaning. My panels sit on a low pitched roof at 3-in-12 or about 11 degrees. The high-desert climate in Central Oregon generates lots of summer dust, which gradually accumulates on my collectors, so I decided to clean them in spite of prevailing expert advice. More disturbing than dust was last winter’s long freeze that piled about 30 inches of snow on the panels and that then refused to melt for six weeks. Even though I projected only a modest loss in production, I hemmed and hawed for several days. It wasn’t until I saw the panels start to bend under the snow’s weight that I knew it was time to clear them myself. In retrospect, I calculated that I lost 200 kwh of production or about 3% of my total for 2016 thanks to the prolonged snow cover. So anything that accumulates on the panels that cannot easily be removed by rain, wind or sun, such as bird droppings or algae growth, may justify the trouble of cleaning the panels, but even that is a judgment call.
If cleaning your panels is required, or if you simply can’t resist the impulse, here are a few things to remember. Never spray cold water on hot solar panels. Thermal shock can shatter the glass. Therefore, clean or clear in the morning or when outdoor temperatures are mild. The simplest and safest procedure for removing dust and debris, which is sufficient in most cases, is to spray water from the ground, using a high-pressure garden hose or power washer. In some cases, using a long pole with a brush or squeegee attached will do the job, but be sure to watch out for power lines. Do as much work as you can from the ground or, if needed, from a sturdy step ladder. Climbing on the roof adds more risk and is best left to the pros.
I wash my solar panels the same way I wash my windows. I get up early on cleaning day so I am finished before the panels received enough direct sun to make them hot. A cloudy, cool day also works great. From an 8-foot, self-supporting ladder, I spray the panels thoroughly with water from the garden hose. The idea is to knock off all the loose grit. Next, I pull out the extension pole that I use to clean high windows. It has three sections and extends to roughly 15 feet. On the end, I attach a micro-fiber brush. With the panels still wet, I run the brush over the entire surface. Be careful not to miss any spots, because most solar cells are wired in series, shading one small spot can reduce the electrical output of an entire panel and one panel can compromise the efficiency of the others in that same string. When needed, I use a bit of dish soap solution for scrubbing and then rinse with a hose.
The greatest thing about photovoltaics is that the fuel they use – sunshine – is free. The next best thing is that most people, with the exception of neat-nicks like me, don’t need to do anything to keep their solar panels pumping out the free electricity.