Copyright 2018-The Energy Show, Barry Cinnamon

When a business or homeowner gets a new rooftop solar installation, the second question they always ask is “how often do I need to clean my solar panels.” We’ll answer that question on this week’s show — taking into account the different effects of rain, dust and electric rates. BTW, the first question people always ask is “how do I read my electric bill;” but that’s a topic for another show.

Rooftop solar panels get dirty primarily from wind-blown dust and pollen. Birds are usually not a problem unless your last name is Hitchcock and you live in Bodega Bay. As panels get dirtier, their output declines. A small amount of soiling — say a light dusty film — may only cause a 5 percent output decline. However, when panels get very dirty — perhaps in an agricultural area or location that does not get regular rainfall — the output decline can be greater than 20 percent. A good heavy rainstorm will usually wash away most of the accumulated soiling.

I use the term “usually” because on panels that are tilted at about 5 degrees or less, the rain may leave a puddle of muddy debris along the lower edge of the panel. When this puddle dries, sometimes a thick layer of dirt accumulates along the lower row of cells (sometimes moss and weeds may even grow in these areas). Depending on the design of the system, this small accumulation of dirt can cause a very significant decrease in output.

So the answer to the question: “how often should I clean my solar panels” really depends on five factors: your location (does it rain regularly or only during certain months), the tilt angle of your panels (steeply tilted panels tend to stay much cleaner than panels that are close to horizontal), the amount of wind blown dust, your electric rate (if your electric rate is high then it is more worthwhile to clean your panels), and the cost to clean your panels.

If you have a large solar array at a low tilt angle in a dry climate with high electricity costs, our basic advice is to clean your panels once a year. Under these circumstances the additional electricity output from clean panels will be much greater than the cleaning costs. On the other hand, if you have a small array in an area that rains regularly, then it may only make sense to clean your panels every five years or so. Here in California it generally does not make a lot of sense to clean your panels in the late fall or winter during the rainy season.

Regardless of your circumstances, please make sure you clean your panels with soapy or treated water to prevent damage from mineral deposits. Contact your solar contractor or maintenance company if you would like your system cleaned professionally. For more about keeping your solar system operating at top efficiency, please Listen Up to this week’s edition of the Energy Show.