A solar panel is a piece of glass exposed to the elements and just like your car sitting outside, dust settles on it. Certain areas have more dust in the air than others due to local commercial activities, wind patterns, etc. Dust is not transparent to light so therefore it blocks light getting to the solar panels, resulting in reduced generation. Since it costs money to wash the solar panels, we don’t say wash them every week. However, lost production also costs you money, so you can’t just rely on the rain to do it for you, so we can’t put it off forever.
Rain is by far the best way to wash your solar panels for two reasons. Rain is soft water, free of minerals so there are no water spots from water drying on the surface from rain water. It is also typically colder so the water doesn’t just dry on the surface. If your panels are dirty and you get just a few drops of rain, like during the summer, they don’t get cleaned very well, but the production doesn’t get much worse either. However, it doesn’t rain for months at a time in many locations, especially in California. So therefore we can’t rely on rain to get our panels cleaned.
You can easily lose 10% to 20% of your production with dirty panels. Depending on the size of your system it can cost $0.50 to $1.00 per panel to get them washed. So how often should you wash your solar panels? 15% of output from a single solar panel will mean every week you lose a day of production. In PG&E territory, with their high electricity rates only going higher, a typical 350 Watt solar panel generates about $0.50 worth of electricity each day. Therefore, if the production is down about 15% according to your monitoring system, it is definitely time to wash the panels.
In Northern California, we wash the solar systems twice a year in the summer months; the first time a couple of months after the rains cease in May and again a couple of months after that. That usually carries us to the rains that start up again in October. However, you might be in a dusty location and find you need to do it three or even four times a year. Don’t bother washing them if it has been raining, because rain is the best way to keep your solar panels clean.
One other note about washing; we have found that additives in the water only seem to make the dust stick better and acts like it is attracting dirt. Also, you never want to wash the panels when they are hot because the water will dry leaving spots just like a shower door glass and those spots are not transparent to sunlight since they are dried minerals and they are very hard to get off once they are there. We have found the best way to wash them is with a two man crew, one washing and one drying with a 36” squeegee. Every 20 minutes, switch jobs and before you know it the job is done. If you have workmen on site you can trust from a safety point of view to handle the project, all you will need is a hose and a floor squeegee you can pick up from any commercial cleaning supply company. It is not worth risking an accident, however, so if you have any question about the safety aspect, it is better to hire a solar panel cleaning company of your choice and setting up a schedule with them.
With CitiCare, we will wash your panels twice a year while also monitoring the output and keeping all other aspects of your solar system running at their peak performance. One thing about generating power from the sun, it is like water under a bridge, once the day has occurred, there is no way to make up for that lost production.