How much does dust effect solar panels?

No humans live on Mars yet, but several robots have. It’s really dusty on Mars, but in spite of that, the solar powered Opportunity rover lasted for 15 years.

There’s no rain on Mars, and Opportunity didn’t have a way to clean its panels, so there were times when its power output was severely degraded. With the help of Martian breezes that would blow dust off its panels, Opportunity managed to generate enough power from its solar panels to far exceed its planned 90 day mission.

If you’re a homeowner, I hope that you live somewhere a little less dusty than Mars and get at least occasional rain. If you do own solar panels and live in an arid climate you might notice that they sometimes - like the Opportunity rover - accumulate a coating of dust after a prolonged dry spell.

You might be tempted to clean your panels or even hire a service to clean them for you. While it’s certainly true that solar panels will generate more electricity when free of dust, it’s not always worth the effort to clean them. In fact, careless cleaning can damage solar panels, leaving you with a system that performs worse in the long run.

Still, there are some situations where cleaning your solar panels might be a good idea. This article will explain the impact that dust and soil have on solar panels, and how to decide if cleaning your panels is worth the trouble.

How much efficiency can solar panels lose due to soiling?

If they’re covered with enough dirt, it’s possible for solar panels to lose 100% of their power output. This has been observed in large solar arrays in desert climates where sandstorms occur.

However, most homeowners live in less extreme environments. (This site is US-based and will focus on conditions typically found in the United States.)

In most cases, you can expect at most around a 5% annual loss of solar electricity generation due to soiling (dust), with many locations in the US experiencing less than 1% losses on average.

This data comes from research by the National Renewable Energy Laboratory, which collects data from 124 stations around the US. They have a handy map of its 124 data collection sites. You can click on a point on the map near you, or use the table to find a site you’re interested in. The sites are concentrated in the parts of the US with the largest markets for solar.

The statistic you’re interested in is IWSR, which is Insolation-weighted Soiling Ratio. It sounds complicated, but it’s the fraction of power the solar panels at that site generate compared to a perfectly clean array. If the IWSR for a site is listed as >0.99, it means that the site produced more than 99% of its maximum, after accounting for soiling losses.

The table includes other data, including IWSR lower and IWSR upper. These are higher and lower values for IWSR collected by the station, which helps to give a better picture of the best and worse case scenarios you can expect.

By exploring the data a little, you’ll find that most major solar markets in the US don’t have to worry too much about dusty solar panels. Sites in Florida, Texas, and Northeast have IWSRs greater than 0.99%, which means they lose very little power generation due to dusty panels.

The worst places for dusty solar panels in the United States

If you want to see the worst places for solar panel soiling in the US, scroll down to the table and click on IWSR column to sort by the lowest values first.

The worst is Kings County, California with an IWSR of 0.936 (or 6.4% annual losses of solar production). Kings County is located in the San Joaquin Valley, and 87% of the land is dedicated to agriculture. This means plenty of dust flying around from farming activities. That’s really a worst-case scenario in the US, so a 6.4% average loss of electricity really isn’t that bad.

The next worse is Los Angeles County - which is famous for cars and smog - with 5.1% losses.

The third worse is in Kern County, California, which includes the city of Bakersfield. It’s dry, and agriculture is a significant part of the economy there too. The NREL site shows 5.1% losses as well.

If you explore the data further, you’ll see that other sites within the same county have lower values. In fact, there are several sites in Los Angeles County with an IWSR greater than 0.99.

The NREL data shows that you probably don’t need to worry about solar panel soiling

According to the NREL data, solar panels located even in agricultural areas don’t get too dusty, with about 6% losses the worst you can expect on average.

In addition, the tilt of the panels in the NREL systems is often low or flat. For example, the Kings County site with 6.4% losses has a tilt of 0°, which means that it’s laying flat. Having a solar panel with zero tilt makes it more prone to collect dust, and reduces the cleaning effect of rain. In comparison, most homeowners mount their panels on a roof with a tilt between 25° and 40°.

This means that even if you are located in a location like Los Angeles where dust might be a problem, having a tilted array will help reduce the impacts. (Of course, some homes do have flat roofs.)

Solar panels have anti-soiling coatings

The top glass layer of a solar panel isn’t ordinary glass. Not only does it need to be strong enough to withstand abuse such as hail impacts, but manufacturers incorporate special coatings to reduce reflections (and thus increasing the amount of light that hits the solar cells) and anti-soiling coatings.

An anti-soiling coating is hydrophobic (water-repellant). This helps solar panels to be “self-cleaning” when rain occurs, as the water rolls off the panel more easily, taking dirt with it.

This is one reason why homeowners usually shouldn’t worry about dirty solar panels. If the windows of your house are really dusty and that makes you think that you need to clean your solar panels, the special coatings that your panels have will help to keep than cleaner than ordinary window glass.

What’s the risk of cleaning solar panels?

If you plan to pay a service to clean your solar panels for you, the main risk is wasting your money because you can expect to pay $150 or more per cleaning. Cleaning your panels yourself will save you money, but then you have the risk of getting hurt by climbing up a ladder to your roof. (That’s one advantage of ground-mounted solar panels.)

Regardless of who does the cleaning, causing abrasion of the front glass and its anti-reflective and anti-soiling coatings is a risk. This means that if you regularly clean your panels, you might end up reducing the performance of the panels by damaging the coatings. A study in 2015 (PDF) showed that abrasion can occur after repeated cleanings.

While coatings may have improved since 2015 and could be more resistant to cleaning, I think there’s still a risk that cleaning your panels will end up being counterproductive.

When should you think about cleaning your solar panels?

If you live in an area that is dusty, you can determine whether it makes sense to do regular cleaning of your solar panels. Look up your average annual electricity production in kilowatt-hours, then calculate 6.4% of that (the worst value in the NREL data). Then multiply that number by your cost of electricity at peak hours. The resulting number is the cost of dirty solar panels in the worst case scenario (on average).

Is that number higher than the cost of cleaning? Keep in mind that cleaning won’t reduce your soiling losses to zero - the panels will still be a little bit dirty between cleanings. More frequent cleanings can help with that, but that will cost you more money and risk damage to your solar panels.

Unless you have a situation where your solar panels are exceptionally dirty, you’ll probably find that doing cleaning as routine maintenance doesn’t make sense for your solar array. Just wait for the next rain to wash your panels.

However, there are a few times when a cleaning might be a good idea. Bird poop is one of them. Sometimes you’ll find that bird poop just isn’t washing off from rain. Another is if you’ve experienced exceptional conditions, such as a nearby forest fire or a dust storm. In those cases, you might find that your panels are more soiled than normal. If you’re in the middle of a drought (as has been occurring more frequently on the west coast), it might be awhile until it rains again.

A cleaning might be warranted cases like this. The general rule is to use soft tools and distilled water if your municipal water is hard. Cleaning solutions usually aren’t needed, but you can use a few drops of dishwashing liquid if you like. You can read my article on cleaning solar panels for more detailed tips.