Do dirty solar panels matter in the Gallatin Valley?

How much does dust cost?

Less than people fear, in most places. A study of 186 solar systems across California found dust cost them about 0.05% of their output a day on average through the dry season: roughly 7.4% by the end of a 145-day drought. A good rain reset them.

California’s dry season is long. Mapping of solar sites across the country found most of the soiling happens in the Southwest, and NREL’s standard solar calculator assumes a 2% loss a year to dirt unless you tell it otherwise. Here, where spring and summer bring rain, a well-tilted array gets rinsed regularly.

Patchy dirt is different

Even dust dims a panel a little. Patchy build-up can do more: a bird dropping, a drift of leaves or a caked corner shading part of a cell. The International Energy Agency’s solar programme warns that uneven, localized soiling such as bird droppings “can lead to hot-spot effects on PV modules,” with risks that include cracked glass or cells and a melted backsheet.

Canadian Solar’s installation manual says shaded cells “can cause localized overheating, which in turn may negatively impact the module service lifetime,” and that a hot spot caused by poor maintenance voids its warranty.

That is the case for clearing droppings and debris before they sit for long, gently and the way the makers specify. It is not a reason to scrub a dusty array every month.

Snow costs more

In Montana the bigger winter loss is snow sitting on the panels. NREL modeled it for cities across the country:

Even so, the panel makers are relaxed about it. REC says clearing snow is “not essential,” Qcells says to remove it only without force, with a very soft broom, and Maxeon strongly recommends homeowners stay off the roof. A December array makes little power either way.

When cleaning is worth it

• Bird droppings, leaves or caked patches. Uneven dirt over part of a panel is the kind that can cause hot spots.
• Low or flat arrays. Canadian Solar says panels tilted at 15° or more generally may not need cleaning. Flatter ones hold the dust.
• After a long dry spell or a smoky summer. Montana’s air quality agency notes smoke affects the state in some summers, and it leaves a film.
• Near gravel roads, fields or construction. More dust, faster.
• When you can see it. REC’s own guidance is to clean when the dirt is visible.

How the panel makers say to clean

Their manuals agree on the essentials: cool panels, clean water low in minerals, a soft brush, and nothing else. REC names pressure and steam cleaners, high-pressure hoses, knives, blades and metal sponges as things that will invalidate the warranty. Canadian Solar says damage from the wrong cleaning voids its warranty, and rules out dry scraping. Qcells does not cover cleaning damage, and says not to clean with water when there is a risk of frost.

If your panels are on a roof, the safest cleaning is one you do not do yourself.