Does Rain Actually Clean Solar Panels? (Spoiler: No)

One of the most common things we hear from Connecticut homeowners is: “I figured the rain would take care of it.” It is a reasonable assumption. Rain washes your car, cleans your driveway, and generally rinses outdoor surfaces. So why would solar panels be any different?

The truth is that rain does not clean solar panels in any meaningful way. In fact, rainwater often makes things worse by leaving behind mineral deposits that accumulate over time. If you have been relying on rain to keep your panels performing at their best, you are almost certainly losing energy production and money without realizing it.

Why Rain Falls Short as a Cleaning Method

To understand why rain is a poor panel cleaner, it helps to think about what happens at a microscopic level when rain hits your solar panels.

Rainwater is not pure water. As rain falls through the atmosphere, it picks up dust, pollen, pollutants, and dissolved minerals. When that water lands on your panels and evaporates, it leaves all of those contaminants behind as a thin residue. This is the same reason your car looks spotty after a rain shower rather than sparkling clean.

Low-angle panels do not shed water cleanly. Most residential solar panels in Connecticut are installed at angles between 20 and 40 degrees, which matches the state’s latitude for optimal sun exposure. While this tilt allows some water to run off, it is not steep enough to create the sheeting action needed to actually carry debris away. Water tends to bead up, trickle down slowly, and leave streaks.

Rain does not apply any mechanical force. Professional cleaning works because it combines purified water with gentle scrubbing action. Rain simply does not have the force or friction to break the bond between stuck-on contaminants and the glass surface of your panels. It can rinse away loose dust particles on the surface, but anything that has adhered to the glass, which is most of what matters, stays put.

The Mineral Deposit Problem

Here in Connecticut, our rainwater has a particular composition that makes the mineral deposit problem worse than in many other regions. As rain passes through New England’s atmosphere, it picks up dissolved calcium, magnesium, and other minerals. When this water evaporates off your panels, it leaves behind hard water spots.

Over months and years, these mineral deposits build up into a hazy film that is nearly invisible up close but significantly reduces light transmission to the solar cells beneath the glass. Studies have measured light transmission losses of 5 to 15 percent from mineral buildup alone, before you even factor in other types of soiling.

For homeowners in Connecticut’s shoreline communities like Guilford and Madison, this problem is compounded by salt spray from Long Island Sound. Rain mixes with airborne salt particles and deposits a saline film that is particularly stubborn. The combination of mineral deposits and salt creates a coating that requires professional-grade purified water and proper technique to safely remove.

What Rain Cannot Remove

Beyond mineral deposits, there is a long list of common contaminants that rain simply has no effect on.

Tree pollen: Connecticut’s spring pollen season coats panels in a sticky yellow-green film. Pollen grains contain natural oils that cause them to adhere to glass surfaces. Rain can move pollen around, but it cannot dissolve or remove the oily residue it leaves behind. If you live near the heavily wooded areas of Hamden or Woodbridge, you know exactly how tenacious pollen can be on outdoor surfaces.

Bird droppings: This is arguably the most damaging contaminant for solar panels, and rain does almost nothing to remove it. Bird droppings are acidic, sticky, and harden quickly in sunlight. Once baked onto a panel, they create a concentrated shadow that forces the affected cell to become a resistor rather than a generator, creating hot spots that can permanently damage cells over time. Areas with high bird activity need prompt professional cleaning to prevent long-term damage.

Tree sap and resin: If you have pine trees, maple trees, or other sap-producing species near your panels, you already know that tree sap is essentially nature’s glue. Rain has zero effect on it. In fact, rain can spread sap residue across a larger area of the panel surface, making the problem worse.

Lichen and moss: In Connecticut’s humid climate, lichen and moss can begin to establish on panel frames and edges, eventually creeping onto the glass surface. These organisms are living things that actively grip surfaces, and no amount of rain will dislodge them. Left unchecked, they can cause permanent etching of the glass.

Industrial fallout and road grime: Panels near highways, commercial areas, or industrial zones accumulate a fine layer of particulate matter that bonds to glass over time. This soiling is often dark colored and absorbs heat, further reducing panel efficiency. Connecticut’s I-91 corridor, which runs through many of our service areas including Wallingford and North Haven, means panels in these areas pick up more road-related particulate than rural installations.

What the Research Actually Shows

The idea that rain cleans solar panels has been studied extensively, and the research consistently debunks this myth.

A widely cited study conducted at the University of California, San Diego found that panels left to be “cleaned” only by rain lost an average of 7.4 percent of their energy output over the study period. Panels in areas with more dust and less rain lost significantly more.

The National Renewable Energy Laboratory (NREL) has published multiple reports confirming that soiling losses are a persistent problem across all climates, including those with regular rainfall. Their data shows that while heavy rainstorms can provide a temporary partial rinse, the effect is inconsistent and never restores panels to their clean baseline.

A study published in the journal Renewable Energy specifically examined the effect of rain on panel soiling in temperate climates similar to Connecticut. The researchers found that while rain events reduced surface dust levels by an average of 20 to 40 percent, the remaining 60 to 80 percent of soiling, which included adherent deposits, biological matter, and mineral films, was unaffected. More critically, each rain event added new mineral deposits, meaning the net cleaning effect of rain over time was effectively zero.

Research from Sandia National Laboratories has also documented that panels in regions with moderate rainfall often develop worse mineral deposit problems than panels in arid regions. This counterintuitive finding makes sense when you consider that frequent rain events create more cycles of wetting and drying, each one leaving behind another thin layer of mineral residue.

Professional Cleaning vs. Rain: The Difference

Professional solar panel cleaning works because it addresses every type of soiling systematically.

Purified water systems: Professional cleaners use deionized or reverse-osmosis purified water that contains zero dissolved minerals. When this water is used on your panels, it leaves no residue behind as it dries. This is the exact opposite of what rainwater does. Purified water actually dissolves and lifts existing mineral deposits off the glass.

Appropriate mechanical action: Using soft-bristle brushes or specialized cleaning pads, professionals can safely break the bond between adhered contaminants and the panel glass without scratching or damaging the anti-reflective coating. This gentle scrubbing is what removes pollen residue, dried bird droppings, tree sap, and other stuck-on materials that rain cannot touch.

Panel-safe techniques: Professional cleaners know the correct pressure, angles, and methods to clean panels without voiding manufacturer warranties. Using the wrong cleaning products or abrasive tools can damage the anti-reflective coating on panel glass, actually making panels perform worse. A quality residential cleaning service protects your investment while restoring performance.

Complete coverage: Professionals clean every panel systematically, including edges and frames where debris accumulates and lichen tends to establish. They also identify potential issues during cleaning, such as cracked glass, failing seals, or wiring problems, that homeowners cannot see from the ground.

How to Tell If Rain Has Not Been Enough

If you have been relying on rain to keep your panels clean, here are some ways to assess whether your panels need professional attention.

Check your production data. Compare your current energy production to the same months in your first year of panel ownership. If production has gradually declined by more than 5 percent after accounting for normal panel degradation (about 0.5 percent per year), dirty panels are the most likely culprit.

Look for visible streaking. On a sunny day, look at your panels from an angle. If you can see streaks, haze, or spots, those are mineral deposits and soiling that rain has left behind or failed to remove.

Compare shaded vs. exposed panels. If you have some panels that receive more rain runoff from upper roof sections and others that are more sheltered, check whether there is a visible difference. Often, the panels receiving more rainwater runoff actually look worse because they get more mineral deposits.

The Bottom Line

Rain is not a substitute for professional solar panel cleaning. Relying on rain to maintain your panels is like relying on rain to wash your car. It might knock off the loosest surface dust, but it leaves behind everything that actually matters, and it adds its own layer of mineral residue in the process.

Connecticut’s climate, with its pollen-heavy springs, humid summers, leaf-filled autumns, and salt-tinged coastal air, creates a combination of soiling challenges that rain alone cannot handle. Homeowners who invest in regular professional cleaning through a residential service plan consistently see better energy production, better long-term panel health, and better returns on their solar investment.

If it has been more than a year since your panels were professionally cleaned, or if they have never been cleaned at all, the accumulated soiling is almost certainly costing you money every single day the sun shines.