Guide · Solar Panel Cleaning
The honest answer is: sometimes a lot, sometimes barely at all — it depends entirely on what's on the glass. A little dust that the next rain will handle isn't worth much fuss. A row of bird droppings shading live cells is another matter. Here's how to tell the difference, what the numbers actually say, and how to clean panels without touching the warranty.
Plenty of "clean your panels" advice skips the part where rain already does some of the job for free. So let's start there. If your panels have a thin, even film of general dust, cleaning them will help a bit — but often only by a few percent, and the next good rain would have washed much of it off anyway. In a rainy climate like Simcoe County's, that self-cleaning effect is real and worth counting on.
Where cleaning genuinely pays off is when something is on the glass that rain can't rinse away: sticky spring pollen, agricultural dust drifting off a nearby field, sap under trees, and above all bird droppings. Those don't sheet off in a shower — they bake on and stay. That's the soiling that quietly costs you production, and that's when a clean earns its keep.
Researchers have measured this carefully. NREL's long-running work puts a typical soiling loss around 5% of annual output, but the honest headline is the range: depending on climate, dust, and rainfall, real-world losses run anywhere from roughly 2% up to about 25%. Dry, dusty regions with long rain-free stretches sit at the high end. Rainy regions — and the northeastern climates most like ours — sit near the bottom, because frequent rain keeps the light dust in check year-round.
So for a typical home in our area, general dust probably isn't costing you a dramatic amount most of the year. That's the honest framing. The reason to clean isn't a scary percentage — it's the specific, localized soiling that rain leaves behind and that hits harder than its size suggests. Which brings us to the birds.
This is the part most people find surprising. A thin layer of dust dims the whole panel evenly, and a panel that's 5% dimmer makes about 5% less power — annoying, but proportional. A bird dropping does something worse. It sits opaque over one or two cells and blocks them almost completely, and because the cells inside a panel are wired in series — like a string of lights — one heavily shaded cell can choke the current flowing through its whole string. A single dropping can knock down far more than its size would suggest, sometimes taking a big share of the panel's output with it.
Rain rarely washes bird droppings off — they're too stuck-on — so they just stay there, shading cells, until someone cleans them. That's why localized droppings, not general dust, are usually the strongest reason to actually get panels cleaned. If you can see droppings on your array, that's the clear-cut case.
Solar glass has a delicate anti-reflective coating — the whole point of which is to let more light into the cells. Scratch it, etch it, or leave residue on it, and you've worked against the panel you were trying to help. So the method matters as much as the timing.
The way we clean panels is the same pure-water science we use on glass: a soft brush to agitate the soiling, and pure (de-ionised) water to rinse it away spot-free. No detergents, no abrasives, no pressure. Pure water has had its minerals stripped out, so it lifts dirt and then dries invisibly, leaving nothing on the glass to shade a cell. If you want the full explanation of why de-ionised water dries perfectly clear, we wrote it up here: Cleaning Windows With a Water-Fed Pole. Panels get the exact same principle.
This is the boundary that protects both the panel and your warranty. Pressure washing, abrasive scrubbing pads, and solvent cleaners are the three things panel manufacturers warn against — high pressure can force water past the seals and damage the coating, abrasives scratch the anti-reflective surface, and harsh chemicals can etch it. Using any of them can void the manufacturer's warranty outright.
A soft brush and pure water sidesteps all of that. It's gentle enough to satisfy the manufacturers' own guidance — many of them specify soft, non-abrasive tools and low-mineral, near-neutral water precisely because it's safe for the glass. One practical note: your specific panel maker may have written cleaning instructions, and some even require periodic cleaning to keep the warranty valid, so it's worth a two-minute check of your paperwork before anyone touches the array.
If your panels are on a ground mount or a low, safely reachable structure, cleaning them yourself is a perfectly reasonable weekend job — pure or distilled water, a soft brush, a cool early morning so nothing dries on the glass, and no pressure. That's genuinely DIY-friendly.
Roof-mounted arrays are a different conversation, and we'll be straight about it: that's working at height on a slope, around live electrical equipment, often reaching over panels you can't safely stand on. The fall risk alone is reason enough to leave it to people set up with the right access and training. This is the one part of solar cleaning where "do it yourself" isn't the neighbourly advice — it's the part worth handing off.
Here's our honest rule of thumb. It's worth cleaning when you can see the reason: visible bird droppings, a heavy pollen or farm-dust coating, sap, or an array that hasn't seen rain in a long dry stretch. It's also worth a look if your monitoring app shows production slipping on clear, sunny days for no other reason. If the glass just has a light, even haze and rain is in the forecast, you can often let nature take the first pass.
Clean for the soiling that sticks, use the gentle method, keep off the roof unless you're equipped for it, and you'll get the real benefit without any of the risk. That's the whole guide in a sentence.
Yes — when they're genuinely dirty. Be honest with yourself about which kind of dirty, though. Light, even dust might only cost a few percent, and rain handles a lot of it. Pollen, farm dust, and bird droppings don't rinse off, and droppings that shade a cell can drag down a whole string — so cleaning those off gives you a real, noticeable bump.
It depends on where they sit. Under trees, near open fields, or in heavy bird traffic means faster soiling and more frequent cleaning. For most homes, once a year is a reasonable baseline. In our rainy climate, rain does a fair share of the light-dust work in between.
On a safely reachable ground mount or low structure, yes — pure or distilled water, a soft brush, no pressure, and skip the tap water so you don't leave mineral spots. Do it early on a cool morning so nothing dries on the glass. Roof-mounted arrays mean height plus electrical risk, so leave those to a pro.
Harsh chemicals, abrasives, and pressure washing can. That's exactly why the safe method is a soft brush and pure water — it satisfies what manufacturers ask for. Check your own panel maker's cleaning guidance too, since some require regular cleaning to keep the warranty valid.
Pure, de-ionised water dries spot-free and leaves no residue. Tap water leaves mineral spots that sit right in the light path and shade the cells — the same reason we use pure water on windows. Distilled water works for a small DIY job; the principle is identical.
Sources: Aurora Solar — Understanding PV System Losses, Part 3: Soiling, Snow, System Degradation (citing NREL), NREL — Performance Parameters for Grid-Connected PV Systems