Solar panels have no way of telling you they are dirty. There is no warning light, no alert in the app, no fault code. A filthy array reports itself as operating normally, because electrically it is. It is just working with less light than it should have.
That makes soiling something you have to check for deliberately. Here are five signs worth knowing, none of which require a ladder.
Five things to look for
1. You can see film or streaking from the ground
Go outside early in the morning or in the last hour before sunset and look up at the array from a low angle. Raking light makes soiling obvious in a way that midday sun completely hides. Dust film reads as a dull, flat haze instead of dark reflective glass. Look especially along the bottom edge of each panel, where grit piles against the frame, and for vertical streaks, which usually mean a previous rain ran dust down the glass and dried in place.
2. Your clear-day peak has dropped
Open your monitoring app and find your highest production reading on a recent cloudless day. Compare it against the same figure from a clear day a few weeks or months back, in a similar part of the season. A steady decline across clear days with no weather explanation is the single most reliable soiling signal you have.
3. You are seeing droppings, feathers or hearing activity
White streaks on your tile below the array, feathers in the yard, or cooing and scratching over a bedroom ceiling all point the same direction. Droppings block light completely where they land and bake solid fast in summer heat. If birds have moved in, cleaning the glass alone is a temporary fix, and it is worth reading up on why bird proofing matters before booking anything.
4. A dust storm has come through since your last clean
One good haboob undoes a cleaning. The silt it deposits is fine enough to form a continuous film across the glass rather than loose grains that blow off. If a storm has rolled over your neighborhood since anyone last touched your panels, assume they need attention rather than waiting for other symptoms.
5. It rained a little, and now things look worse
Light rain on a dusty array is not a free cleaning. It wets the dust, runs it into streaks and dries into mineral spotting that is more stubborn than the dust was. A brief shower during monsoon season frequently leaves panels in worse shape than it found them, which catches people out every year.
Checking output without fooling yourself
The monitoring app is the most useful tool you have here, and also the easiest to misread. A few rules keep the comparison honest:
- Compare clear days to clear days. A hazy sky or high cirrus will drop your numbers on its own.
- Compare similar times of year. June production is not supposed to match December production.
- Use peak instantaneous output, not daily totals, since day length skews totals across seasons.
- Watch for one string underperforming the rest, which often means localized soiling or droppings rather than general dust.
- Take the reading before and after a cleaning. The difference is the number that tells you how often your roof needs attention.
Do this once a month on a clear day and it becomes a thirty-second habit that catches problems while they are still small.
What to do once you have spotted one
One sign is worth acting on. Two or more means it has probably been building for a while.
Resist the urge to grab the hose. Tap water in the Valley carries enough dissolved mineral content that it dries into spotting on hot glass, so a quick rinse trades dust for scale and leaves you scrubbing harder later. A deionized water cleaning leaves nothing behind when it dries, which is the difference between glass that stays clear and glass that goes hazy again in a month.
If birds are part of what you found, handle both jobs at once. Clearing the underside and installing bird proofing while the crew is already up there costs less than two separate visits and stops the cycle rather than resetting it. Our services page covers how we bundle the two.
Not sure what you are looking at up there? Describe what you are seeing and we will tell you whether it needs a visit. Drop us a line any time.