Phoenix does not really do four seasons, but your solar panels experience four distinct ones. Each brings a different kind of material onto the glass, and each arrives at a different point in your production year. Getting the timing right means you are clean when it counts, rather than clean in January and filthy through June.
The Valley's soiling calendar
February through April: pollen and bloom
Olive and mulberry pollen kick things off, and anyone with allergies in this town knows exactly when that starts. Then the palo verdes bloom and drop yellow flowers over everything in sight for a few weeks. Pollen is sticky in a way dust is not. It bonds to glass, holds onto dust that lands afterward, and turns into a film that resists a plain water rinse.
May and June: the dry, dusty run-up
This is the stretch that matters most. The ground has not seen rain in months, wind picks up ahead of monsoon season, and dust moves constantly. It is also when your array is at its absolute best: longest days, highest sun angle, maximum potential output. Dirty glass in June wastes more energy than dirty glass at any other point in the year.
Mid-June through September: monsoon
Storms bring a mix of rinse and mess. Some of it helps. Much of it does not, especially the dust-then-rain sequence that leaves mud spotting baked onto the glass. We covered this in detail in our monsoon post, but the short version is that storm season adds material more often than it removes it.
October through January: settled dust and low sun
The air calms down and dust accumulates slowly and evenly. Production drops with the shorter days and lower sun angle, so the stakes per dirty panel are lower. This is also the stretch when birds go looking for nesting spots for the year ahead, which makes it the best window for bird proofing work.
The two visits that do most of the work
For most Valley homes, two cleanings a year handle it, placed deliberately:
- Late May, before monsoon. Clears the spring pollen load and puts you into your highest-production months with clean glass. If you only ever do one cleaning a year, do this one.
- October, after monsoon. Takes off the mud spotting, storm debris and whatever blew under the array during storm season. This is also the natural time to clear the underside and get bird proofing installed before nesting picks up again.
Those two visits bracket the periods that matter: one sets you up for peak season, the other cleans up after the messiest one.
When a third visit earns its cost
Some homes need more, and it is usually obvious which ones:
- You are near a dust source. Open desert, farmland, a dirt road, a horse property or active construction nearby all mean faster accumulation than a mid-subdivision roof.
- Trees overhang or sit beside the array. Palo verde, mesquite, olive and mulberry all drop material directly onto the glass, and a mid-spring visit after the bloom is worth scheduling.
- Your panels sit close to flat. Low-tilt arrays hold everything. Steeply pitched ones shed at least some of it.
- A major haboob came through. A big storm justifies its own visit regardless of where you are in the schedule.
- You are on a time-of-use plan. Summer on-peak production is your most valuable, which makes an extra midsummer clean easier to justify than it looks on paper.
Building a schedule you will actually keep
The best schedule is one that does not depend on you remembering. Book both visits at the start of the year rather than deciding month by month, and check your monitoring app once a month on a clear day so a slow decline does not go unnoticed for a season.
One thing not to build a plan around: rain. It is tempting to wait for a storm to do the job for free, but Valley rainfall is too infrequent and too light to rinse an array properly, and it leaves mineral spotting when it dries. The cleanest your panels will be after a rain is still noticeably dirtier than after a deionized water cleaning, which leaves nothing behind at all.
If you are trying to work out where your home falls on this calendar, tell us where you are in the Valley and what is around you. Send us a message and we will suggest a schedule that fits your roof rather than a generic one.