A solar array is a large, angled sheet of glass installed in a climate that can go from April to November without meaningful rain. Everything that lands on it in those months stays there. Dust, pollen, salt, ash after a fire season, and bird droppings all build into a film, and that film sits between the sun and the cells doing exactly what you would expect.
The part that surprises people is that the loss is not proportional and it is not even. A panel is made of cells wired in series, and a series string is governed by its weakest member — so a band of droppings across one row of cells does not cost you that row, it drags down the output of the entire panel. This is the same reason a single shading branch matters far more than its size suggests. A little bit of dirt in the wrong place costs more than a lot of dirt spread evenly.
Where you are changes how fast it happens. Inland in Santa Clarita or out in Bakersfield it is agricultural and construction dust, and it accumulates fast through the dry months. On the coast in Malibu or Palos Verdes it is salt, which is worse than dust because it is hygroscopic — it pulls moisture out of the air, goes slightly tacky, and then catches and holds everything else that lands on it. An array under trees anywhere gets pollen in spring and leaf litter year round, plus the birds those trees bring.
There is a second cost that nobody notices until it is expensive. Debris that collects along the bottom rail and in the gaps between rows holds moisture against the frame and the mounting hardware, and on a roof that means moisture sitting where you least want it. Clearing the edges and the row gaps is part of the job for that reason, not just because it looks better.
How it gets cleaned matters as much as whether it gets cleaned. Panels have an anti-reflective coating on the outer glass, and that coating is the whole reason the panel captures light at shallow angles. Abrasives take it off. So do a surprising number of household detergents, and so does dragging a brush across grit that should have been rinsed away first. Hard tap water is its own problem: it dries into exactly the mineral spotting described above, so you finish the job having swapped dirt for a deposit that is harder to remove than the dirt was.
We rinse loose grit off before a brush touches the glass, wash with soft brushes, and rinse with deionised water — which has had the minerals stripped out of it, so it dries clean with no spotting and no need to squeegee a roof. No detergents, no abrasives, and nothing that puts a panel or roof warranty at risk.
For most arrays in our area, once or twice a year is right. Under trees, near the ocean, or out in the Valley dust, more often. If your monitoring app has been drifting down all summer and you have been assuming the panels are ageing, it is worth cleaning them before you assume anything more expensive.
Over the Top Window Cleaning cleans residential and commercial solar arrays across Los Angeles County, the South Bay, Santa Clarita, Santa Barbara and Bakersfield. Call (310) 972-8217 for a free quote.