Why the dry season is the whole issue
California's rainfall is concentrated in winter. From roughly May through October, most of the Bay Area gets little to no meaningful rain — which means nothing rinses the panels for five or six months straight.
Dust, pollen, and airborne particulate accumulate uninterrupted over that entire stretch. By September, panels that looked fine in May are carrying a full season of buildup. That's why end-of-dry-season is the highest-value time to clean: you're removing the maximum accumulation, and you're doing it before the shorter, lower-angle days of winter when every bit of light matters more.
What actually reduces output
A uniform film of dust and pollen is the most common issue and the easiest to miss, precisely because it's even — there's no obvious dirty patch to catch your eye from the ground.
Bird droppings are the more severe version. They're opaque and concentrated, and depending on how a panel's cells are wired, shading one section can affect output disproportionately.
Leaf and needle debris collecting along the lower edge of an array, common on roofs under tree cover, both shades cells and holds moisture against the frame.
How much output you get back
Honestly: it depends entirely on how much buildup is there, and anyone who quotes you a specific percentage without seeing your array is making it up. A system that's gone a full dry season under trees will show a meaningful difference. One in an open location that was cleaned six months ago won't.
If you have monitoring on your system, you can see it yourself — compare output on clear days before and after.
What not to do
Don't use abrasive brushes, scouring pads or harsh chemicals. Panel glass scratches, and scratches are permanent and reduce transmission forever.
Don't spray cold water on hot panels in the middle of a summer afternoon. Thermal shock can crack glass. Early morning is the right time.
Don't use plain tap water and let it air dry — mineral content leaves spotting on the glass, which partially defeats the point. Filtered or deionized water dries clean.
And think carefully before doing this yourself on a roof-mounted array. It's roof work, usually on a slope, often two stories up.