Short answer: at least once a year, usually after the dry season, and more often if you live next to fields, orchards or dirt roads.
Fresno's long dry stretch
The National Weather Service's 1991-2020 normals for Fresno Yosemite Airport list about 0.03 inches of rain in July, 0.00 inches in August and 0.05 inches in September. Most of the year's roughly 11 inches falls from November through March.
That means dust that lands on your panels in late spring usually stays there until the first real storms in fall.
What the studies found
A UC San Diego study of 186 California solar sites (Mejia and Kleissl, 2013) measured an average loss of about 0.05% of output per day without rain, which added up to about 7.4% over an average 145-day summer drought. About a quarter of the Central Valley sites lost more than 0.1% per day.
An earlier study of large systems in California (Kimber and others, 2006) measured Central Valley loss rates of about 0.11% per day at rural sites and 0.19-0.20% per day at suburban and highway sites. Flat panels (under 5 degrees of tilt) lost about five times more than tilted ones in the 2013 study.
So when should you book?
For most Fresno and Clovis homes, a wash in late summer or fall, after harvest dust, catches the worst of it. Homes next to orchards, open fields, dirt roads, freeways or construction often benefit from a second wash in late spring before the hottest months.
Every roof is different. Check your monitoring app: if output on clear days has dropped compared to the same weeks last year, dirt is a likely cause. Send us a photo and we will tell you straight if it needs a wash yet.