Two ways to schedule
The national PV O&M best-practice guide from NREL and Sandia says cleaning "may be on a defined interval or 'condition based'," and that soiling can be measured by instruments to trigger a cleaning.
For a commercial rooftop, carport or ground-mount site, that means you either book cleanings on a calendar or let your monitoring data tell you when output has dropped.
Why there is no single answer
The same guide says a site-specific cost-benefit check should decide whether routine cleaning is worth it, and that the right frequency "may be seasonal, depending on local rainfall and dust characteristics."
In the Central Valley, the long dry season matters. Fresno gets most of its rain in winter, so dust from fields, roads and construction can sit on glass for months.
What we suggest
Start with your monitoring: compare clear-day output to the same weeks last year. Then set a schedule with your O&M plan, and adjust it after a season of data. We work around your site rules and safety requirements.
