Solar panels do not need to be spotless to be profitable. The Engadget explainer notes that soiling causes roughly 4–7% of global photovoltaic energy losses, according to the IEA, and an NREL study measured pollen cutting output by up to 15% at some North Carolina sites. But those figures are not a universal trigger for cleaning: rain, panel tilt, and local dust levels determine whether a dirty array is actually losing meaningful revenue.
Cleaning only makes business sense when the value of recovered generation exceeds the cost of the wash. UC San Diego researchers found that panels left 145 days without rain or cleaning lost 7.4% of efficiency, yet cleaning a typical 5kW residential system halfway through that dry spell would have produced only about $20 in additional electricity. With professional cleaning typically costing $142–$310 and averaging $209 on Thumbtack, a one-off wash rarely pays unless the dirt problem persists for months or output drops sharply after a dry period.
Improper cleaning can be more expensive than the grime. The article warns that abrasive pads, pressure washers, and hard water can scratch glass, leave mineral deposits, or void warranties, citing Qcells' warning that damage to the anti-reflective coating or delamination may be excluded from coverage. For utilities, automated options exist—such as Ecoppia's water-free robots for ground-mounted projects of 50MWp and larger—but for most residential owners, the rational move is to monitor production data and clean only when a measurable drop follows a prolonged dry spell.