Soiling losses on California ground-mount arrays
What California studies measured, and why rain is not enough.
Oct 27, 2026 · Premier Solar Service
California research on soiling
Kimber et al. (2006) studied soiling on large grid-connected PV systems in California and the Southwest, and Mejia & Kleissl (2013) measured soiling losses across California PV sites. Both found measurable energy losses that build up during dry periods.
Rain does not reset the array
NREL's analysis of soiling station data could not find one rain amount that reliably cleaned panels, and panels did not always return to full output after rain. In the Central Valley, the dry season runs for months.
Turning it into a plan
NREL's O&M best-practice guide says cleaning can follow a fixed interval or be condition based, and that a site-specific cost-benefit check should decide the frequency. Track clear-day output against the same weeks last year, and schedule cleaning when the loss justifies it.
Sources
- Kimber et al., The effect of soiling on large grid-connected PV systems in California and the Southwest (2006)
- Mejia & Kleissl, Soiling losses for solar PV systems in California, Solar Energy (2013)
- NREL, Time series analysis of PV soiling station data (2017)
- NREL, Sandia & SunSpec, Best Practices for O&M of PV and Energy Storage Systems, 3rd ed. (2018)
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