How Much Does Dirt Actually Reduce Solar Output?
The real-world impact of dirt and soiling on residential solar production — with San Diego numbers.
Independent research and our own before/after monitoring put typical San Diego soiling losses at 15–25%. Here's the data behind the number and why it gets worse over time.
Industry research on soiling rates
NREL and Sandia have published soiling research for years showing losses of 5–25% globally, with semi-arid climates like San Diego sitting at the higher end. The exact number depends on rainfall, dust composition, panel tilt, and surrounding land use.
Our own before/after numbers
Across hundreds of San Diego County cleanings, our before/after monitoring consistently shows 15–22% production recovery on systems that haven't been professionally cleaned in 12+ months. First-time cleanings on three-year-old systems regularly exceed 25%.
Why the loss grows over time
Soiling compounds because each layer of dust makes the surface stickier for the next layer, and bird droppings, salt, and pollen actively bond with the glass on hot summer days. A panel that's 6% soiled in month three is often 18% soiled by month nine — and the cleaning ROI grows with it.