Measured dirt losses from published studies, why rain matters, and how to decide whether cleaning pays for your system.
By the SolarCleaningRated team · Facts checked against the sources listed below on November 1, 2026. General information, not professional advice.
At one 86.4 kW commercial site, researchers measured a loss of 0.21 percent per day: efficiency fell from 7.2 percent to 5.6 percent over a 108-day dry spell in summer 2010, then recovered to 7.1 percent after rain (Mejia, Kleissl and Bosch, 2014).
Sandia National Laboratories' modeling guide describes the Kimber model, which assumes losses build at a steady rate until rain above a minimum threshold resets the panels to clean; its authors estimated rates of 0.1 to 0.3 percent per day at southwest US locations.
Researchers at the National Renewable Energy Laboratory (NREL) report that annual soiling losses in the United States have ranged from 0 to 6 percent, and a later NREL mapping study of US sites found the greatest losses in Southern California counties, because of high particulate matter concentrations and long dry periods.
What matters is your own system: its tilt, how dusty your area is, how often it rains and whether birds or sprinklers leave deposits. If your inverter app shows output dropping through a dry season and recovering after rain, dirt is costing you something. Compare that with the cleaning price; the prices companies publish are on our published prices page.
41 companies in 5 areas, scored by the same published criteria.