Muller, who has extensively studied the soiling problem, and his colleagues measured the effects of the utility-scale solar plants located in four counties in North Carolina. Their findings ran counter to a common assumption that frequent rainfall would result in negligible soiling impacts. Instead, they determined the performance of the solar panels decreased by as much as 15% during peak pollen season, with most pollens coming from cypress, juniper, pine, and oak trees. While these are peak losses, the data suggest that without planned cleaning, annual production losses can be on the order of 10%.