Soiling is the accumulation of dust on solar panels that causes a decrease in optical efficiencies of CSP systems. However, geographically widespread data is only available for solar photovoltaic (PV) systems. The changes in efficiency of a large commercial site (86.4 kWdc) was quantified during the summer dry period over the course of 2010 with respect to rain events observed at a nearby weather station (3.4km away) and using satellite solar resource data. Soiling losses were found to be 0.21% per day. The site was observed to have a decrease in efficiency from 7.2% to 5.6% during a 108 day dry period in the summer at which point a rain event occurred that recovered most of the lost efficiency going back to 7.1%