A system for measuring the power or energy loss in a photovoltaic array due to soiling, which is the accumulation of dust, dirt, and/or other contaminants on the surfaces of photovoltaic modules, comprising: a pair of photovoltaic reference devices placed within or near the photovoltaic array and co-planar to the modules comprising the array, wherein one reference device is a module similar to those of the array and is allowed to accumulate soiling at the natural rate, and wherein the second reference device is a module or a cell and is periodically cleaned; and a measurement and control unit which measures and compares the electrical outputs of the soiled reference device and the clean reference device in order to determine the fraction of power lost by the soiled reference module due to soiling.