Soiling is the accumulation of material on light-collecting surfaces in solar power systems. The accumulated material blocks or scatters incident light, which leads to a loss in power output. Typical soiling materials include mineral dust, bird droppings, fungi, lichen, pollen, engine exhaust, and agricultural emissions. Soiling affects conventional photovoltaic systems, concentrated photovoltaics, and concentrated solar (thermal) power. However, the consequences of soiling are higher for concentrating systems than for non-concentrating systems. Note that soiling refers to both the process of accumulation and the accumulated material itself.

There are several ways to reduce the effect of soiling. The antisoiling coating is most important solution for solar power projects. But water cleaning is the most widely used technique so far due to absence of antisoiling coatings in past. Soiling losses vary largely from region to region, and within regions. Average soiling-induced power losses can be below one percent in regions with frequent rain. As of 2018, the estimated global average annual power loss due to soiling is 5% to 10% percent. The estimated soiling-induced revenue loss is 3 – 5 billion euros.

Soiling is typically caused by the deposition of airborne particles, including, but not limited to, mineral dust (silica, metal oxides, salts), pollen, and soot. However, soiling also includes snow, ice, frost, various kinds of industry pollution, sulfuric acid particulates, bird droppings, falling leaves, agricultural feed dust, and the growth of algae, moss, fungi, lichen, or biofilms of bacteria. Which of these soiling mechanisms are most prominent depends on the location.

Soiling either blocks the light completely (hard shading), or it lets through some sunlight (soft shading). With soft shading, parts of the transmitted light is scattered. Scattering makes the light diffuse, i.e. the rays go in many different directions. While conventional photovoltaics works well with diffuse light, concentrated solar power and concentrated photovoltaics relies only on the (collimated) light coming directly from the sun. For this reason, concentrated solar power is more sensitive to soiling than conventional photovoltaics. Typical soiling-induced power losses are 8-14 times higher for concentrated solar power than for photovoltaics.

Soiling losses vary greatly from region to region, and within regions.

The rate at which soiling deposits depends on geographical factors such as proximity to deserts, agriculture, industry, and roads, as these are likely to be sources of airborne particles. If a location is close to a source of airborne particles, the risk of soiling losses is high.

The soiling rate (see definition below) varies from season to season and from location to location, but is typically between 0%/day and 1%/day. However, average deposition rates as high as 2.5%/day have been observed for conventional photovoltaics in China. For concentrated solar power, soiling rates as high 5%/day have been observed. In regions with high soiling rates, soiling can become a significant contributor to power losses. As an extreme example, the total losses due to soiling of a photovoltaic system in the city of Helwan (Egypt) were observed to reach 66% at one point. The soiling in Helwan was attributed to dust from a nearby desert and local industry pollution. Several initiatives to map out the soiling risk of different regions of the world exist.

Soiling losses also depend on meteorological parameters such as rain, temperature, wind, humidity, and cloud cover. The most important meteorological factor is the average frequency of rain, since rain can wash soiling off of the solar panels/mirrors. If there is consistent rain throughout the whole year at a given site, the soiling losses are likely to be small. However, light rain and dew can also lead to increased particle adhesion, increasing the soiling losses. Some climates are favorable for the growth of biological soiling, but it is not known what the decisive factors are. The dependence of soiling on climate and weather is a complex matter. As of 2019, it is not possible to accurately predict soiling rates based on meteorological parameters.