Soiling loss

Accumulation of dirt and its effect on system performance are uncertainties that strongly depend on the system's environment, rainfall conditions, and similar factors.

In moderately rainy climates (such as Central Europe) and residential areas, soiling is typically low and can be neglected (less than 1%).

In rural areas with agricultural activity, soiling can be significant during certain seasons. In industrial zones, noticeable effects of several percent are common. Häberlin reports effects of metallic dust near railway lines, which can trigger further pollution and moss growth.

The accumulation of dust and the growth of mosses and lichens along module frames create partial shading on lower cells and tend to trap more dust. Moreover, these pollutions are not removed by rainfall. Therefore, with low tilt angles, frameless modules are recommended when possible.

Birds droppings represent a serious problem, as they are usually not removed by rainy events. But their impact is reported as relatively small (less than 2%).

Soiling losses are strongly dependent on rainfall, of course. Therefore, PVsyst allows definition of soiling loss factors in monthly values. During simulation, soiling loss is treated as an irradiance loss.

Some reference articles provide information on soiling^{1 2 3 4 5}, as does a review by Thevenard (2010)⁶, p. 22. However, they do not provide definitive answers.

Snow

Snow is not included in PVsyst weather data, and its effects are very difficult to predict (when will it leave the array?). When snow is a concern, you can account for it by defining partial or complete soiling attenuation during certain months.