The photovoltaic modules installed in the system must be electrically and mechanically safe for the entire expected life of the plant. In addition, the module must produce the expected energy with the lowest costs for maintenance and possibly no maintenance.
Factors that may decrease the revenues are broken modules, extraordinary maintenance costs and a decrease in energy production due to the degradation of power.

The life cycle of a photovoltaic module installed in a system is shown in the following figure.

In the first years takes place the so called infant mortality which is caused by defects during product, transportation damages or defects caused by improper installation. Afterwards, the probability of failure is constant. At this stage failures are caused mainly by climatic stress such as snow, hail, wind and less important by product defects. Then begins "wear out" phase, i.e. the degradation during which both the possibility of failures and  the degradation of the power may increase.