It is sometimes useful to foresee system failures or maintenance stops in the production expectations.
You can define a system unavailability as a fraction of time (or a number of days). For these failure hours, the system will be considered inactive (OFF) during the simulation.
You have the opportunity of defining specific periods of unavailability of the system. As this is usually unpredictable, it is also possible to ask PVsyst for creating these periods in a random way (up to 5 periods of any duration along the year).
The effective energy loss is of course depending on the season, the day/night, or the weather during the unavailability periods. Therefore the energy loss evaluated by the simulation will not correspond to the specified unavailability time.
Therefore the unavailability loss has only a statistical meaning.