The filling of the upper chamber is dependent on varying conditions such as the air conditioning setting and how long it is in use for. In normal use, the upper chamber will fill sufficiently during the day therefore negating the use of mains water.
The lower 6-litre chamber of the cistern is always full and ready to flush. For back up, if there is an unusually high number of flushes in quick succession or the air conditioning is not in use, the cistern is filled in the conventional way using the mains water supply.