In the Mediterranean area, water scarcity represents a critical issue due to the increasing water demand related to the population growth and the expansion of urban and industrialized areas. Rainwater harvesting (RWH) may be an effective alternative water supply solution to deal with water scarcity in order to reduce non-potable water needs. The reliability of RWH systems is greatly affected by the intensity and the temporal distribution of rainfall events. The purpose of the present study was to identify the optimal tank capacity, in terms of water saving efficiency, of a RWH system installed to supply water for toilet flushing, garden irrigation and both uses with reference to a single-family house in a residential area of Sicily (southern Italy). A water balance simulation of the rainwater storage tank was performed to define the tank release rule. The optimal capacity of the RWH tank was evaluated considering three different catchment surfaces, namely 100, 200 and 300 m2. Results showed that, in some areas of the region, the system could be able to provide significant water savings, even with the installation of collecting tanks of less than 10 m3, thus ensuring important environmental and economic benefits to the householders.