Water saving and stormwater retention benefits from Rainwater Harvesting (RWH) tanks can be evaluated by the use of behavioural models able to simulate the long-term water balance of the rainwater tank. However, simulation results may be affected by the computational time step, that is normally chosen according to the aim of the analysis and to the available data. The objective of the paper is to analyse the influence of the time-step on the evaluation of the performance of RWH systems. Results of the investigation may help modellers deriving indications to select the appropriate time scale resolution when evaluating water saving and retention efficiencies of rainwater tanks. The analysis was carried with reference to a household case study in the south of Italy. High resolution rainfall data were used to run the water balance simulation of the tank at different daily and sub-daily time steps. In parallel, event-based data of toilet flushes collected during the field monitoring of the household residential water demands were used to derive long-term toilet water demand patterns at the different time scales of aggregation. Simulations of the tank showed that the daily time step may be reliably chosen for accurate evaluation of both the water saving and the volumetric stormwater retention efficiency of the rainwater tank with the exception for small tanks and high water demand values for which inaccuracies may occur unless higher time resolution are adopted. (C) 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the CCWI2013 Committee