The main objective of the paper is to identify the appropriate temporal scale for modeling the behavior of rainwater harvesting tanks in relation to the purpose they are built for, i.e., water saving, stormwater retention potential, etc. A tank water balance model coupled with a specific procedure to determine long-term series of rainfall (tank inflow) and toilet flushes (tank outflow) at different daily and sub-daily resolution timescales was developed. The model was applied to a household case study for which detailed water demand data are available from measurements. Simulations show that the daily scale may be reliably chosen to evaluate the tank water saving efficiency. In contrast, sub-daily resolutions (at least the hourly time step) are needed for the evaluation of the tank retention efficiency to limit inaccuracies, especially for small tanks and for high values of the water demand. Moreover, preliminary results at the 5 min time step show that rainwater tanks can help in reducing the rainfall intensity peak, basically depending on the tank storage and on the rainfall event characteristics.