Urban water systems throughout the world are under recurring and increasing water scarcity, given demand growth, aging infrastructure, variability and uncertainty imposed by climate change. Rainwater harvesting systems (RWHS) represent a promising alternative to increase flexibility and robustness of water supply systems. Given the importance of the tank size in establishing a reliable RWHS, we propose an implicit stochastic approach to assess performance and aid designers. The main objective of this study is to simulate common non-potable water demand and roof area typologies and verify how the demand and roof area affect the efficiency of the RWHS. To highlight the usefulness of the model to aid in the definition of tank sizes, we have also performed an economic assessment. Results are specific to the study region's climate and might vary under different climate. However, we have chosen a rather unfavourable climate, with poor rainfall distribution along the year. Under such conditions, results are likely closer to lower bound benefits. This indicates potential water savings benefits of a RWHS even for unfavourable scenarios. The RWHS indicated to be more efficient at meeting demands with smaller Demand-Roof Area ratios. Furthermore, it was possible to determine the efficiency of the RWHS under various scenarios, identifying the minimum and maximum likely benefits. Several scenarios were compared directly allowing for the establishment of a general maximum tank size per unit roof area in a given region of interest, beyond which no significant benefits are likely. The economic assessment indicates positive net present values for investing in RWHS, especially for large rooftop areas and small Demand-Roof Area ratios. Under such conditions, investing in a RWHS has very low risk and will most likely have short payback times. (C) 2017 Elsevier Ltd. All rights reserved.