Because water is a key at-risk resource, improved water management is essential. In terms of quantity, the two major alternatives in water management can be grouped into: (i) improving efficiency in water use; and (ii) exploring alternative water sources. Rainwater harvesting (RWH) is one of the most promising alternative water sources, since rainwater can easily be collected and used without significant treatment for non-potable purposes. However, the economical viability of these systems is not always assured. This paper examines the most relevant technical and economical issues in designing domestic RWH systems, evaluating the technical feasibility and economical viability of this technology in the particular weather and water use in Portugal. The evaluation is performed for a single-family residence, where the water use pattern was monitored. The precipitation pattern is characterized for two locations in Portugal, Porto and Almada, since they represent two different scenarios for RWH. The RWH and water savings efficiency were assessed and payback period estimated for both cities. It was found that, for an optimum rainwater tank, the water savings potential are similar for both locations, despite the differences in the average annual precipitation. A simple rule for estimating the optimum tank capacity for single-family households in Portugal is proposed. A sensitivity analysis shows an important influence of water fees on the economical viability of RWH systems in single-family houses in Portugal, namely when compared to changes in the consumption pattern.