Rainwater harvesting has been recognized as an alternative water supply method with many environmental benefits. This method can also produce drinking water for people who cannot access safe water resources. In this study, we evaluate the newly developed rainwater for drinking (RFD) system built at Ly Nhan Hospital in Vietnam. Most evaluation methods are not suitable for the RFD system because they focus on given conditions and overlook the potential of the system via operation and management practices. The hydrological performance was evaluated based on the system supplying drinking water sustainably, with zero no water days and a rainwater utilization ratio of 22%. Methods for improving performance indicators under adverse conditions were determined using sensitivity analysis and include increasing catchment area and tank volume and maximizing water use by utilizing rainwater overflow. Among them, an additional tank should be prioritized considering the cost, or it can be replaced by a plastic bag. The RFD system can be designed based on system monitoring data despite a lack of daily rainfall data and unexpected changes in the conditions. Appropriate regional RFD guidelines can be established with the continued evaluation of the RFD system worldwide.