Demographic growth that will take place on urban centers for the next decades may constitute a new challenge for water providers, which may then have to rely on more distant and/or poorer quality sources, or opt for energy-intensive technological solutions. In this context, the water–energy nexus will assume great relevance in near future water management policies and rainwater harvesting systems (RHS) may arise as an appealing alternative. The goal of this paper was to describe RaINvesT, a holistic tool developed to assess the viability of RHS, helping to adequately size the system, while allowing estimating its embodied energy per cubic meter (kWh/m3). Additionally, considering the independence towards the public water supply network, an investment analysis is performed for both the water provider and the final user perspectives. RaINvesT was tested for a university campus: evidencing, in terms of embodied energy, a positive ratio of 0.013 kWh/m3. The investment analysis has proven the RHS to be economically viable, revealing an investment return of about 12 years. The promotion of RHS can therefore represent a viable business model for water utilities, and can be a step forward in water systems decentralization, contributing to this sector sustainability for future cities.