In many developing countries, the stress of rapidly growing populations, mismanagement of resources and changing climate has created a burden on already compromised water resources. In Africa, where a significant proportion of the population is without access to improved water source, the urgency for clean available water sources to sustain healthy and productive human and natural populations has become a priority. As a water scarce country, Kenya has seen an increased investment in rainwater harvesting (RWH) projects to harness the vastly untapped rainwater resource, particularly in rural areas. Most of RWH literature is centered on the potential and implementation of rainwater harvesting systems, however not much focus has been placed on examining the demand satisfaction of these systems. This study investigates the reliability of rooftop rainwater harvesting (RRWH) as a key priority source of water supply for domestic use in three towns in Western Kenya: Kisumu, Nakuru and Lodwar. This was done using two approaches (1) the fraction of time water was available and (2) the fraction of time that a minimum demand was met, with acceptable reliability of 0.95 or higher. Actual rainfall data and RRWH parameters were used to produce supply/demand simulations of the system under Constant-Demand and Responsive-Demand scenarios over ten years. It was observed that all towns achieved acceptable reliability values for RRWH in terms of water availability however Lodwar only achieved demand satisfaction below 0.95. This study concluded that though RRWH cannot satisfy the minimum demand requirement through all days of the year, it is more than able to provide an alternative water supply for the domestic household in periods of long dry spells or when primary water source are inadequate.