The increasing demand for water in urban areas of developing countries has been given more importance to rainwater harvesting techniques in the last decades. However, there is still a lack of general approaches to show and compare the potential to benefit effectively from direct roof runoff at different locations. Past studies mainly focused on specific design conditions, making it difficult to interpret and upscale their results in zones with similar conditions. This research explores the influence of rainfall characteristics and design parameters: storage tank size, catchment area, and daily water demand on rainwater harvesting systems designed for water supply. Sixteen locations in Ecuador with varying hydroclimatic characteristics were selected to evaluate our methodology. A mass balance method using satellite records of the Tropical Rainfall Measuring Mission was employed to estimate optimal dimensions based on the reliability of the system. Our results suggest that locations with similar annual rainfall and seasonality have similar performances of rainwater harvesting systems. Based on that premise, regional approaches can be formulated. They can be employed as a tool to support decision-makers promoting policies related to rainwater harvesting at local and regional scales.