Rooftop-based solar and rainwater harvesting systems as a sustainable urban solution for the water–energy nexus

The technical, financial, and environmental sustainability of a 70.20 kW grid-connected photovoltaic system combined with rainwater harvesting is investigated in six geographically varied regions (Algiers, Banjul, Monrovia, North Nicosia, Tarfaya, and Tunis). These locations, which represent various climatic zones across Africa and the Eastern Mediterranean, are especially selected to provide a comprehensive assessment concerning how environmental factors affect combined water-energy performance. Rainfall analysis reveals significant regional variation: Monrovia has the highest rainwater harvesting potential (RHP), ranging from 392 to 571 m3, followed by Banjul (138–232 m3), while arid Tarfaya has the lowest RHP, ranging from 10 to 42 m3. Moreover, the results reveal that latitude has a major influence on optimal energy generation. In all the Mediterranean regions analyzed herein (Algiers, Tunis, and North Nicosia), the yearly output varies between 105 and 115 MWh when the angle of inclination is between 30° and 40°. A tilt of 10° to 20° yields the best results for tropical regions, while in the arid desert climate of Tarfaya, the best performance of the PV system is obtained for a tilt ranging from 20° to 30°. Furthermore, the results of the economic assessment demonstrate that LCOE varies by region, ranging from a minimum of 2.37 cents/kWh for Algiers to a maximum of 6.04 cents/kWh for Tunis. North Nicosia and Tarfaya had the shortest payback periods, at 2.34 and 2.02 years, respectively. Additionally, the Mediterranean zone’s lifetime GHG savings ranged from 1136–1344 kg CO2-eq to the semi-arid Tarfaya region’s 2339 kg CO2-eq. Consequently, the combined assessment highlights the necessity of climate-adaptive design to maximize the sustainability of integrated PV-rainwater systems.