Economic and environmental factors play a crucial role in the transitioning of water desalination technologies from conventional energy sources to alternative sources, solar energy being in the forefront. This paper investigates the technical and economic merits of using three different types of powering systems: the National Electric Grid (Grid), a Standalone (Off-Grid) Photovoltaic (PV) System, and a Grid Connected (On-Grid) PV System to power a 100 m³/day reverse osmosis seawater desalination plant in the Mediterranean city of Tripoli-Libya. The cost analysis of product water showed significant savings in favour of On-Grid PV as a power source. Also, in this paper, the effect of interest rate on the cost of the water produced was studied. An additional finding of this paper was the impact of Off-Grid PV system oversizing to account for times of low solar irradiance on the level of utilization of the total electricity produced.