This work shows the techno-economic comparison of the design of two solar photovoltaic systems: 1) on-grid (G-SPVS) and 2) off-grid (SPVS). Both schemes aim to supply electricity to a model water desalination plant located in Floreana Island, Ecuador. The annual load profiles and other operational details of the case study were previously obtained. For this research, a period of 15-years was analyzed. During this time, the maximum power of water pump system remained constant and represented the highest percentage of the electrical load, which changed each year influenced by the drinking water requirements of the population. Results from the HOMER PRO simulations showed that the SPVS produced higher surpluses of electricity. In contrast, the G-SPVS exhibited lower net present cost (NPC) and cost of energy (COE).