Constant energy supply for decentralized wastewater treatment plants (DWWTPs) is crucial in order to ensure its functionality and prevent contamination of rivers and human illnesses due to pollution. However, power blackouts are a common problem in rural areas, which can affect the reliability of wastewater treatment plants. This paper presents a simulation study of sizing of solar photovoltaics and Sea-Salt batteries for powering a DWWTP working in 100% off-grid mode. The analysis is performed for two different DWWTPs: a prototype membrane bioreactor (MBR) and a Bever III compact wastewater aerobic system. The study is performed using the simulation package DEMKit developed at the University of Twente in the Netherlands. Results show that a solar photovoltaic system of 15 kWp coupled with a 20 kWh Sea-Salt battery may provide 100% of the electricity necessary during summer and up to 75% during winter in the Netherlands for the Bever III. In the case of the MBR, a photovoltaic system of 30 kWp in combination with a Sea-Salt battery of 50 kWh meets 100% of the electricity needs during summer and up to 65% during the winter in the Netherlands. Furthermore, in order to power the DWWTPs during the months of low sunlight, the dimensions of the solar photovoltaic system and the Sea-Salt battery needs to be increased by a factor of three.