Drinking water and wastewater treatment industry is rapidly expanding in Romania, as a result of European regulations, and also of funding by European Commission for investments in the development of this public service. Water industry is an energy intensive industry, so an analysis from this perspective has the potential of significant economies of energy. In this paper the analysis of energy efficiency is based on the analysis of technological procedures of wastewater treatment and on population forecast. Furthermore, there are suggested optimal solutions to use PV systems in small wastewater treatment plants. Finally, the simulation results confirm the proposed methods.