HESS is currently engaged in its biggest, core project: the construction of five hydroelectric power plants (HPPs) on the lower course of the Sava River. HPP Boštanj, HPP Arto-Blanca and HPP Krško have already been completed and are in full operation (HPP Krško trial operation ended on 1 April 2014). The project will continue until HPP Brežice and HPP Mokrice have been completed.

HPP descriptions and activities are available on site at respective hydroelectric plants.