Despite its rich hydropower potential, Slovenia is having difficulty fulfilling its obligations concerning the utilization of renewable energy sources. This situation could be improved partly by the positive effects of economic growth and by a turnaround in the construction of small power plants after re-establishing the support scheme. The sites of abandoned water mills offer hydropower potential with mostly ultra-low head, which can be efficiently exploited by an Archimedes screw. For assessment of electrical energy production, hydrological data are taken into account in combination with closely examined conditions that can be expected from water streams with ultra-low head, located in a region with relatively abundant precipitation. Several simulation models were developed for constant speed and variable speed small hydropower plant (SHP) installations, exploiting a combination of manufacturer's data-based equipment models, in-situ measurements and published data. Surprisingly, the variable speed operated SHP produces less than the constant speed operated SHP. The seasonal limitation of hydropower potential exploitation, which is defined in order to minimize the influence of SHP operation on flora and fauna in the river, as well as the technical limitations for operating an Archimedes screw given by its manufacturer, have only moderate or no effect on production. (C) 2018 Elsevier Ltd. All rights reserved.