The power output by a small hydropower generation unit (SHP) is conventionally estimated as a product of the net head, admitted flow, and a typical value of efficiency. In case of run of river (RoR) units however, sub-daily variability of stream flow together with consequent variation of efficiency make the conventional estimate open to errors. This paper presents closed form expressions for sub-daily mean output power, sub-daily maximum likelihood output power, and sub-daily output power variability; all of which serve as useful measures of RoR unit performance in presence of fluvial micro and macroturbulence phenomena. As part of the development, bimodal (alternately, multimodal) logistic distributions are established as dependable statistics for sub-daily flow admitted to a RoR unit. The concepts are substantiated through an operational RoR case study from Malaysia, and detailed Monte Carlo simulations based on the same. The three sub-daily performance metrics are shown to form good basis for comparison between candidate turbine configurations for RoR projects in operation, or alternately, those under consideration at a planning stage. (C) 2019 Elsevier Ltd. All rights reserved.