Thanks to a new sensibility to renewable energy and to local and smart electricity production, traditional water wheels are regarded again as a clean and accessible way for pica-micro hydro electricity generation, especially in presence of very low heads and small flowrates. In particular, among the different kinds of water wheels, the overshot ones exploit the lowest flowrates with the highest efficiency (efficiency up to 85-90%). Therefore, in order to determine the performance characteristics and to estimate the power losses and the output power for overshot water wheels, theoretical and experimental analyses are here developed. By a numerical optimization process, the power losses and the mechanical output power are theoretically quantified. A critical angular velocity is also identified (about 65% of the runaway speed). When the wheel rotation speed approaches the critical velocity, volumetric losses at the top of the wheel begin to increase linearly with the rotational velocity, while the other power losses start to decrease, due to the lower amount of water available to the wheel. (C) 2015 Elsevier Ltd. All rights reserved.