Tidal stream farm sitting in estuarine areas constitutes a complex and iterative process, which is usually subject to a large number of uncertainties. In order to shed light on the site selection procedure, it is necessary to make available reliable metrics and indicators to facilitate the decision-making. To this end, the new Hydrokinetic Energy (HE) index, based on the of the exploitable and reference energy resource concepts, is proposed and applied to a case study in the Shannon Estuary (W Ireland), illustrating its applicability to an informed tidal stream energy site selection.