Due to the high cost of a tidal stream plant it is essential to select turbines that will have an optimum performance at the site in question. The objective of this work is twofold: (i) to present a procedure to compare the performance of different Tidal Stream Turbines (TSTs) at a given site, and (ii) to illustrate it by means of a case study. A new parameter, the site-specific turbine efficiency, is defined ad hoc and used alongside the availability and capacity factors, and the energy output over a reference period of time. These four performance parameters are determined for each turbine on the basis of the characteristics of the flow at the site. The procedure is illustrated with a case study in which two TSTs, the Gorlov Helicoidal Turbine and the Evopod, are compared for a site in an estuary. It is found that the availability and capacity factors of the Gorlov machine are higher (implying more operation hours and equivalent hours per annum) but the Evopod has larger site-specific efficiency and total energy output. In conclusion, a parametric approach that allows to compare the performance of different TSTs at any site of interest is presented and illustrated with a case study.