Abstract The renewable resource of tidal current is presently attracting the interest of many companies. Several systems are in the development stage and SAIPEM is participating in one of them. The tidal streams are predictable well in advance but are variable along the hours and the days. In order to determine how much energy can be produced during a year, a model has been written. It takes into account the local current characteristics and the astronomical evolution of the tide during a whole year. This model indicates how to select the nominal electrical capacity of the tidal turbine in order to optimize the energy cost of the scheme. The model teaches that a turbine located in a tidal race off the European coast has an equivalent number of producing hours of about 3000h per year as an average. In some locations, the site may be exposed to the swell in addition to the tidal currents. A tidal current turbine reacts almost instantaneously to the velocity variations, because the inertia of the turbine is small compared to the inertia of the water flow crossing the rotor. This results in power fluctuations of a single turbine, at least as long as the water velocity does not exceeds the nominal rating of the machine. Some examples are shown to illustrate the different cases. The local wave climate should be taken into account in order to correct the yearly production on a statistical basis. The potential influence of a strong swell may alter the maximum power predicted based on the sole astronomical parameters. Clusters of tidal turbines have also been studied. The detrimental influence of the swell can be mitigated through an optimum design of the turbine array. When the current is slow and the swell important, the cluster can even produce more energy than without the waves.