Open sea experience is essential to develop wave energy technology and reduce costs. One of the goals of the EU H2020 OPERA project was to advance the development of the biradial self-rectifying air turbine from TRL4 to TRL7. For this purpose, a 30 kW turbo-generation prototype was fabricated and tested at IST variable flow test rig, the shoreline Mutriku wave power plant and the offshore IDOM MARMOK-A5 spar-buoy device. The paper presents and analyses detailed operational data of the OPERA’s 30 kW turbo-generation set at the oscillating water column device of the Mutriku harbour breakwater and unveils operational data of the Mutriku’s Wells turbines collected during the testing period. The sea trials at the Mutriku wave power plant demonstrated that the biradial turbine exhibits a mean time-averaged peak efficiency 37% higher than the Wells turbines in operation at the power plant.