A tidal turbine near the mouth of Strangford Lough has begun producing electricity at full capacity for the first time.
The SeaGen system now generates 1.2MW, the highest level of power produced by a tidal stream system anywhere in the world.
The system works like an "underwater windmill" but with rotors driven by tidal currents rather than the wind.
It has been undergoing commissioning trials since May.
SeaGen will now move towards full-operating mode for periods of up to 22 hours a day, with regular inspections and performance testing carried out.
The power generated by the system is being purchased by Irish energy company, ESB Independent, for its customers in Northern Ireland and the Republic.
The turbine has the capacity to generate power to meet the average electricity needs of around 1000 homes.
Martin Wright, managing director of SeaGen developers, Marine Current Turbines, said that having the system generating at full power was an important milestone.
"It demonstrates, for the first time, the commercial potential of tidal energy as a viable alternative source of renewable energy," he said.
"As the first mover in tidal stream turbine development, we have a significant technical lead over all rival tidal technologies that are under development.
"There are no other tidal turbines of truly commercial scale; all the competitive systems so far tested at sea are quite small, most being less than 10% the rotor area of SeaGen."