Up to 300,000 homes in Northern Ireland could have their electricity supplied by offshore wind and tidal energy as part of a new deal with renewables companies.
Areas off the north coast and South Down have been earmarked for development, said the seabed's owners the Crown Estate.
The projects could deliver 800 megawatts of electricity and power more than a fifth of homes, experts said. Development is expected to begin before the end of the decade.
Energy Minister Arlene Foster said: "Today's announcement by the Crown Estate represents a major milestone for Northern Ireland."
The Estate, which grants seabed lease development rights for offshore renewable projects, said agreements had been signed with First Flight Wind, Tidal Ventures and DP Marine Energy of Ireland, and DEME NV (Dredging, Environmental and Marine Engineering) of Belgium.
The companies secured leases for a stretch of seabed between Torr Head and Rathlin Island and another section off the coast of Ardglass, Co Down. Two north Antrim sites would contain two tidal stream projects generating 100MW each. An offshore wind turbine project is planned for south-east Co Down creating 600MW of energy.
It is part of the Executive's plan to significantly reduce reliance on fossil fuels by 2020. The Executive requires 40% of electricity to come from renewable sources by 2020.
Ms Foster added: "The development of offshore renewables is an excellent opportunity for Northern Ireland in so many ways, energy security and diversity, climate change mitigation, a contribution to the Executive's 2020 (renewables) targets and the business supply chain opportunities for local companies over the next few years as these projects come on stream."
The consortium First Flight Wind, which includes one of northern Europe's leading energy companies (DONG Energy), has been selected to develop an offshore wind farm off the south-east coast of Co Down. It could deliver up to 600MW of generating capacity, supplying up to a fifth of Northern Ireland's 2020 electricity requirements.
The other two tidal stream projects will together deliver 200MW and the total new planned capacity now announced is equivalent to demand from between 200,000-300,000 homes.