The Irish Republic's energy giant, ESB, has completed a £1.2 billion (1.4 billion euro) deal to buy Northern Ireland Electricity (NIE).
The agreement finalises plans announced in July for the leading electricity company to take over the northern business.
ESB chief executive Padraig McManus said the company was determined to meet commitments to green energy.
"We are delighted with the acquisition of NIE, an efficient and successful company," he said.
"ESB is committed to making the capital investment required to match future growth in the economy and to meet the Northern Ireland government's targets in relation to the development of renewable energy."
Under the terms of the deal, NIE will continue to operate as a stand-alone business with its own brand and identity.
Patrick Haren, Viridian Group chairman, said the sell-off would see NIE provide more renewable energy to customers.
"Under Viridian Group's ownership, NIE has strengthened its network capacity and improved significantly its service delivery to customers.
"Under its new owners, it will meet the challenge of grid development and delivering increasing volumes of renewable energy to customers."
The deal also sees ESB acquiring associated companies of NIE, including NIE Powerteam Limited and Powerteam Electrical Services (UK) Limited, which provide electrical construction and maintenance services.