The inauguration ceremony was attended by Portuguese Prime Minister António Costa, Iberdrola chairman Ignacio Galán and other officials.
The 1.1GW project was built over a period of eight years with an investment of more than €1.5bn.
It features the Alto Tâmega, Daivões and Gouvãe power plants and can store 40 million kWh, which is enough to meet the domestic energy requirements of 11 million people for 24 hours.
The Tâmega Gigabattery project’s launch is expected to help reduce annual oil imports and eliminate 1.2 million tonnes of carbon dioxide a year.
In a statement, Iberdrola said: “This major engineering project, on which several thousand people have worked, will prevent the importation of more than 160,000t of oil a year, consolidating its position as a socioeconomic and job-creating driving force in the region.”
The company added that the project’s construction was accompanied by several social, cultural and environmental initiatives benefitting seven municipalities.
At the event, Galán reaffirmed Iberdrola’s commitment to Portugal and said that the company will double the investments made in the gigabattery to €3bn over the coming years.
This will be invested in new wind farms and solar plants that are already under development or construction.
Galán also said that the Tâmega Gigabattery project showed that the European Union’s goals of energy independence and decarbonisation ‘are perfectly compatible as long as work is done to electrify the economic and production system’.
Earlier this month, Iberdrola subsidiary Neoenergia won two bids in a transmission line auction in Brazil.
One of the projects involves installing 1,707km of transmission lines between Minas Gerais and São Paulo, while the other project will deliver 291km of lines in Mato Grosso do Sul.
The two projects will involve a combined investment of around $1.02bn and generate more than 11,000 jobs during their construction.