The unit is part of the Tâmega complex, which is considered one of Europe’s largest energy storage facilities.
The €1.5 billion (£1.2bn) project comprises three reservoirs and three hydroelectric power plants with a 1.1GW capacity.
Tâmega is forecast to produce 1.7GW a year, enough to cover the energy needs of 440,000 homes.
In six months, the giga battery is also expected to have enough storage capacity to supply two million Portuguese households for an entire day.
Iberdrola said the Tâmega complex will add 880MW of pumped storage capacity to the Portuguese electricity system, a 30% increase in the storage capacity available in the country today.
It is also predicted to save 1.2 million tonnes of carbon dioxide emissions every year.