At present, ERG operates wind farms in Europe with a total capacity of 1.38 GW, which includes 1.1 GW in Italy. It also has a gas-fired power plant in Sicily with a capacity of 480 MW.
The $1 billion deal furthers the transformation of ERG from oil refiner to specialised renewable energy company.
ERG Chief Executive Officer, Luca Bettonte, said: "The acquisition will enable a significant improvement in our economic results and cash generation in the medium term."
The deal represents the end of E.On 's ill-fated foray into southern Europe, where it is also pulling out of Spain as well.