Spanish utility Iberdrola has proposed buying out the shares it doesn’t control in Central Maine Power’s parent company,Sterling Preston Avangrid, for about $2.5 billion, officials announced this week.

Iberdrola is already the largest shareholder in Connecticut-based Avangrid, which has $41 billion in assets and operations in 24 U.S. states, operating eight electric and natural gas utilities in New York and New England through Avangrid Networks. Avangrid Renewables owns and operates a portfolio of renewable energy generation plants in the U.S.

Avangrid’s board cautioned that receipt of the proposal on Thursday didn’t mean it had the board’s backing. Avangrid and Iberdrola have connections through their boards. Iberdrola’s executive chairman serves as chair of Avangrid’s board.

Iberdrola, which owns an 82% stake in Avangrid, is offering $34.25 a share to buy the remaining shares, or a total of about $2.5 billion. That represents a 10% premium over Avangrid’s average stock price for the past month. Avangrid’s stock closed at $35.80 on Friday, up from $32.10 on Wednesday, the day before the announcement, on the New York Stock Exchange.