In its ruling the Privacy regulator said it had received hundreds of complaints from consumers over unwanted sales calls in Enel Energia's name, and general difficulty in exercising rights over data protection.
The watchdog said that besides the fine, Enel Energia also had to introduce measures to make sure it complied with national and European rules on the safeguarding of data.
In a statement on Wednesday, Enel's retail energy sales unit said it had always acted in full respect of existing regulations and was "not involved in any conduct of unsolicited phone calls".
($1 = 0.8814 euros)
(Reporting by Stephen Jewkes; Editing by Jan Harvey)