The AGCM watchdog said it had carried out searches at Enel Energia and Servizio Elettrico Nazionale's headquarters on Thursday.
The probe stems from dozens complaints from customers, Enel said in a statement.
"The companies believe that they have always acted in full compliance with primary and industry regulations and are confident that they will be able to prove the fairness of their actions during the proceedings," the firm added.
The two companies were fined in 2021 for the same reason, but Enel said it had appealed the ruling and a final decision on that case was still pending.
(Reporting by Federico Maccioni Editing by Gianluca Semeraro and Mark Potter)
