Electricity retailer Simply Energy has been fined $20,000 for signing up a customer with a brain injury without his consent in a case described by the energy regulator as "disturbing".

Victorian energy regulator, the Essential Services Commission (ESC), fined Simply Energy after its sales team repeatedly called a man with an acquired brain injury until he signed onto the retailer.

The man with an acquired brain injury was signed up Simply Energy by a telemarketer who ignored his requests not to switch retailers. Credit:Reuters

The regulator said the fine was issued after the retailer's sales agent ignored the man’s requests to call back later, even after he demonstrated signs of being confused and said he was unable to properly consent to switching retailers due to his brain injury.

ESC chairman Ron Ben-David said Simply Energy failed the customer.