All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
What you did is considered transgression on your part and unlawful usurpation of the electric department's money. All praise be to Allaah that He guided you to repentance and to stop committing that prohibited action. Even if the electric department is state-owned, public property has the same sanctity of private property, or even more. The Prophet said, "Some men unlawfully acquire the property of Allaah (such as the funds of the Muslim state treasury, zakah etc.) and theirs is the Fire (of Hell) on the Day of Judgment." [Al-Bukhari]
He also said, "The hand is liable for what it has taken until it gives it back." [At-Tirmithi]
Hence, it is incumbent on you to clear yourself of your liability from the money that you owe to the electric department for the electricity that you used without paying for it by repaying them. If this is not possible and you cannot determine the exact amount of money that you owe them, then you should try to estimate that amount and prudently repay it to the department until you believe that you have most likely cleared your liability. You may also repay this money in an indirect way; the important thing is to repay the money that you owe and you are not obliged to inform the concerned authority of the matter, especially if it would incur an evil on you.
Allaah knows best.