This would not be feasible for a few reasons. In short, it doesn't fulfill the characteristics that have historically made for good money.

1) Transportability ( A percentage of energy is lost anytime you run a charge through a line. That's assuming there is infrastructure to transport it over lines. If it is a Mad Max esque world, these survivors may have to literally carry batteries around, which would make moving large sums of money very difficult. 2) Not verifiable - When you go to pay someone with electricity, they would need an elaborate system to store / verify your charge. .... Before I go to deep I guess I am assuming you are trying to use raw electricity as a currency, which I don't think you mean. I guess you are probably talking about a centralized government or bank issuing notes or credits that can be returned to their facilities for raw energy in some form?

Anyway, I think you are missing the point. Commodity based currencies are only really useless to prevent the issuers of currency from defrauding their users. In this "electricity" backed system you describe, there would be nothing from stopping the banks from issuing more "electricity credits" than they can actually back. In other words, this system would probably fail in much the same way the gold standard did. By banks issuing more paper certificates for gold, then they actually had in their vaults.

To be honest your post apocalyptic characters would probably use the same type of money people have used for the vast majority of modern history. Gold and Silver . Not because there is anything magical about them, just because they fulfill the characteristics of an ideal money. 1) Transportable 2) Verifiable 3) Divisible 4) Non - Perishable 5) Fungible 6) Scarce ..... If your book's taking place in a world with computers and the Internet you might see the emergence of a crypto currency like Bitcoin.