Well, the basic hydrocarbons come from the ground in the form of crude oil. But that has to be pumped out of the ground, into the tankers or through the pipelines. That alone adds a significant amount of electricity consumed. Then the refining process. That uses a lot of energy. A large part of that comes from burning even more oil, so the environment impact is huge. But it wouldn't appear on the electricity bill indeed. But still, the refineries eat up a lot of electricity too. All the pumping required, the big destillation facilities might be heated by burning oil, but the control and the operation is electric. And then the gasoline is pumped around even more. Until it is pumped into your car. All of this uses electricity. Finally, even the gas stations themselves use a lot of electricity as for illumination.

I can't name precise numbers - my calculation was really only the worst case szenario. Some calculations claim, going fully electric with cars would only add about 10% to the electricity consumption for all the electricity saved in the fuel supply.