There are several factors to consider:
1) How efficient is the generating station? In general, if any attempt is made to capture "waste" heat and use it for domestic heating or some industrial process, the answer is "very".
2) How efficient is the transmission system? This can vary quite a bit, both from location to location and over time. For example, a system under heavy load is less efficient than one under less of a load. As load increases, you get more heating of the cables.
3) How efficient is the charging system? Modern battery chargers are more efficient, but any time you change voltages or convert alternating current to direct current (or vice versa) you have losses.
4) How much is lost as the battery sits there? NiMh batteries are far better than lead-acid. Some of the newer technologies promise to be better still.
5) How efficient is the motor and drivetrain of the car?
6) And last, but certainly not least, how much energy went into building the car? The least impactful product is the one that has already been produced!
Of course, ultimately, the best answer is to ride mass transit, if you really must travel.