I agree with your point, though I guess its probably more like 2x the cars. Also, in colder countries / states, the equation doesn't look as rosy. In winter, you use the heat generated by the combustion engine. Hence, efficiency goes way up.

Without knowing any numbers, I would assume, that at the moment, an additional, fossil fuel based, heating system would do the best job for an electric car. Webasto [1] for instance sells those.

[1] http://www.parkingheater.com/