The energy required for charging is the energy input required to fill the battery, plus whatever additional energy is needed to condition the battery. If 50% of the energy you use for charging (which you pay for) goes towards conditioning the battery, then then the energy efficiency of the vehicle is 50% times whatever efficiency you get through the drivetrain. When it gets cold enough, all of the energy draw goes towards conditioning the battery, so the car doesn't charge at all. Since none of the input energy goes towards moving the vehicle, the efficiency is zero, though if you wait long enough it'll take a low value as the temperature increases and the vehicle begins to take charge. It's probably more correct to say that the energy efficiency of the vehicle approaches zero as the temperature drops.

This is different to a standard car, because losses during refueling are negligible, but losses in the drivetrain obviously aren't. And of course for both types of vehicle you have additional upstream effects, e.g. production and transportation of fuel versus production of electricity, production of the vehicle, etc.

The reason I raised the issue is because I hear a lot about well-to-wheels efficiency of BEVs, but I've never heard anyone take into account the effect that conditioning the battery has in certain climates. It's one thing to say that the well-to-wheels efficiency of a BEV is around 60% compared to 16% for ICEVs, but if you're neglecting a large multiplicative efficiency hit in your chain, things like hybrid electric vehicles suddenly compare a lot more favorably.

And if you're comparing mileage on the basis of miles per gallon using energy input at a pump or cable that you paid for, it seems like a reasonable question to ask. Is that mpg figure calculated using the energy of the battery, or the energy that was paid for?