My 2018 model BMW i3 reported about half the normal range when we drove it during -20C without pre-heating it this winter. When pre-heated the range reduction is primarily due to the occupants using electricity to heat up the cabin.

But, as mentioned that doesn't mean half the range is gone, after a while the batteries heat up and you get more available range. But it can take about an hour or more, depending on driving, so you're looking at a fairly substantial hit regardless.

Another point is that the car won't supercharge if the batteries are too cold. If you regularly see -15C or below, you might want to consider a car which allows you to remote start the heating and run the battery heater from the batteries themselves, like Tesla does.