The best way to heat using electricity is to run an air-conditioning heat pump in reverse, so that it cools the outside and exhausts heat into the interior. That delivers 4 to 5 times as much energy to the interior as is requiring for pumping.
This means a given amount of electrical energy is about 4x more valuable to run a heat pump than simply to heat the cabin through resistance heating.
Recent models of the Nissan Leaf have a reversible heat pump for heating as well as cooling, which yields significantly reduced cold-weather range loss. Strangely, the premium Tesla Model S does not provide this function with its heat pump.
But heat pumping loses effectiveness as the outdoor temperature drops, so auxiliary heat is required for the coldest conditions.
This is often provided by electrical resistance heating--which only delivers 20% of the energy that could have been provided by a heat pump.
For the same reason that electric vehicles are a great idea, heating electric vehicles with electrical resistance heating is a terrible idea.