“eat up prime motive power running heat or air conditioning.”
So does the gas engine- but the energy in gas is so concentrated/lb (over 50 times the energy density of a lithium battery) that it can be efficiently carried.
And that’s why ‘all-electric’ cars are just not inherently practical. They’re “concept cars”.
At best a fuel based climate system is needed even if the electric motor is to provide the motive power, and since the fuel is there anyway it’s reasonable to also have a gas engine for power when needed.
Though it may sometimes be efficient to have only the energy for climate control provided by fuel. If a small propane heater would extend the range dramatically it seems a responsible ecar builder should integrate one safely- and PR fantasies be damned.
The heat is a by-product. It must be rejected from the engine to somewhere; in the winter it gets rejected into the cabin.