Damn, what EV do you have? I have a '23 Bolt and I only see about a 30% reduction in efficiency in below freezing. The performance of mine in winter was fair, and even in blizzard conditions in Chicago at 20F it still got the job done. But yea, the heater will consume a lot of battery regardless. Charging is my only concern, and that's solved by the fact that I have a supercharger not too far I can use once a week early in the morning for cheap $0.35 per kWh.