My experience is that the decrease during cold weather is more a function of the lowered performance of the battery than the result of the cabin heating.  The battery draw at freeway speed is maybe 30kw.  The baseline draw from the accessories in the vehicle ( radio, console ) and no heating&#x2F;cooling&#x2F;defroster is around 0.5kw.  With the heat cranked up all the way and all the various heaters turned up to high the additional draw above baseline is 2-2.5kw.