Are you talking about the A/C blower fan, that uses maybe 200W of power on high?
Or did you mean the compressor, which is a 400V DC device?
What part of the BMS do you think uses lots of 12V power, and why?
Actually I miswrote in my reply. I am definitely not an expert so I sometimes make elementary errors. Please accept my apologies for that. I do not know how much energy is used in any part of the Tesla systems, nor any specific loads placed on the 12V systems.
The specific systems that have been mentioned in the documents I have seen, and reread just now, are:
1. Steering;
2. Driver assistance systems (the reference discussed "decision management" (the term included database processing, analytics, external image processing and decisioning and decision presentment) both the actuators and controls for steering, navigation, braking, speed control plus "sensors" (they listed six types (including radar and Lidar so not all apply to Tesla);
3. active suspension;
4. air conditioning and ventilation (this excluded compressor but mentioned seat heating and cooling);
5. vehicle displays;
6. vehicle controls (this referred to windows, doors, seats, sunroofs and other actuator initiated physical movements);
7. Audio and other entertainment systems.
This came from another manufacturer, not Tesla. I left out the myriad ICE and Hybrid-specific issues. This manufacturer listed ICE (differentiating between several types conventional OTTO, Atkinson and variants, Diesel); hybrid and BEV. Each of the three other than BEV had a number of engine management tasks deemed to be 48V candidates. Although it was too technical for me in several respects the interesting general issue is that they thought the change from 12v to 48v would incorporate retention of some 12v systems in transition (just as Audi and Bentley are doing now) but expected to have 48v as a standard for most non-motive-force items by 2025 at the latest. They also anticipated 400v (some debate about other voltages) for high-power systems.
One interesting generalization is that this manufacturer is trying to reduce the role of ICE as much as possible, replacing it with a fairly wide variety of electrical systems in an effort to reduce the energy loses implicit in historically engine-driven components. To that extent this seems quite similar to the "more electric" campaign Boeing employed in the B787 and is now applying in other programs. Those are all intended to minimize the losses in traditional engine driven compressors, pumps, hydraulic systems etc.
In a couple of supplier documents i saw there was expectation of savings in weight and cost for wiring, connectors and power transmission as a result of change from 12V to 48V in the categories other than active suspension. Those varied from 12-20% cost savings. Active suspension was presented separately from the other topics and i did not see that piece.
I cannot directly quote any of these references, but there are numerous industry white papers on the 48V transition, as well as a fair number of symposia and conferences.
I hope this clarifies my thoughts. Without any question more and more automotive electrical use will be higher voltage than in the past and much will be 48V. The only question is how long 12V endures. I think it will not endure for long, if only because of the weight reduction and electrical distribution efficiency that comes from reduced amperage. We shall see.