I was basing my statement on the following from the
owner's manual: "
Whenever Model Y is plugged in but not actively charging, it draws energy from the wall outlet instead of using energy stored in the Battery. For example, if you are sitting in Model Y and using the touchscreen while parked and plugged in, Model Y draws energy from the wall outlet instead of the Battery." I assumed that the screen is powered by the 12v. Is that not correct? I figured that sometimes we wake the car for various things, so there would be some additional battery use.
That statement from the Owner's Manual is an over simplification. When powered on, with the climate control running, the power used will exceed 1.5kW, even up to 8kW when preconditioning. This is well beyond the power that can be delivered to the Tesla Model Y when plugged into a 120V receptacle. When using a 15A circuit the maximum power is just 1.4kW. In this example the only available source for this level of power is the high voltage battery.
The other point is that there is a low voltage side and a high voltage side to the Model Y's electrical system. The AC compressor runs on high voltage, i.e. 360V. The only cabling to provide the high voltage power that the AC compressor requires is from the high voltage battery.
The on-board charger can operate at up to 11.5kW (requires a 240V, 60A circuit.) There would be conversion losses and excess heat generated by the on-board charger if this was used to provide power for the AC compressor. There would have to be additional high voltage wiring from the on-board charger to the AC compressor. This would increase weight, complexity and cost of the wiring in the Tesla Model Y, i.e. it would not be a good solution. Better to draw all power from the high voltage battery, run the AC compressor directly off the high voltage battery. Use the power conversion system (PCS) to step down 360V from the high voltage battery to the needed 13V to 15.5V DC for up to ~2kW to run the 12V accessories including the infotainment screen, computers, HVAC blower, lights, wipers, doors and windows. The PCS maintains the 12V battery whenever the Tesla Model Y is powered on. The Tesla Model Y will detects the voltage drop in the 12V system when parked, then the Tesla Model Y will power on so the PCS can charge, top up the 12V battery.
A different approach to maintaining the 12V battery was taken GM. The GM Chevrolet Volt has a dedicated charging circuit that periodically will use power from the charging station (either 120V or 240V) to charge, top up the 12V battery in the Chevy Volt when the Volt is plugged in. When the Chevy Volt is powered on all power comes from the high voltage battery and the accessory power system (APC) the GM version of the Tesla PCS.) The Volt can be actively charging at either Level 1 or Level 2 while powered on. If power is being drawn from the high voltage battery faster than it can be put back into the battery while charging then the battery would not be able to be fully recharged until the preconditioning cycle has completed. Charging could continue for an additional 20 or 30 minutes after preconditioning completes to put back the all of the energy that was consumed during preconditioning.