I'm rather thinking of being plugged in. 1500 watts is about all you can plan on drawing without blowing a breaker. The context, if I understood correctly, was that plugging in to a 120v household outlet was a net loss because 1500 watts was insufficient to warm the batteries to charging temperature and that presumably some numbskull in Tesla's software group decided to run the heaters even if it resulted in a net loss of power.
The explanation of the anecdotes is probably complicated, but I believe that the Tesla chargers monitor the voltage at the input to the car and reduce their current draw to keep a minimum voltage. So if your 120v transformer is a little on the low side (they vary) and maybe you used that 100' 16g extension cord you had handy, and maybe your house was wired when aluminum house wiring seemed like a good idea and that outlet has a bit more resistance between the wire and the terminal screw, you might be getting a lot less than 1500 watts. There could well be a point in the charging curves where there is a problem, but I don't think it is because 1500 watts isn't enough.
