I just found out from Tesla support that the built in thermal warming for the batteries in cold weather is only functional if there is grid power. This is an issue in my area where I live in the mountains and subject to cold in the winters. My batteries are mounted on the outside wall of my house. As it happens, this past winter I was without grid power for about a week. Well, my batteries ran my house for two days and then ran out of power forcing to borrow a generator to run critical appliances. Even though the days were sunny and my solar could have run the house and charged the batteries during the day, the powerwall switch output a frequency to prevent the solar production because the batteries were to cold to accept a charge. This pretty much cancels out the purpose of having the powerwall batteries. The thermal heating using a minimal amount of energy and, I think, should be able to warm the batteries to accept a charge even if there is no grid power. This certainly seems to make more sense to me. IN the meantime, I am building an insulated battery box around my batteries which will be heated by a couple of silicone rubber 25W heaters which I will set to come on when grid power is out.