[tcas]

Car and Driver did a test using an older Model 3 with a resistive heater (the newer models use a much more efficient heat pump), and found that it used around 2.2% an hour to keep the cabin warm.

https://www.caranddriver.com/news/a38807463/tesla-model-3-cl...

In your example with 15% left, you'll use ~9% battery while in traffic for 4 hours keeping the heat and car on, leaving 6% to get to the next charger. At ~300wh/mile you'll arrive with ~4-5% left. There's also buffer under 0%, but it's not guaranteed.

4-5% is not a comfortable number to be at, but I think it's acceptable in a worst case scenario like this. That being said I would definitely turn down the heat, and drive slower for the next few miles (and check for alternate chargers) to minimize power usage.

[cogman10]

Man, the heat pump hvacs are impressive :). My 2018 model LR 3 has a resistance heater and a heat pump heater is the one thing I really wish I had.

[jhoechtl]

I really wonder how a heat pump can achieve so much efficiency gain. Heat of electric resistance is about 99% energy efficient: almost all energy is converted into heat, nothing else.