The pack heater is variable power, but it can draw as much as 6 kW. The cabin heat can also draw a similar amount of power. But this only happens when you first start up a cold soaked car.
Tesla doesn't tell us the actual battery temperatures, but if you cold soak the car below freezing the battery will be heated when you initially power up. You can avoid most of that by preheating the car (using the remote App) while the car is plugged into a charging station. It will draw AC power to heat the pack. This will have a huge benefit in preserving range.
Once you are underway, waste heat from the battery and drive train provides most of the heating. I believe the waste heat is also used via a heat pump to warm the cabin (it uses a resistive heater when the drive train is cold). So once the car is at equilibrium it's actually very efficient in terms of heat usage, even in pretty extreme cold.
At that point the loss of range is mostly due to aerodynamic effects (cold air is denser) and some rolling resistance (rubber gets harder). I've driven 303 km (190 miles) at -20C (-4F) in a blizzard and arrived with a decent safety margin (speeds around 50-55 mph mainly due to the road conditions) - while keeping the cabin toasty.
At a few degrees above freezing there is little if any impact on range. Certainly at +5C (41F) there appears to be no impact. As you drop through 0C (32F) you start to see some loss of range, perhaps 10%. As the temperatures drop further the loses increase to about 20%.
Somewhere around -25C or so (-13F) it appears the pack heater starts to run continuously, even at highway speeds. At that point you have more substantial range impacts, perhaps as much as 30%.