I don't have fancy graphs for you, but I do have gleanings from a good ol' logbook.
First, most important point: Don't look at the forecast for the day and read the high. Take the low from the previous day or from current day's evening as appropriate. This is more representative of the state of your battery temperature (assuming it's parked outside and/or you are travelling in the morning/evening rather than the warmest part of the day).
As you probably know, generating heat for the cabin is the main result of efficiency loss in Winter. Something to keep in mind is that moving slow while heating the cabin will generally require more power for heat than if you travelled faster, since you will get to your destination slower and thus be heating it for longer. I'm not advocating to move faster in poor conditions, just to be aware of what you will use if you need to go slower.
My best example of this was in -10°C (14°F) going just over 60km/h (almost 40mph) took double the normal energy, aligning with the
50% efficiency loss others have noted above. In colder temperatures (-20°C, -4°F) I could travel faster (90km/h, 56mph) with "only" about
40-45% efficiency loss. A full day of on/off city driving would certainly be worse than 50% loss, but I'm not sure by how much.
At about freezing (0°C, 32°F) I get about 30% loss in both city and highway driving, and at +10°C (50°F) I can see about 20% loss as well.
For us, 30-35% is typical for what we get in Winter (dancing around the freezing point most days).
The absolute worst thing is short hops multiple times per day separated by many hours which cools things down. This requires a lot of heat pumped into the car that isn't put to much use. A pizza delivery driver on a slow Monday shift might be better off driving an efficient gas vehicle, for example (in terms of $/km).
This forum previously led me to believe "30% at absolute worst" before we got the car, I'm glad to see people have become a bit more realistic of just how much Winter can hurt the range. 30% is more like the average if you see anything that resembles freezing temperatures or snow.
Note about graphed results from third party services: Not all of these include the power used to preheat your car. If you preheat without being plugged in, you're still using that energy whether the service (or car) counts it or not. Preheating uses even more power due to a recent update which will aggressively warm the battery pack to provide regen as well. In addition, things like standby usage for Sentry really start to matter since every percent capacity gets precious, so you'll be more aware of that.
For my LR AWD, this is now my process for Winter:
- Try to keep between 20% and 80% for maximum battery health. This gives for 60% usable capacity, or about 300km rated range (186mi).
- Assume half efficiency if it's cold or slow going (which will give some of the buffer for preheat, standby losses, etc.). This brings my 300km rated usable range to about 150km effectively (93mi).
- If more range is needed, use 10-90% which will effectively give 200km (124mi).
120km is what we need the car to do every day at a minimum, so an LR normally rated for 500km turns out to be absolutely necessary in our case.
