Robert with #304 posted a great article on the heater in the Mini E. It says the "PTC" heater (I think that means Positive Temperature Coefficient) can put out 3000 watts.
Mini said at first that the battery has 30,000 watt hours of usable capacity. (Later reports indicate slightly less if I recall.)
So running the heater at maximum for one hour should consume 10% of the range. I have never run it at full for more than a few minutes, I usually back off to half at that point. But theoretically, this means the two hour driving range (usually enough to drain the battery fully) might shrink to an hour and a half roughly, if it were cold enough to run the heater at maximum the whole time. At lower settings, the range reduction should be less than 20%. Maybe much less.
I measured heater power usage at low fan settings and posted them way back in September here. But this morning I measured again at the highest fan and heat settings with the windows open.
After 80 minutes at max heat the charge meter dropped 18%, which is a bit more than theory predicted above, but reasonable. It was in the upper 40s Fahrenheit and I left the hatch open. Temperature one inch from the heater vent was above 110 F with the fan at max, and the temperature was higher when the fan was slowed down. At fan setting #2, the temperature went off my scale, above 130 degrees F.
I have stayed away from using the heat on long drives. This is partly because of the reports other bloggers have posted about reduced range, but mostly because I'm from Minnesota where only the weak use the heater until the weather is below freezing. Ha. But I'm starting to think I can afford to use it more on my long commute. These numbers are encouraging.
While the charge gauge is pretty reliable, I find it to be pessimistic early in a drive and I almost always have more range than it indicates. I wonder if the calibration gets worse when the heater is on? This might explain some of the concerns that seem worse than the numbers I took.
One technical point about a "PTC" heater. It is designed not to overheat. So if you run the fan on low and turn the heat all the way up, the resistance of the heating element increases so that won't over heat and burn out. My measurements showed that the heater can maintain a 60 to 70 degree increase in temperature at the highest fan setting. But often I run it on lowest fan, and I notice it heat up and then cool down to just warm. This is what it is designed to do. Like Mini says, use at least the second fan setting with higher heat settings or the element will not reach maximum heat output.
By the way, 3000 watts is about 4 horse power. The 120 volt AC heater that I use to preheat my Mini E with "shore power" is 900 watts, and I usually run it for a couple hours by timer before I leave for work.