Running the fan at its lowest speed with the heater at maximum for 1:55 drew the charge meter down 9%. I don't expect to need max heat often, so I suspect I'll have enough range this winter. My drive is typically around 1:35, and I have often preheated my cars in the morning during winter with a 900 watt automotive electric heater. So I probably won't need heat the whole drive.
I took the measurements this morning with the car fully charged, windows open and in the garage. Ambient temperature in the garage started at 62 degrees F, same as cabin temperature. After the one hour and 55 minute test, the ambient temperature read 65 and the cabin was 79.
Mini announced a while back that there are some extra high power chargers available for lessees to install at work, now that all the high power chargers have been installed at the homes of lessees. I use the low power portable charger at work and have always had a comfortable margin of range for my 3+ hour commute, even in the hot weather where I was running the AC a lot. But I don't know about the winter when aerodynamic resistance increases, rolling resistance increases, batteries get "stiff" and the heater is on.
At this rate of heater power consumption, I'm not worried. But it will be nice if they grant me a second charger. But if they don't, I will deal with it. I am driving the Mini E this winter, whatever it takes. I expect it will not be a problem, since everything else has performed as expected or better.
I tried making measurements a couple weeks back using the "Amp Hours per 100 miles" reading on the dash board. There is too much filtering however, so I could not get numbers that looked trustworthy. My method was to zero all the gauges, coast in neutral down a small hill for 0.1 miles and then run the heater or AC for ten minutes. If it worked, it would have given a result in less time. I'll make some more readings with the longer method described at the beginning, but at different heater settings and post them later.