It's been getting colder here in Seattle, and so I've started using the cabin heater more. It's really noticable how much battery it uses.
I did a little experiment upon arriving home tonight. After pulling in the garage (no worries about carbon monoxide here!), I put the car in park and looked at the current draw. It was 1-2 amps. I turned on the heater full blast, and after a few seconds it went up to 9-10 amps. AC showed 4-5A. So, the heat uses about 8A, while the AC is only 3A, nearly three times cheaper.
I got to wondering why the car couldn't just run the AC backwards and have a heat pump like my house does. This results in significantly more efficient heating than just running a big resistor, which I assume is how the Roadster works. I guess car AC units don't have the de-icing cycle that heat pumps need, but I don't see why they couldn't if they were properly designed.