[oil pan 4]

I have a 2011 nissan leaf with electric resistance heating.
There is a display showing how much power the climate control system draws.
So I can post my observations based on conditions.

Lately I have only used the defrost. With the A/C off of course. If I use the defrost I always leave the A/C off. With gasoline vehicles i have noticed using the A/C with defrost combines the worst parts of summer and winter MPG.

So far using fresh air (the leaf will only allow me to select fresh air when using defrost), defrost on, A/C off, blower turned down just enough to keep the windshield clear in upper 40s to low 50s I'm still ruining about 1kw to 1.2kw.

That's about 3,400 to 4,100 BTUs per hour. That's really not much.
Just using the defrost on a cool wet day shouldn't take enough heat away from a gasoline powered cars engine to effect mpg once it's warmed up. If you run the heater blower while the car is warming up and it takes longer to warm up all the way then yeah.

If I put it on recirculation when it's upper 40s to 50s with no fogging then the heater will stabilize some where around 500 to 700 wattts.