How much power does the heater use
 

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.

^^
Do you have seat heaters? If so, how much wattage do they consume?

I have a seat heater. It uses about 2 amps, or about 26 watts.

Seat heaters consume next to nothing on the vehicles I've measured (25-80w each seat depending on setting). Simply running the HVAC fans on low consumes more power.

In other words, in an ICE vehicle, it takes less gasoline to run seat heaters than to run the fans to move the free waste heat into the cabin. It seems counter-intuitive, but that's what I've measured on my Prius and Acura.

In my old camry, at cruising speed you could get airflow in without use of the fan.

Wonder if aerodynamics can be modified a bit to do that.

Iirc: place called FROST(king?) That has tape defrosters like the rear window ones, but in many different sizes and DIY. Since the defrost is only heating the window above dewpoint, all itneeds is just be warm.

I have something like that on my suburban.
But it's not intended for passanger vehicles.