Wow. Quite a discussion. So how much does FE suffer if your charging system is loaded with an extra 30-80 watts? You guys are really worried about your carbon footprints
Running coolant through the heater core will lower the engine's operating temperature some. We used to use that fact in my younger days when we were driving POS cars that needed radiator work and we were running straight water since anti freeze cost money. Turning the heater control (assuming it worked) to high would sometimes allow the vehicle to get where we were going without overheating too much. Assuming the heater core didn't leak.
Back to the subject, a lot of cars as stated by Frank Lee do turn on the ac compressor when in the defrost mode. Some even when in the heat mode. All to dry out the air.
In the mid '80s I had an 81 Mustang in Scotland for a couple years. Petrol was about 98 pence a liter if I remember correctly. $/gal I don't remember exactly but it was a lot more than in the US. Since Scotland is a place you need your defroster a lot, I decided to try to disable the ac in defrost. The easiest thing would have been to pull the connector off the clutch at the front of the compressor. I didn't want to disable the ac as there are maybe seven days in the summer that it is hot there (to a Scot) and some of the local lasses liked to go for a drive to cool off. Some even wanted a ride out of the deal
I pulled the HVAC control out of the dash and swapped the vacuum hose from the defrost position to the heat position. This made the blend air door put all air to the defroster vents in the heat position. No air to the floor. This created a vacuum leak at the hvac switch defrost position that I believe I dealt with by using a rubber ear plug. This did improve my dismal FE somewhat as the defroster was on most of the time I drove there.
This 81 Mustang 2.3/auto rarely got better than 20mpg and it was a total sloth as to acceleration. Never did figure out why.