[MetroMPG]

PaleMelanesian: it's for that reason (higher electrical loads in winter) that I don't mess around with alternator optional running at this time of year. Also, the cold temps degrade battery performance. So on top of the added draw, you also have lower capacity. Bad combination.

Hondaworkshop: I understand that if you underdrive an alternator, the voltage regulator will simply compensate for the reduced output by increasing the field current.
So it's a wash in the end. The only (small) benefit from underdriving then is limited to belt losses & less aero drag from the pulley fan.