How much energy does an alternator use?

[Daox]

No, I live in quite a cold climate. I do, however, have a garage that I keep the car in for most of the winter. This reduces the requirement for immediate defrosting and engine warm up. When at all possible (and I can't even remember if I did use it last year) I do not use the rear window defroster. I wait for the car to warm up and then use the car's heat to defog the rear windows.

Headlights are probably the biggest hit (the fan on high is a close second), and it will take its toll on the battery. However, I have driven on cloudy rainy days with the headlights on and have had no problems besides the deep cycle battery needing a bit more juice when I get home. Radios use very little power and are of little concern thankfully.

I should qualify that my commute is not the shortest either. I have about a 42 mile (62 km) round trip to take each day. The trip can take anywhere from 30 minutes to an hour if traffic is bad. I am using a used group 24 (which are usually 70-85Ah) deep cycle battery. When I plug it in for recharging the battery usually is at 70-80% charged. The day I ran headlights/wipers I think I was down to 60% or so. So, if you had a longer commute, you could simply go with a larger battery, or two in parallel.