Originally Posted by jarcher
I only have a single 90 AHr house battery. I figure turning off a light that is not needed never hurts.
Yup I would be afraid to turn much of anything on especially if you are sailing off the hook or live on a mooring.
90ah to 50% depth of discharge (max draw down) = 45 usable amp hours. If on the hook or charging using only an alternator figure you'll only ever get back to roughly 80% of that 90ah capacity. So you now have only 72 ah's total available leaving you only 22 usable ah's before you hit a 50% depth of discharge.
A 22 ah bank is barely enough to be safe if you needed your running lights. Of course you can always go below 50% DOD but you'll be shortenting the battery life..