Go to the local chandlery
or online and buy two of the books
on twelve volt boat systems. They'll give you a more complete picture of the entire SYSTEM because looking at just any one or two pieces will pretty much ensure you miss some important parts
of the bigger picture.
And take a look at the forum archives
, here and elsewhere, with that bigger picture in mind. All the questions have been asked and answered many times before, the whole point of a forum is that the answers are already there, posted forever, for anyone who bothers to look for them.
All the rest is simple math, amps in and amps out.