How big a battery bank do I need?
For most people to answer to this is "as big as possible". Charlie Wing's book the "Boatowners Illustrated handbook of Wiring" has a very good explanation of how to calculate the capacity you need. The calculations start by working out what your daily load will be both at anchor and while underway.
How much charge is left in my battery?
Keeping accurate track of battery state of charge is essential for the high performance marine electrical system. Today’s expensive heavy duty batteries are too valuable to leave without proper monitoring. Traditional ways of assessing state of charge don’t always work with these batteries and more sophisticated measuring systems are now available at relatively low cost. It makes good economic sense to upgrade to a modern amp hour meter.
Lithium Ion Batteries
"Super Cycle" AGM Batteries
Conventional AGM batteries