How much power is us is dependent on how many toys you have on board.
My wife and I lived on our boat for nearly ten years. During the time
at anchor we used about 165 amp/hours per day. Most of which supported
our refrigerator system (Glacier Bay). Our batteries were rated at
880 amps. We could go for about four to five days with out recharging.
We also had 130 watt Solar panels and a wind generator. Our charging
system was two 120 balmar alternators, one on each engine.
You said you are thinking about a 100Ah battery, that's not enough for
one day if you have refrigeration. A small refrigerator will consume
50-60 Amp/hours in a day. Your batteries should never fall below 50
percent of their rated value.