Just a couple of points to add. Many new boaters overestimate how much power you can get out of batteries.
Firstly, you should not routinely discharge them beyond 50% charge. Secondly, most engine charging systems will not charge to 100% capacity and finally most batteries will only be working at a % of their original rating anyway.
So, say 4x 100ah = 400ah then suppose they are now at 90% of original capacity = 360ah and charged to 95% = 342ah
50% of this = 171ah
So you will be lucky to get 40% of the theoretical capacity of your batteries, and it may well be less than this.
Now go round your boat adding up your appliances. The fridge may be the biggest battery killer as it’s on 24hours a day and running for a good proportion of this.
Finally, beware the inverter. Fine for running small electronic items, laptops etc but anything with heating and motors designed for a normal home will kill your batteries in no time. Don’t even think of washing machines and dryers! ( They are a whole different subject.)
The whole subject of batteries is endlessly fascinating to some of us boaters and intensely boring to others! If you are in the second category then get an expert to check over your whole system. Ask them to calculate your loads as a guide and to fit some kind of monitoring system to avoid discharging your batteries too far and wrecking them in a few weeks. Our share boat has a smartgauge which is simple enough for us all to use and does the trick nicely.
Lastly, if Tony Brooks says different then he’s the expert so believe him rather than me!