Before replacing batteries, figure out what your amp/hour usage is over longest period of discharge is (eg 1800 to 900 = 13 hours). This calculation assumes discharge doesnt exceed 20 hour rate of batteries. If you exceed 20 hour rate use Peukart's formula tables to arrive at discharge time.
Ensuring long life the usable portion of wet cell lead acid battery is 35% if charging by genset as the last 15% takes many hours (eg 100 a/h battery has 35 a/h of usable capacity).
Using the above will give the size of battery bank required.
Ensuring long life the usable portion AGM's is 45% and can be pushed further...
On my next battery replacement im going exotic with Firefly battery, its a new design/chemistry in AGM family.
There about 4 X times the price wet cell lead acid.
Cycle count is in the many thousands instead of hundreds if discharged to 50%
Can be dischaged to 20% and still get 1000 cycles.
Usable capacty 50%
There sulphation rate is very slow. So slow that they can be left partially discharged without sevre damage and be brought back to 100% capacity.
If your on the hook a lot or do not have room for more batteries they may a good choice.
Dont overlook solar. 400 watts of panels works out to 200 amp hours charging daily!
Plus with solar you can bulk charge with genset and let solar do absorption charge gaining another 15% of battery capacity.