Trever,

The answer to your question is kind of relative to a few things. One thing you can count is, is that any standard alternator will not do a good job, as its not designed to, recharge a depleted house battery bank. With the engine running, you should not experience any issues with the alternator keeping pace with the system, but if you spend a great deal of time running the system while at anchor, a larger battery bank is a plus and you will become more dependent on shore charging.

Im guessing you have a main cranking battery and then the 2 batteries in parallel are the house battery? Are these banks isolated by a dual-battery switch?