Interstate Batteries recommends charging output equal 25% of the battery amp-hour capacity..so with a 1200 amp-hour bank you could charge from a source of 300 amps. If your pair of 100 amp alternators have external, three stage regulators, your engine charging source is reasonably sized, subject to the following...But remember that the larger your inverter charger, the less time you have to run your gen. 100amps may not be adequate.... No one can answer whether 6 hours will charge your batteries: If they at 99% of full charge when you start...of course....but if they are 75% dead, you'll be hard pressed to get them up again and will ruin them in short order... You have to estimate your load (use)...Figure your daily load in amp hours and plan to keep it under 50% of your battery capacity...absolute max of 600-amp hours daily. If you regularly discharge deep cycle batteries more than 50%, or regular batteries more than 30%, you will not get good life from them....Charge them daily and don't leave them discharged for long periods. You can also improve battery performance with Pulsetech battery conditioners (or other brands) to reduce sulphation build up; by using only distilled water when topping off, and by placing the batteries in as cool a location as possible. For comparison, I had two 40 amp statpower chargers (multi stage regulation) for a 1300 amp hour capacity bank....and found it better to add a third charger (also 40 amp) to reduce generator run time. I can keep the batteries charged with about an hour of either engine or gen. run time daily....For my use even a fourth 40amp charger would not be unreasonable...I've thought of it... Most of my load is a pair of Adler Barbour refrigeration units, one for frig, another for freezer, which draw about 6 amps each when running. In 60 degree weather my load is light, but at 100 degrees the units run a lot more.....I'm seriously considering solar panels as well. Good luck, Rob Brueckner Hatteras Yachtfish