Do you have refrigeration
? How you use the boat can make a big difference too. The idea is to not let the batteries fall below 50% of capacity then charge them back to 100%. This would be a perfect situation. The other part is to have enough battery
capacity so you can fit within those first goals. If you add a lot of batteries you eventually make a situation where you never can get them all charged to 100% again.
A bigger alternator can put out more power but a smart regulator
will cut the power output of the alternator to match the charging
profile of the batteries. Batteries accept charge based on type and by the percent capacity. A low level battery
can take more charge than one nearly full. That means as you get up to 80% full the larger output alternator may not be putting out as much as it could because regulation as throttled it back. The bigger alternator only uses full output when your batteries are low and your need for power is great and even then only a short period of time. Charging
up a cold plate in a fridge while making water
when the batteries are low would be a great time to run the engine
with a bigger alternator. AGM
batteries have a lower internal resistance and can accept more charge faster so could perhaps take advantage of a larger alternator longer but it's not a huge difference. The key is if the increase in alternator cost is going to come back with savings for you. This is one of those "it all depends" questions.
You need to determine how you will use power and when you will recharge the batteries and how long that will take based on the charge profile for your batteries. You have both solar
input, alternator input, and shore power
input. You may find a 70 amp alternator does more bang for the buck than a 150. You may also find that the 40 amp does the same job in only a slightly longer period of time. In your case the 150 amp alternator will never be able to cut your time to recharge as a function of the increase in size alone. It won't let you reduce engine time by 75% because if you added 150 amps all the time the batteries would fry quickly. Large alternators are really great when you plan on using the generated power right then. Alternators do not run for free. They convert engine power to electrical
power and not at perfect efficiency.
If you look through the electrical
postings in the forum you can find a large amount of details on this subject. Electrical use aboard is an interesting and important topic. DC power is not something most people understand well if at all. It does not work like the AC power in your house. A lot of details come into play and you need to step back and look at the total system to get a plan that suits your style of using electrical power use and to understand the costs and burdens required. When you understand the bigger picture you can better see where the alternator fits into the the overall plan.