I agree with Jim that if you use your engine everyday your batteries will stay charged. However, we cruise Lilliana, our Willard 40, off the grid in Mexico. Sometime we wish to stay at anchor for several days at a time. We normally would use about 100Ahr per day from our 900Ahr battery bank. The distance between anchorages may involve 2-6 hours of motoring. We have a Balmar 100A alternator because that is all that will fit in the impossibly small space between the engine and the back of our fuel tank. Even if we could get 100A out of that alternator, we would need at least five hours underway to replace the Ahrs that we used over five days at anchor. So to make the arithmetic work out we use two solar panels that drops our usage to about 60Ahr/day when at anchor. We also have a huge generator on board because , (I think) some PO had an electric stove. In fact we have never used the generator and I wish it would disappear. It is a monster designed to run an electric "get home" engine. Why would I want to "get home" when I went to all this trouble to get to Mexico? So it all depends how you use the boat to determine the alternator needs. I wish I could squeeze in a 200A alternator but that is not in the cards. Richard P