With the more economical flooded batteries, bigger is better if you have the room and can handle the weight. Group 24s are a bit small for my liking, although they may meet the minimum requirements for your engine. The Deka Group 27 is pretty standard OEM installation for Yamahas. There are a lot of threads on this forum that address the topic and everyone has an opinion. Check your engine manuals for the minimum specifications. I have Dekas, group 31s and 27s. I have two 27s in one bank and two 31 deep cycle in the other bank. I have had good luck with Interstate Group 29s also. I like the deep cycles for running the house load on overnights when we don't run the generator.