If your routinely in a situation where fighting current all day long is an issue or if you fish a lot of open water that gets rough on a regular basis for long periods of time then you might consider the 100ah. For a normal day of 8-10 hours of off/on use of the trolling motor throughout the day you will be fine with 60ah. I know a guy with a 24V system and he can get 10 hours of normal fishing out of two 50ah batteries but he did say he has killed them one time when the wind was blowing 10-15 mph and he was on a good strong bite and fished all day.