This is totally based on where and how you fish and what boat you have. I have 3 100ah batteries for my 36v ultrex. I go home under 20% almost every time while spotlocked in spring river current. I'll have to run my kicker motor going forward at the same time if I'll be out all day to not run out of juice. For me personally, I wouldn't even think of 50s and could actually have used 125s.