While Optima blue top batteries are good quality, their price doesn't necessarily mean they have more capacity than other batteries available for less money. Optima batteries are sealed, so no electrolyte level checks are required. (Not a big deal to me.) You're paying a lot for that feature, but it doesn't make the battery last longer.
To comprehend cranking ability of a battery, look at CCA/MMA rating where higher is better.
To understand low amp drain support ability, look at Reserve Capacity, where higher is better.
Those two specs will help to compare batteries than price or sheer size. Though, all other things equal, larger batteries do tend to have more capacity. My first recommendation is to go with as big of a battery as you can fit, regardless of the size you have now. Many boats are delivered with a small Group 24 battery. Many of us up-size that to as much as a Group 29 or 31. (31 is generally the same footprint as a 29 but a bit taller).
Edited by Flippin-Out (01/29/16 03:10 AM)