It also is a function of how often you open the box and for how long. In my view, the affect of the efficiency of the well designed, constructed, and installed refrigeration equipment system is less effect than the construciton, location, and use of the box.

If the box is well insulated ALL around, of moderate size (say 18"x24"x30" deep=7.5 CF) , has one well insulated and sealed "lid" on the top, he doesn't go into to very often, etc....? Maybe between 60 and 110 amp hours a day would be my wild ass guess.

If he has a sepearte freezer (former ice box) and refigerator section, want's to keep ice cream (close to or below 0° F), etc. -- then I'd guess he'd be closer to the uper end of my wild ass guess.

The how cold you keep the box vs. the relative outside temp has a major impact.

Another factor: Older boats with ice boxes often have very poor insulation compared with boxes that were designed to be constantly kept "COLD". For example, open cell foam (not blue board) or spray-in foam usually gets saturated with condensation and mold over time of being constantly cold. Having the exterior of box proximate to the sun baked hull or next to an engine enclosure can change what real conditions that the box has to overcome in terms of the "delta T".

In the old days, most boxes were used for weekends or maybe a couple of months, and then decommisioned until the next time. Unless you use a closed cell (or vacumn panel) insuallation, you're going to have the "vapor point" within the insuallation. Unless it dries-out before mold starts.... Aside from that, soggy insulation transfers heat like a wet towel.

I've seen "quality" boats that have had their galley's redone after a few years with open cell foam or expanding foam insullation. It has the look of the "gooh of life". ;^)))