I have a boat similar in size to you (26.7 ft). I use a simple $150 large truck battery
(990 cca) with a small $40 1.1amp smart charger
(chipped, five phase) thats always connected to the battery and plugged in. It has a memory so when i plug
the boat back in it automatically starts testing the battery for its state and picks up at the right phase and voltage.
Because its always plugged in when the boat is at rest it gives the battery an amazing charge (these smart chargers can revive old batteries that no longer hold a charge) and im good with a similar setup to you for three days as long as im not blasting the stereo amd running all the lights all night.
My load is a vhf radio
, sounder, small gps
, stereo, some 1A interior
lights and i have a small inverter
for phones/ tablets that can output up to 100w burst or 45w steady.
Ive contemplated a bigger battery as i ran out of juice last year at a dock
without shore power
, but since ive ditched my old 15A dumb charger
and gone with two 1.1A smart chargers (one is on my truck as a battery maintainer as i dont drive it) i dont think ill need it.
Depends how long you figure your going to need to go without access to power. If you plan on long voyages or a lot of time in areas where you cant overnight at a marine
without shore power
you will want more capacity than my rig. If your gunkholing around doing coastal cruising where there are usually decent marinas
nearby you will be fine.
Also, you dont mention the capacity of your alternator
which is key. If you have a 6-12 amp alternator
you can always run the motor
for a while if the juice is getting low, plus there are always going to be times when you are either too lazy to hang the rags or there is no wind
and you want to get someplace. A days worth of motoring with a 12A alternator would be enough to take a nearly dead battery to half way if your using a truck battery, enough to get you closer to shore power for a night.
In conclusion id say the litmus test for you would be how long you expect to be away from a dock
with shore power. If most of your trips are coastal gunkholing and weekenders just get a smart charger for when the boat is at rest and between that and your alternator you should be fine if the battery is in decent shape.