I would think with your load that 6 batteries and maybe as few as 120-200 watts of solar would handle your load, BUT I don't know where you are located and how you use our boat. When it comes to solar, more is merrier but it's also more expensive, and you have to draw the line somewhere.
A panel rated at 120 watts will in theory give you 10 amps of power but you'll rarely get this much. A lot depends on latitude, cloud cover, temperature, panel inclination relative to the sun, battery state-of-charge, etc. And just because the sun is up for 12 hrs a day doesn't mean you'll get 12 hrs of EFFECTIVE sun to the collectors. For a clue on how to estimate all this go to http://rredc.nrel.gov/solar/codes_algs/PVWATTS/.
Anyway, if you don't have enough, the batteries may go dead; too much, it'll simply shut down when the batteries are charged. Your call.
Bear in mind if you only use your boat for 3 days a week you don't need enough solar to carry the full load; you just need enough to RECHARGE your batteries over 7 days what you took out in 3. How you recharge the batteries doesn't really matter; you can use solar, a windmill, a gas generator, your engine alternators, or a combination of all of them.
Using the engine alternators only makes sense if you're moving, then it's "free". Running a big engine at fast idle for hours to charge your batteries will get expensive. That's why we have dedicated gensets. If you cruise a lot the alternators can provide a lot of power; if you like to go out to some cove and anchor up for the weekend they don't.
Running ANY gas engine at the dock or at anchor presents a carbon monoxide problem. You have to be careful where you locate your generator (actually it's exhaust) and a carbon monoxide detector is a MUST. Your life depends on it. Then there's the noise factor. The constant buzz of a genset (quiet as some are) can still get irritating, if not to you then other boats in your vicinity.
All the above is about generation. Now about consumption. On your boat with the gas fridge your lights are probably the biggest energy hog. Replace everything you can with fluorescent lights or better yet LEDs. Fluorescents use 1/4 the power of regular bulbs. LED's are better yet but still pretty expensive. Lights you rarely use aren't on enough to matter, but lights in the galley and living area can eat up a lot of power. Look into replacing them with something more efficient.
Pay attention to your instruments. So often I hear about someone going out for the weekend and Sunday morning they get up and everything is dead as an anchor. That should NEVER happen, but only proper power management will prevent it. That's why the battery monitor is so important. A simple voltmeter is NOT enough. IMO the battery monitor should be the FIRST thing you install, it'll help you determine how much you are using, and how/where you can conserve. Then you can size your battery bank. Then you can figure out how much $$$$ solar or other charging capacity you need.
Let's say after all is said and done you figure you use 100 amp-hrs per day. For a three day weekend that's 300 amp hours. You don't want to discharge your batteries more than 50% so you need 600 amp hours of batteries @ 12 volts. That'll work out to 6 golf cart batteries, about 660 amp hours.
You know you won't use the boat for the rest of the week, so you need to recharge at least 43 amp hours per day. Not every day is sunny so let's say 80 amp hrs per day x 12 volts or 960 watt hrs. You go to the site above and discover that for a 1kw array you can get 4 kw-hrs of energy per day. working backwards, 960 watt hrs/4 hrs =240 watts of panels required.
However if you only use 50 amp hours per day you only need 4 batteries and 120 watts of solar. And save some $$$.
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw
07:39:02 AM by