I just put a single 230 watt panel on, mounted it on the dinghy davits, with an Morningstar MPPT controller. The most we have seen it put out is 14 amps, the controller tops out at 15 amps. We just did two weeks holidays and never had to run the engine just to charge batteries, we normally run the engine about a half hour to get off anchor and out of the anchorage and about the same to get on anchor in the evening. The 90 amp alternator combined with the solar kept things topped off nicely. The panel even makes power (some) on cloudy days.

A lot I think will depend on where you are, hours of sunlight, angle to the sun, etc. We are also putting a wind gen on because we get a lot of wind here.

If I wasn't doing the wind gen I would probably be adding a 2nd 230watt panel and controller. Our controller comes alive as soon as the sun is up and it starts making a couple of amps and gradually builds, it peaks around 10:00 am and holds that until about 4 pm then it starts going back down. So your absolute best case scenario in our latitude is it could possibly maybe theoretically make 90 amps, but it won't. Don't take the maximum figures as a guide, you won't get them.

Our fridge runs about 1/3 of the time in normal weather and draws 6 amps when its on, so we figure about 50 amps a day just for the fridge. We are all LEDs now so lighting draws very little but we everything in (pumps, entertainment, yada yada yada) we figure close to 100 amps per day on a 464 amp/hour bank.

We have four 120W panels and that has allowed us to run all our electrical needs for a 100 day cruise, where we did have abundant sunshine, I must say.

If all you want is to have to run your engine less to recharge batteries, then anything over 200W will certainly help (depending upon the sun in your area). To be as independent from engine or shore-power charging as possible, I think you will need 400W or more.

You need to be a lot clearer.

What is your design goal?

Complete independence or a short, small supply of amps.

Are you a marina-hopper or an anchor out?

Do you have a larger externally regulated alternator or an OEM?

What do you want to do with your boat?

Answer those questions first and then we can help.