Fully charged batteries can last over the winter without recharging. You can estimate a drop in charge at about 2% per month, give or take a percent or two. By the end of a long winter you still should have three quarters of the charge left. But if you want to have full charged batteries when you take the covers off, estimate how much replacement charge you need and get a solar maintenance charger to supply that amount. For every 100 amps battery capacity you will have to supply about 5 amp hours per month. Assuming a reasonable amount of sunlight you can count on 4 to 6 hours a day of solar cell power. In a typical situation that should add up to about 120 hours per month of charge. Roughly speaking, a 5 watt maintenance charger placed so that it could be illuminated by the noonday sun should suffice to keep 500 amps of batteries topped up. But most of the maintenance chargers sold in auto parts stores are smaller than that, perhaps supplying only 150 ma of current at 14 volts. You might need a couple of those. The big problem is where to put the solar panel. You want it to get full sunlight but to remain uncovered in a moderate snowfall. Fastening it to a mast or to the top of the winter cover will usually do. The last couple of years I had my boat shrink wrapped with uncolored translucent polyethylene. Enough light passed through so that I could get away with simply placing a solar panel on the cabin top. But if you keep your boat in a shed, just plug in a regular auto charger every couple of weeks. Larry Z