Notice:
The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.
The rule of thumb is that you need a 5 watt solar panel to trickle charge one battery. The charge is just enough to counter the battery loss of charge during the summer and winter and not require a solar controller since very little chance of overcharging. If you have a flooded battery, always required to periodically check water level and add as appropriate. If you have light loads on your boat such as occasional use of a fishfinder, nav lights, etc then need at least a 10 watt solar panel for one battery. Any addl loads or loads used for many hours during the week, then best to go with a higher wattage solar panel. Any solar panel above 5 watts per battery will require a solar controller to ensure you do not cook the battery during days the loads are not used on the sailboat.
Here's basic story if I remember correctly: A single battery will discharge just sitting there at around .5 Amps/day in the summer and about .25 Amps/day in the winter. A 5 watt solar panel will charge at ideally at about .3A/hour but usually less than that and not for the entire day and then there are days that are overcast, rainy, etc. So figure it charges less than ideal at .2Amps/hr for 5 hours/day and ~ 4-5 days a week = 4-5 Amps/week charging. Meanwhile in the summer the battery is discharging at .5 AMPs/day x 7 days = 3.5 AMPs/week. So, you can see that a 5 watt charger just makes it during the summer months. During winter months, the sun is lower/angled more and there are less daytime hours of sun but battery when it is cold outside loses less charge, so these sort of offset each other and a 5 watt charger will be also okay for winter months as well.
I use the same Solar Grip as Ryan with a 10 watt panel, Keeps my single 105 amp battery fully charged. We will go out all day (10hrs) Motor into the slip, (around 20min on the motor 10 amp charger) Then I will angle the panel to the rising sun so by the time we get to the boat the next morning the battery is fully charged and ready to go. I also angle it to the sun as we sail. Easy to do with one hand. If I had two batteries I would have gotten a larger panel but I didn't want it to be in the way of starting the motor or scraping the admiral when she comes up the swim ladder. This set up works really well for us.
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.