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.
It will work for long term float charge to keep a charged battery topped off. It will not bring a deeply discharged system back to life.
I use 2 United Solar Flex 11s to recharge my system while cruising. Here's a mail order site that I have done business with that has deep discounts on solar stuff (cosmetic seconds).
If you can spring for slightly more money, get the 11 watt panel. It is more useful, having enough output to recharge a mildly depleted battery after a few days, whereas the 5 watt panels can only keep a full battery topped up; as Jim said, they can't pump up a battery that is in a less-than-full state of charge.
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.