I have a 12 watt one and gave up on it. It was too much trouble keeping it aimed at the sun to get near the 1 amp it is supposed to provide. It was useful for charging flashlight batteries with some trouble hooking them up.
Our 100 watt panel flat on the roof of our class C motorhome, with Renogy charge controller, provides all the electric energy we need in summer but I can’t use it much in winter due to snow on the panel. I simply disconnect the house batteries and check voltage once a month. If down to 12.4 volts I put a charger on them (usually once every 2 or 3 months).