Being I run an mppt set up I think I can say that you need a higher voltage then the batteries as stated by the others, the peak for the one I use is 17V mp, 22V Voc, You need at least 30 to 60 watts befor you gain any advantage.
I run my system at 49V Voc, 32V Mp, and my aray is rated at 120W but actual wattage is average 75 to 90w (Wich is about expected for my area)