BFL13,
Of course you don't have watts if you don't have current.
Don't high quality controllers have a low enough in-out voltage differential spec such that the solar panels can sink to very low output voltage levels (on a very cloudy day, for instance) such that their output is sill high enough to supply - say, 1/2 amp - into the batteries? Even 1/2 amp provides 4-5 amp hours back into the batteries per overcast day.
Remember that some folks solar power drycamping goal is to putoff having to run their generator as often, or as much ... not to bring their batteries up to full charge for ultimately long battery life. Full charge needs to be attained other ways during extensive periods of no sun.