Note that the specifications on solar panels can be misinterpreted, especially the wattage. The wattage rating is typically the absolute maximum wattage that the panel can produce in the very brightest and most direct sunlight, like in Nick Gatel's back yard. By the time you get out to an overcast day, you will be struggling to get even 5% or 10% of that power. Plus, if you have one of these hanging off the back of a pack while headed north, it may be swinging east and west out of the most direct sunlight, so you might get only 1% or 2% of maximum.
They may be better than nothing, but you may not get but a fraction of what you seek. If the charge voltage or current drop below a certain point, your device may not charge at all. In that case, having a panel with attached battery pack starts to become more practical.
Some systems give you specifications on a solar panel with an attached battery pack, so you might see some sizable wattage numbers, but it might take forever to recharge that attached battery pack.