That tiny panel isn’t going to do anything useful.
Solar power is roughly 1kw per square meter, and what can collected by a real solar panel is about 15%, so 150W per square meter. From there we deduce that if we want to collect 5 watts, we need 33cm^2 = ~6x6cm. This is assuming an optimally positioned panel, in the blistering, cloudless sun, producing only enough power to function as a low end USB charger that would struggle to charge at all something like a Samsung Note, let alone a tablet, and ignoring any conversion and charging inefficiency.
From that we see that for any useful performance we need a panel several times larger, making the minimum size that would be useful somewhere around the size of the entire back of the backpack, rather than a tiny bit of it.
