On a side note, as a fun side project - Ive started creating a backpack which is intended as a day pack with a number of uses, of which one is hiking. It is mid-development, i.e I have a prototype in hand - yet will perfect this to reach a final pre-launch product.
It is equipped with a detachable & portable solar panel & charger capable of 12 full phone charges with a few extra features.
Good luck with your pack project.
I don't understand why you would need 12 full phone charges if you are developing a day pack. Or indeed a solar panel at all.
Important factors in selecting a day pack for me are:
Comfort - well padded and well fitting shoulder straps and waist belt. I would never buy without trying.
Capacity - 30 litres say for an all season day pack.
Durability - it's got to be able to take the knocks.
Drawstring type closure of the main compartment, covered by the usual flap with straps. - useful for stuffing a jacket under.
Small pocket on top and a map pocket on the back.
Ice axe loop and attachment point for crampon straps.
Mesh pockets on the side for water bottles.
Back ventilation - although I find that I'll sweat whatever!
Weight (low down on the list for me - might be more important on a backpacking pack).
You'll notice that solar panels are not on the list. Neither are hydration bladders etc.