That's fairly simple. Example: my Note 3 battery is rated 7.4 Wh, so a 7W panel I have (Goal Zero Nomad 7, 9"x6.5" folded) should charge it in a bit over an hour under perfect conditions. In reality, I cannot draw 7W out of its USB port, as it's rated for 5 Watt only. Plus, unavoidable loses due to various voltage conversions in the phone, not ideal atmospheric conditions, etc, so I would double or even triple that time. But 3 Hours is not that bad at all, especially if you have a battery buffer, such as the Guide 10 (11Wh) to accumulate the solar power all day (in fact, 3-4 hours, so a second batch of 4 AAs can be charged). By the way, I plan to make an adapter, fitting the Nomad 7's pocket to charge my phone's spare battery on the go.