Or if you want reliability, there’s an Anker folding panel only that you can plug into your own battery. Just done a calculation on the efficiency.
It says it’s 70x34cm unfolded, less a bit for borders. Assuming 20% efficiency and you’re at 50N ish, that’d give a peak of about 40W on a sunny day. If you integrate for the angle of the sun from 0-180 degrees over say 12hrs (it’s more than that in summer obvs) I make that about 80Wh of power that you could generate, in Scotland, in the sunshine. Li-ion batteries charge at near 100% (apparently) so I’m going to say 80Wh of available power in to the battery, which is about 6,600 mAh. Plenty for a phone, on a sunny day (which is about 3,000 mAh ish, and that’s a conservative estimate since you have more than 12h of light – much more, in Scotland. As you move south later in the year there’d be less daylight but more chance of sun and higher intensity. And even if it’s not enough on dull days, you can still get some, and it’d make your power bank last longer. You might only need to charge it every 10 days instead of 3.
Yes, he’d be way better off with a dynamo but he’s already on his way, so that might be harder to organise 🙂
EDIT someone check my maths I have forgotten how to integrate