UKH

We're planning a bit of a trek in the sunny French Alps, and wondered if anyone would care to recommend a solar charger for our phones? Efficient, lightweight, waterproof and cheap preferably all 4!

As per every one of these threads: your phone won’t like the variable charge from a solar panel. Neither will a cheap regulation power bank. If you buy something cheap it won’t work. If you buy a small solar panel, it won’t work. Even on a sunny day you’ll get, at most, 2/3 of the max panel value. 

My recommendation, as always on these threads, is to get a Goal Zero panel and power bank. Use the panel to charge the power bank, and the power bank to charge your phone. I’m not super up to speed with the current product line, but I’d go for a panel which is 14W or higher. Battery wise: buy one that matches your requirements. Yes, they are expensive, but they have charge circuits designed for the inherently variable solar input. I’ve used these systems all over the place to great effect, and wouldn’t recommend anything else.

I use an Anker 21W 4 panel with various Anker power banks when car camping. Works well when sunny. We generally charge a bank through the day and top up our devices in the evening. A day with ok sun will give us enough power to top the banks up for our two phones plus solar lights.

Not sure I'd bother if walking, a decent 20k mAh bank will do me for a while with sensible charging.

How long will you be without power? a power bank plus mini wall charger might suffice?

 I used a XStorm AM121 Evoke on my Cape Wrath Trail walk in 2019. It's a solar panel with a built in batter pack. Even in Scotland which isn't famous for its sunny days my phone was never lower than 80% battery. 

these are pretty good (work for me that is):

https://www.mobilesolarchargers.co.uk/shop/details/travel-solar-charger

https://www.mobilesolarchargers.co.uk/portable-solar

I have a choetech 12W made in China one. I use it charging my phone, inreach and camera batteries. I like doing long trips to the north so having the sun go down isn't a problem. I generally just set it up in the evening and leave it to do it's thing. I don't think hanging it on the back of your pack works very well. I have enough power in the batteries, so that a few rainy days shouldn't be a problem. My trips have generally been two or three weeks away and I've not had any problems keeping things changed.