My guess is you want a 30 watt panel, stop and charge it in the sun for a couple hours a day without using the phone. You’d really need to be careful with it also.
I monitor my phone and ipad and see the, take as much as 30 watts almost empty and the tapers back to 10 watts when full. I’ve also used a 100 watt panel charged with while using the device and that takes a couple hours to charge. Between the panel and the charger was a buck converter.
IME experience the 100 watt panel charger is perfect for keeping thosebdevices charged as I lounge around the RV, but I would not want to take that hiking. The Charger and buck converter are not heavy, but the panel weighs 25 LBS.