What features should you look for in a solar charger?
Power: Make sure you get a charger that is powerful enough to charge the device you want in an adequate amount of time. That means you should look for a charger with at least 7 watts to charge a smartphone, and closer to 15 watts or higher for charging multiple devices or more power intense ones like an iPad. Skimping on watts also may mean a slower charge rate, so if you can afford to go a bit higher, go for it!
Portability: If you don't mind added bulk, a folded solar panel can likely result in a larger surface area and more power than a single panel charger, though you'll likely have to trade some power output for portability. Another plus for larger chargers is that they usually charge the devices more quickly (though wattage is a better indicator by far).
Connections: Make sure the solar charger is capable of charging the device you own, and that it has enough ports to charge multiple devices if that is desired (and the power to be able to do so! Smaller electronic devices, like those you can usually charge with a USB, need roughly 5V output rating. Larger electronics may require 12V-24V.
Backup battery: If you don't know if you'll have exposure to the sun (i.e. at night or when it's super cloudy), make sure your solar charger is able store your charge by juicing up a rechargeable battery that in turn charges your mobile later when you'll need it, as opposed to one that directly charges your phone.