Many factors affect how quickly your portable solar charger will charge up your phone or other devices. Solar power chargers are most efficient when charged in direct sunlight, so if you’re moving around with one strapped to your backpack then the speed will be slower because you’ll likely be alternating between direct sunlight and shady, indirect light. High temperatures can also lower the performance of the charger, so charging earlier in the day when light is strong but temperatures are lower can result in faster speeds.   In strong, direct sunlight, your phone can be charged in as little as three hours, as long as there is no cloud cover or anything else that interferes with charging. An even faster way of charging your phone is to use the solar panel charger to charge up a battery pack and then use that power up your phone.