A:Answer In regards to efficiency, all our PowerCores burn 30-40% of their power when they are charging other devices. So the PowerCore III 20000 has 20,000mah of power, but the power left for charing your device is in fact 12,000 - 14,000mah.
Since your device has a battery capacity of 3100 mah, it should be able to charge 3-4 times with that PowerCore.