Power banks

[wanye]

as a rule of thumb (and taking some loss/waste of power during the charging) an average phone and tablet these days will have a 4000-5000mAh capacity battery (most phones are around 3000, but with a 10-20% loss during transfer as well as tablets generally being higher capacity) then you can use this as a ballpark to work out how many full charges you can get from a battery pack.

so a 10000mAh unit should be enough to charge a device 2-3 times from empty before it needs recharging itself, a 20000mAh unit is double that capacity, so should be enough to keep a phone going for a week before needing to be recharged (good for camping/festivals/etc)