Gee, yet another power brick which may (or NOT!) actually provide 20AH of power.

The real limitation on most of these are the same for most small electronics, the battery chemistry used is only good for <900 (most less then 300-400) charge/discharge cycles, cold temperatures may have a large negative effect on the AH stored and lastly getting energy in/out of the devices is not 'free' and involves losses so don't expect to get an additional 20AH of capacity for the device you want to charge.

Don't get me wrong, having a device like this CAN be very helpful. Given if it was actually charged (or still has a charge) when you need it.