DX XIAOMI Genuine 10400mAh Power Bank
This looks like a good find; http://dx.com/p/311565?Utm_rid=65939668&Utm_source=affiliate
The benefit of this power bank is that it can do charging, whilst it is also being charged. For usage with a solar panel this means that the battery bank itself can be left on the solar panel and charge whenever solar power is available, yet you can continue to charge other devices in the mean-time.
Most battery banks can only charge or be charged, but can't do both at the same time.
With this you can make maximum use of the available solar power, without being limited to just charging devices at night whilst the battery pack isn't charging.
Great review of this battery pack here; https://www.youtube.com/watch?v=daKPjv_LaWU&feature=youtu.be&t=3m38s