Just started using a Sinoband S-400 Waterproof Power Bank Bluetooth Wireless Speaker. I didn't really need an external speaker for my iPhone but the Sinoband S-400 has an internal 3.7v 7000 mAh power bank (Li-Ion batteries) which can be used for charging other devices, such as that iPhone.

The spec sheet lists the input power to recharge the batteries as 100-240 volts, 50-60 Hz AC, but the input is a mini-USB port (USB charging cable included), so whatever AC to USB adaptor you have should work. Output is a standard USB. So for a little more than the cost of two 3400 mAh 18650 batteries, you get that level of mAh power and a BT speaker system too. The sound is okay and the casing is supposedly shockproof and waterproof (haven't tested it, I'll take their word for it).

Next time I break out the Goal Zero Guide 10 with its Nomad 7 solar charging panel, I'll see how long it takes to charge this power bank. This is just another item that can run off-grid and provide power to other systems.