I got a Anker Astro E4 13000mAh External Battery on Amazon $43.00 US, really works great I use it a lot when I'm using my phone for Internet sharing or hotspot. Really charges fast even while using the phone for a hotspot. The only drawback is that it doesn't come with a wall charger, but your 920 charger will work just fine, you'll probably only have to charge it up once a week if you use it a lot.
Specs
Huge capacity (13000mAh) delivers 6+ charges to most smartphones or an 80% charge to an iPad 3 / 4.
Super-fast charge output (5V / 3A total) through dual USB ports (5V / 2A and 5V / 1A) allow for simultaneous charging of your iPad, iPhone, Android, and more at high speed.
Smooth, ebony varnish allows you to power up in style, while 4 status LEDs keep you informed of remaining capacity. Built-in flashlight for dark or low-light environments.
Premium lithium-ion battery ensures quality, supplying 500+ recharge cycles over a lifetime. With a charge input of 5V / 1.5A, use a 1.5A adapter (not included) for fastest charging (6.5 hours).
Package contents: Anker? Astro E4 External Battery, connective adapters for smartphones, USB cable, instruction manual.