So you want backup power for your devices when you're out and about, but you don't want to go too small because you might need more, or too big (with it's additional weight and size) because you might need less? The exovault plus from EXOGEAR is a stackable battery pack so that you can customise your reserve power based on your needs.  Start with the main battery pack which will in itself supply 5200mA and is 3.7" x 3.7" x 0.6" (95 x 95 x 15mm) and weighs 156g (5.5 oz) and add additional battery packs as required. The system will put out 5V ± 0.5V / 2A max so will support your more power-hungry tablets as well.
The Gadgeteer , Exogear Exovolt Plus â" Stackable Power, Exogear Exovolt Plus â" Stackable Power