A simple and inexpensive solution is to use a 'mobile power bank', which is now widely available at low cost (from 5 €). The capacity of these systems is very high, almost always greater than 2000mAh, although some Chinese manufacturers print values little true, as is the case for the batteries.
It is used to give energy to our smartphone or tablet, in cases in which our cellular battery is discharged or we can’t use the electricity grid. These devices incorporate, in a small volume, one or more rechargeable polymer battery, a charger (5V to 4.2V) and a step-up switching power supply to generate the 5V output from 3.6 V battery.
Normally it has two USB connectors female: output type A to power the phone or tablet and a micro USB for charging the internal battery, it connects to a normal power supply 5-5,5V 1 or 2 A, which is now a standard for the last generation mobile phones o tablets.
Usually they are provided of one or more cables to adapt to phones and tablet, typically with the micro USB connector or with standard iPhone. With the cable terminated with USB Micro, we can supply different cards, including Arduino Due, Yun and Teensy.
To power Arduino Uno we need a common USB cable A / B USB, like that used for printers.
This type of power supply is particularly suitable for portable and very compact systems, and it has an operating time of several hours. I use these devices for some years.
The photo below shows an example of an application with Arduino Yun.
At the top you can see a 'power bank' with a capacity of 2600 mAh.
In the picture below you see the USB connectors on the power bank. A short USB A to micro USB cable is used to power Arduino Yun.