In this tutorial I would like to show you the three ways how to connect power to the Arduino Uno. I will emphasize when should you use which type of powering depending on the state of your electronic project.

Supplies

Arduino Uno (or clone)
USB Type-B Cable
Power Jack Cable (also known as barrel jack)
AA or AAA Battery pack (4 pack)
Male Header Pin (x2)

Step 1: USB Type-B Cable

This cable usually comes with the Arduino Uno board. You can use it to power up arduino uno while uploading and testing the code. It's Essential because you will use it to upload the code no matter the state of your project.

When designing a housing for the electronics it's recommended to leave the opening for the USB, you will need it to reupload the code.

Step 2: Power Plug

Recomended power source is from 7 - 12 V. The current goes through the built in 5 V voltage regulator. You can also use a 4x AA/AAA 1.5 V Battery Pack, which totals of around 6 V, but as the batteries lose capacity the voltage drops, which makes powering with the 6 V last only for a short time.

Barrel Jack is the best way to power up the arduino after the code is uploaded and the electronic project is ready for use. It's not often that you can find the barrel jack on the batteries so you will need to get a barrel jack adapter and connect the wires to it with screw terminals.

Step 3: VIN + GND

The same power supply like with the barrel jack can be provided directly to the pins of the arduino uno. These pins are VIN and GND. VIN is used to supply the positive voltage from 7 - 12 V and GND is ground (or negative voltage).

Disadvantage of this connection in comparison with the barrel jack is the loose connections. The barrel jack is always more firmly connected then the pins on the arduino.

To connect the wires with the male headers, you should solder them together and isolate or simply twist the wires around the male headers and isolate.

You can also decide to solder on the VIN and GND and resolve this loose connection issue.

It is also possible to power up the arduino using 5V and GND but this is not recommended because you will need to regulate the voltage to 5V before connecting the wires with the arduino, which basically means having 2 voltage regulators (1st on the arduino and 2nd before the arduino).

I would recommend using VIN and GND to power up arduino and 5 V or 3.3 V pins to supply voltage to external electronic devices such as potentiometers or sensors, and that only if you are testing a single electronic component (sensor).