There are many options for powering an Arduino board.  The designers did a great job of designing an auto-select circuit that makes it simple to power the boards.  However, there are trade offs in how much power is available and how much power gets wasted.  Australian Robotoics has a great breakdown of what an Arduino looks like under a thermal camera, when powered from USB and various external sources.  I don’t agree with everything that is said in this article, but the pictures are very revealing (and cool to look at).

Fuelling Your Arduino – Why you should use an External Power Breakout | Australian Robotics.