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).