A new forum member @voltage inquired if the Arduino Nano could be powered without USB cable, since it doesn't have a 5V input pin. I responded yes, I think so, since it has that VIN pin. (see link to other comment for details). You just connect it to 5 volts from wherever. 

Here is proof, of a sort. The circuit is powered, the lights blink.

I normally plug it into a USB adapter, since I have a few of those lying around. I just soldered in the 2 screw terminal because I need the practice soldering with perfboard. But now I want to actually test if this would work. I'm willing to read the manual, watch the video, study the pinout diagram. But I feel best when I can actually try it myself and see if it really works.

Warnings:

* it worked for me, this time. I think it would work in all similar circumstances, long term. YMMV.

* The VIN = 5V because I used one of these handy little regulator thingies, which I saw in Bill's other video on power supplies. Be sure to set the little jumpers correctly, since it can supply 3.3 or 5V, or both at the same time.

* This uses the regulator, which itself connects to mains via, I think, 9V wall wart. (This is how I connect an Arduino Pro Mini, which does not have a USB connector). If I weren't using mains power, but a battery, I would still need a regulator of some kind, since the VIN needs to be 5V.

Cheers, John