As far as I know, there isn't a way to power the camera from a usb and still use it. I believe it has to be battery powered. I'm not sure how long the battery will last for something like that....I've never tried. But I don't think there's anything you can do besides set it up fully charged and hope, or try to change the batteries when it dies and try to get it in the same position.
As far as I can see the "tether tool" being advertised is nothing more than a normal high quality USB cable. If you use it on the Sony a5000, then you must set the camera to "PC Remote". At that point I do not know what the capabilities are, but it is clear that it is meant to be use while attached to a working, active computer and controlled by the computer. So if you simply want to use the camera by itself, but powered externally, I do not think that it will not work.
On the other hand, I have seen in a manual that there was an accessory called "AC-PW20" available that works in the Battery Compartment. I have never seen this device, so beyond that, I have no more information.