Tessel can be unplugged from your computer and powered from alternative power sources, like a USB battery.
While Tessel is connected to the computer, you can push code into Tessel's Flash memory with the following command:
tessel push script.js(Try it with blinky.js from here if you just want to test it out.)
Once the push command completes, you can disconnect Tessel from your computer and connect it to a different power source.
(You can read more about pushing vs running code on Tessel here.)
For the simplest power solution, all you need is a USB charger of some kind:
USB battery | USB wall adapter
(There are various other ways to power your Tessel as well. You can read about them here.)
Once you've pushed your code to Tessel's Flash, plug in your Tessel to power.
Tessel should boot up and immediately start running your code.
Make something cool? Show it off on the Projects Page!