If you use a personal roboport, and the port runs out of power, you can hold down right click to pick up the robots around you. When placed in your inventory, they gain full power for free, meaning you can technically power a power armor full of roboports with nothing if you're willing to hold down right click and spin your mouse a bit.
Just to be sure, I tested it just now by taking out all the fusions out of my power armor, and I was able to run my 9 roboports just fine even after all the robots ran out of power and flew really slowly. Just pick em up and they're ready to go
Tested in 1.12.20
