system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] Do Prothesis, Droids and Lightsabers have to be charged? If no, why?
gpt
A droid the size of R2-D2 up to a droideka (destroyer) with a standard battery can last 100 hours on a single charge. 

Droids larger than a droideka will generally have the same proportional battery life (around 100 hours), but any larger than that and they are likely to use vehicle engines instead, which last as long as they have fuel; they need to refuel the same way a vehicle would. 

Lightsabers last virtually indefinitely, due to their highly efficient use of energy. If one did run out, you would have to rebuild it and replace the power cell. 

Standard medical prosthetics will use the body's natural electric output to power their operations. If you wanted an arm that was super strong or had a hidden blaster, you would need the equivalent power source as you would find in a tool that had the same purpose (energy cell, or energy cell and gas chamber for blaster). 

Although it's never explicitly stated, I've always assumed that a total conversion cyborg (like Darth Vader or General Grievous) would have to either power himself like a droid or have an extremely expensive extended-life battery.