Yeah they are very power hungry, usually 24 killowatts is what they'll suck down at max on AC power at least if it's available (it isn't always), if they can't get up to that they'll still charge at lower power but it just takes much longer.
Usually to run their visor, their cybernetic systems (some of which are critical support systems that keep them alive), if they have synthetic/mechanical limbs they power those too and are the primary source for movement. Some protos have x86 CPUs and high end GPUs in their visor and for them the batteries run those as well.