Power is the god of… wait for it… power.
Well, that was easy. Specifically, he represents Zeus's power over the other gods. As you might imagine, he is one tough, unforgiving dude who loves to throw his weight around. He's only onstage for one scene in the play, but that's enough to make a memorable impression.
In the play's opener, he's one of the three figures (along with Hephaestus and Violence) leading Prometheus as a prisoner to the Scythian rock. And does he feel sorry for the poor guy? Not at all: "it does no good to lament over this fellow" (45), he says, because it is not "possible to disobey the word of the Father" (40).
So we don't learn too much about Power from this little exchange, but we do learn more about Zeus's power. (So to speak.) Power suggests that Hephaestus should be "more in terror" of Zeus's wrath than sorry for Prometheus (40-1), so we know right from the beginning that (1) Zeus is powerful, and (2) Prometheus is a really brave guy for standing up to him.