I'd never heard of Power Girl before seeing these photos. It looks to be a sexier version of Super Girl. Watch out though she's related to Superman so he might beat yo ass if you treat Super Girl wrong. Here's some fan costumes from various Comic Con's and other costume parties.
This first one reminds me of Joan Holloway from Mad Men.
This last one is the "Official Fan Power Girl" as seen in these fan made films
And then we um... Power Mom? Points for trying though.