I guess I should explain a little about what she is like because talking to her will probably do nothing. She is immature. If I told her to stop she would likely do it more often and more extreme.
For one, she's already not talking to me. I don't know for sure why but I can only still assume it's because about a month ago (seriously) it was about 3:30am and we had all just got back home from the bar. Everything was fine as it was just a couple of her friends staying the night (a guy and a girl). No problem. Then out of no where all these people start showing up at my house (a bunch of guys) and they just start walking in without knocking or anything. I don't know most of them and I had never seen most of them before. I'm not too pissed off yet at this point, i'm still up just playing guitar hero or something and cooking chicken wings so I walk over to my porch to see who all these people are and I say to the one guy "what's going on?" in just a 'what's up' kind of way. He looks at me and is just like "nuthin'" in a quick voice and just goes into my basement. I'm like, wtf? So I go downstairs and theres all these guys and like 2 girls in my basement (like 10 guys 2 girls). Not a good ratio, and I'm not having it. So I basically told them to GTFO. I wasn't as blunt as that but I didnt put it much nicer either.
Anyways, since then she has barely spoken to me and she's pissed off at me all the time. She's acting like she's 15 years old, hence why she wouldnt understand it if I simply told her to stop leaving the lights on (not to mention throwing her clothes and other junk on the floor in my bathroom and in the hallway, turning the kitchen into a disaster area, and leaving the house unlocked and windows open when no one is home).
Now do you see why it's more fun and probably more effective to do something more creative?
oh and electricity is included in our rent but my landlord has already raised it once for the cost of electricity.