Simple, tell her that for 2 months, the both of you are going to do things YOUR way. Turn off lights when not in use, don't wash your laundry twice, etc.
Make a list with specific changes you want made. Be
specific and make sure she'll cooperate. Tell her that for
2 months you're going to do it your way, and if there's no change in your utility bills you'll never bug her about her habits again. BUT, if there
is a change in the bills (and there more than likely will be
) she has to permanently change her habits. She'll be able to see a tangible result of the changes, and it'll be a "challenge"
Also, if you approach it as a "two month" trial thing, it'll seem like less of a "demand" because it's only two months..... When she sees the change in the bills, she'll realize their really is a difference!