How do I tell my roommate that I can't afford to pay for air conditioning in April?

It is beautiful outside and I have struggled with many things with my roommate, cleaning, sharing, and now electric bill. I have told her about leaving lights on, and she needs to clean not wipe off little areas and call it clean when something that is usually white is now pink with mildew. I came home on two very nice and cool nights to find the air conditioner cranked up to the max, it can't be more than 65 degrees out. I can't afford to pay it and she couldn't care less. What do I do? Do I make her pay for the ac or do I just keep complaining. I am more than happy with a fan blowing in the summer. Help!