Roommate uses a/c while I'm gone

I am so mad at my roommate.

A few months ago, we both agreed to save money by reducing our electricity bill.

Some of our new rules we both established is turning off lights early, and turning off all unused electricity using devices. We also agreed to stop using the air conditioning system for now until we can both have a bit more money. We barely had enough money to pay off the electricity bill and I was tired of worrying every month about whether we would have enough. Although we both agreed on this deal, it seems my roommate has been still using the a/c unit while I am away from the apartment. I thought it was odd that my electricity bill was only slightly reduced, especially since I thought the a/c system was now off, when it was previously used a ton. I guess my roommate didn’t like the heat, so instead of being a mature adult and coming to talk to me about the heating and cooling issues, she instead decided to go behind my back and use the cooling device while I was gone, and would turn it off right before I got home. That was something else I noticed about our apartment, it was still cool even though it was above 80 degrees outside. I brushed it off though since I saw the air conditioning component was still off, and now I know why. Me and my roommate had a huge fight, and now I know I can’t trust her and I’m not sure how I am going to resolve this issue.

HVAC maintenance