Tenants refusing to pay utilities in full

[Rita Rosa G.]

Hello BP members,

Need advice on what to do with tenants who are refusing to pay utilities in full. I own a duplex, the upstairs unit is being rented by a married couple and the downstairs tenants moved out by July 1st. Unfortunately, the duplex utilities are combined as one (the duplex shares the ac unit, water, and electricity). (1.) The upstairs tenant is claiming that their electricity bill is too high because they are cooling two spaces. The AC unit has a smart thermostat that allows them to adjust whatever temperature they would like (the smart thermostat is located in their unit). They have the ac unit cooling the house between 71-73 degrees and working between 14-16 hours per day (readings found on the smart thermostat). Since the downstairs tenants moved out on June 30th, I made sure nothing was connected to power source and the vents were closed to provide full air circulation in the upstairs unit.

(2.) They also said they do not have full control of the thermostat and were not provided with a user's manual or instructional video on how to use the thermostat. Although, the physical thermostat is in their unit and they have been using it since January 2019. The thermostat is smart, and will learn people's preference and they are saying the thermostat goes from 71 to 76 degrees without their consent (i'm assuming from their previous setting adjustments and the thermostat adjusted accordingly).

They said they will turn off ac unit and just use their window unit if they are being charged so much to cool two apartments.
(Note:They installed a window unit in the month of June since they overworked the AC unit and had to have someone come repair it.)

After inspecting their space, I noticed that they cook/bake a lot, do laundry alot, leave lights on when not in the house and have a room with lights for their plants running 24/7. (house is completely electric).

NOTE: She mentioned that her job gives her unlimited hours with a lawyer over tenant-landlord disputes but they don't want to have that type of relationship.

The overall cost of their utility bill for July is $617.38, and they said they will be willing to pay $475. The electricity bill was $370.60.

I don't have any tenants lined up for the downstairs unit and I'm afraid they will proceed to talk to their lawyer for the utilities for the next few months while their lease is active (ending December 2019). Our lease clearly states that the utilities would be paid by the number of people living in the entire space. This is the first month they have gone without having someone to split the bills with. Although it was mentioned to them that it would be their sole responsibility to pay the bills after downstairs tenants left.

How should I proceed? Should I negotiate to have them pay more? Have them turn off central air? 

Thank you!