One gas furnace for two tenants

[Breanna Barajas]

I have a duplex where there is one gas furnace. In the lease it states that gas and heat is included in rent. The tenants are responsible for the electric bill.   I have never had this problem in the past. The current tenant states they will not pay the electricity that is being used to run the gas furnace.  Does anyone know how to figure out how much electricity a gas furnace uses, or any ideas how to handle the situation.  

Thanks in advance