Meter for Office

There is a main house and an office with bath on the side. I rented the house and office to two separate tenants. They share the electric bill. I am fishing for suggestions on how and what are the options to set up some sort of meter for the office. Even if I know how much power is used the power company charges are so complex it is not clearly apparent how one would divide up the charges based solely on usage.

Has anybody had a similar situation and solved it? I would love to hear about your ideas and what works and what does not.