Splitting out utilities for duplex & garage

[Tyler Davis]

I recently just purchased a duplex that has a garage in the back. 

Both units are rented and the garage is rented to the neighbor. The first floors electric is tied into the garage, and I'm looking to put the utilities on the tenant so they have to pay them.

My question is what is the best way to handle this scenario, as the neighbor who is renting the garage is using electric inside the garage. Should I knock a percentage off the rent for the first floor tenant and then up the rent for the neighbor to account for electric? 

Obviously the easiest approach is to keep the electric in my name but would like to avoid that if all possible.