Passing Oil Heating Costs to My Tenants?

[William Conrad]

Hi all. I own a four unit multi. There are two oil tanks in the building, each heating two of the apartments. So, it's impossible for me to know who is using how much oil and the heating cost ends up on my side of the ledger. Any cost effective solutions out there for how I can fairly and legally get the tenants to pay for their own heat? Can you submeter an oil tank? Help! Thanks!