Using Ratio Utility Billing System for gas?

[Shannon Brill]

Hello BP!  I currently own a duplex and have a question regarding using RUBS for gas usage.  This formula is already in place for water, but here's the issue with gas -- both units use gas heat, however only one unit has a gas range/stove (the other unit has electric).  How to you properly charge the tenants seasonal usage (high in winter/low in summer) and when there's not equal gas appliances between the units?  I'm unable to come up with a solid solution here (adding a separate gas meter is too expensive.) I'm hoping that I'm not the only one that's encountered this.  TIA!