How to fairly distribute utilities to tenants in triplex

[Caroline Packard]

We own an old home that was created into a triplex many years ago.  It’s not separately metered for electric (needs major electrical upgrading to do so and will do in the future) and there’s only 1 furnace.

Had intended on using square footage calculation for RUBS to pass off utilities to tenants but one unit has 2 occupants and a window A/C- so clearly the usage is more than an single occupancy unit without an A/C unit.  Any suggestions on how to pass of utilities fairly to these tenants in this situation?

Thanks in advance