Multi-family utility bill back models for centralized systems

[Trent Currie]

I am looking for different models to bill back utilities in multi-family complexes with centralized systems.

For example a 50 unit single building complex with a centralized boiler system. What are some models to bill back the utilities to units with different room mixtures and square footages. Some units may be a 3/2 with 1100 sf while others could be 2/2 with 800 sf.

We were planning on doing it by sf for each of the units using RUBS, but I was curious if there are any other models commonly used.