How Do I Split These Utilities?!

[Catie Fihn]

Hello!  We just bought a duplex, which has separate meters for electricity.  That's easy enough for the tenants to put their bills in their own names.  But the gas and water need to stay in our name as landlords (per the water and gas companies), and we will bill those amounts back to the tenants.

We have 1 single man who has 1 child who will live with him 1/2 time in the upper level, and we have 1 single woman who has 3 children who will live with her 1/2 time in the lower level.

I don't know if this makes it easier or more complicated, but our upper level may not have any gas services at all (husband is going to the property tomorrow to see if upper water heater is gas or electric, and whether washer/dryer in each unit are gas or electric). Upper level stove is electric and upper level heat is electric. So if anything at all, our upper level tenant would only have gas water heating, and gas washer/dryer.

Lower level has electric stove, gas heat, and we are also checking the water heater and washer/dryer for gas vs electric. How on earth would I split up the gas bill?!?

Water needs to stay under our name, so I can split that 50/50.  But in doing this, do I factor in the upper unit will have 2 tenants (1 adult, 1 child) and the lower unit will have 4 tenants (1 adult, 3 children)?  I don't want to drive myself nuts on this (I think I already have, lol) but I do want to be fair to my tenants and charge them their appropriate amounts.