Shared Utilities - best way to divide costs

[Emy Bernardo]

Hi all - we have an ADU in our single family home and have been including the cost of utilities in the ADU rent. We recently started charging our tenants a $100 flat rate to help cover some of our soaring utility costs here in San Diego. We are moving out of state in a few months and plan to rent out the house and keep our same tenants in the ADU. What is the best way to split the costs of the bills between tenants? The main house is about 1100 SF and the ADU is 500 SF. The main house has 3 bed/1.5 baths, washer/dryer. The ADU has a stackable washer dryer, 1 bath. Both have only one gas wall heating unit. 2 tenants in the ADU and not sure how many we will have in the main house. I am hesitant to charge a flat rate because it doesn't incentivize people to be mindful of energy use. Maybe 1/3 to ADU and 2/3 to main house? Or should we go by # of tenants per unit? Thanks for any suggestions!