How to split heat and hot water amongst tenants?

[Darrick Lowe]

I'm used to multifamilies where each unit has its own water heater and furnace.  But what If there is a home with only 1 water heater and 1 boiler in a multifamily home, how exactly would one split the bill almost tenants? Metering each unit costs too much money. Could I have bills sent to me, and then every month send each tenant a text message telling them how much to add to their monthly rent? Or perhaps have them pay an extra $50 a month, and then credit them if they overpaid at the end of the year? Or is there a more efficient method to do this? I'd love to hear if you have any pro tips.