How to split the water bill with only 1 meter?

[Gene Baldwin]

Hello all!

Thank you in advance for your time, advice, and experiences!

We recently purchased a property in Wisconsin with 6 units— 2 commercial and 4 residential. One of the commercial tenants is a hair salon, and one of the residential is a 3 bedroom with a large family living there, the other 3 residential are efficiencies. 

The building was built in 1898, and has only one water meter. It is city water/sewer which costs roughly $2,250 per year. We are looking to split the bill between the tenants, but do not have an accurate way to know how much water each unit uses without sinking lots of money into re-plumbing it all. 

As the property owners, do we have the right to decide who pays how much percentage on the water bill? With the hair salon and the large family, it seems only fair that they would pay a larger percentage of the water bill based on assumed consumption versus the single person living in the efficiency... ie, have the salon pay 25% of the bill, 3BR 20%, other commercial 25%, and each efficiency 10%? Or, is the only option to split it equally among all 6 units?

Has anyone else ran into this problem? Any suggestions?

Thank you again!

Gene and Leah