I am currently purchasing a condo that has a studio unit and a 2bd 2bth unit, not metered separately. I’d like to bill the tenants back by number of people in each unit for water, and sqft for electric. The reasoning behind this: water usage is pretty much going to be an average amount per person, regardless of unit size, whereas with electricity, a larger unit typically uses more electricity: more outlets, gadgets, lighting, heat/air. This would be the breakdown:

Water

2bd 2bth unit (2 tenants) = ⅔ of the bill 

Studio (1 tenant) = ⅓ of the bill

Electric

2bd 2bth unit (1,000 Sqft) = 1000/1350 or 74% of the bill 

Studio (350 Sqft) = 26% of the bill

Here’s what's tripping me up. The current property management has warned me not to bill tenants back in this manner. They said that  they “cannot legally” do this because of landlord-tenant laws and the liability of tenants complaining, then suing due to paying an unfair amount of utilities because another tenant over consumed. However, I have heard other management companies using this method. Property is located in Eastern North Carolina in New Bern. Is this an atypical, legally loose way to bill back tenants? Or is the property management being overly cautious in their advice?