Greetings:
What I have been doing with my 1br ADU has been charging a ratio by square footage & circumstances. So for example, if my 3 br house was 1000 sq ft with one tenant and the other house is 500 sq ft with one tenant, I have been charging smaller 1/3. The house larger 3 br sometimes have guests, esp during the holidays & usually chilly here. Lgr home has all electric, the other electric and gas. So I only charge ADU 1/3rd. It seems to work, no squabbles past 2 years.
However, currently here in CA, PG&E is getting extremely expensive, that is why do not include electric/gas in their rent. Only garbage, water/sewer & Wi-Fi. Unfort. water is getting very expensive as well but I have not altered billing at present. Rents have not gone up much to justify a larger rent pmts. Also have great bill paying tenants...Plus, psychologically, I think if a tenant thinks he's paying x, including utilities it doesn't SEEM as much... :)
Also, when I rented rooms on another one of my properties, there were always utility squabbles. So I just let the tenants figure it out among themselves (I paid trash/water) and it worked great 95% of the time. That way I don't get stuck with the high PGE rates.