utility billing for tenants

[Remone R.]

So i have two duplexes that are mastered metered(2 meters) so i'm unable to individually track each tenants usage. BUT i have spoken to a company that does sub metering so I'm in the process of getting that installed. MY only question is the way the utility bill is structured is different. One month I'm charged only base rates for water,sewer,garbage and the next I'm charged for water, sewer,garbage PLUS consumption. So should i only charge the tenants for the months of base rate plus consumption? Or also the months of just base rates? I'm just trying to make the best decision costs for me and also the tenant but decrease my expenses to increase the NOI.

To give a brief Outlook: Base rate bill comes to $310 per month. Base rate plus consumption bill averages $530 per month although this past month was $845!! 

any suggestions appreciated