The service charge is based on the amount of impervious area on a property. Impervious area is the best indicator of how much you use the storm water system. Single-family homes provide the basis for the rate. The average amount of impervious area for a single-family residence is 3,163 square feet or one (1) equivalent resident unit (ERU). ERU’s for businesses are calculated by dividing the impervious area by 3,163 square feet.