Most single family homes will be charged a rate of $4.30 each month.  Very small homes and very large homes may pay slightly lower or higher rates respectively.  Other properties will be charged by the number of Equivalent Runoff Units (ERU) times the monthly charge of $4.30.  An ERU is equal to 3,120 sq. ft. of impervious surface and represents the median impervious surface area found on a single family property.  The number of ERUs for non-residential properties is determined by dividing the amount of the property's impervious surface by 3,120.