The stormwater charge is based on the amount of impervious area on your property as calculated by Equivalent Residential Units (ERUs). One ERU equals 2600 square feet of impervious area. Single family residential properties are charged one ERU and duplex properties are charged two ERU's. All other properties have the impervious area measured based on aerial photographs. The total area is divided by 2600 square feet and rounded to the nearest ERU. So a property with 26,000 square feet of impervious area of impervious area (rooftops, sidewalks, driveways, parking, gravel, etc.) would pay 26,000 square feet/2600 square feet/ERU=10 ERU.