The Equivalent Runoff Unit, or ERU, is the base unit for the stormwater user fee, similar to the base unit for other utilities. An ERU is a measure of the amount of impervious surface on a property. One ERU is equal to 3,770 square feet of impervious area. This is the average amount of impervious surface found at the average single-family residence in Georgetown County. The charge for one ERU will be $52 per year.