An ERU is a unit of measure which serves as an index to compare the runoff generated by different size properties with different amounts of impervious surface. Impervious surface is any surface which does not allow water to pass through to the soil. Rooftops, driveways, parking lots and streets are familiar examples of impervious surface. In Orlando, the average residential unit (a weighted average of both single-family and multi-family parcels) consists of 2,000 square feet of impervious surface. One ERU is equal to 2,000 square feet, two ERUs are equal to 4,000 square feet, etc. A non-residential property which has 12,000 square feet of impervious surface (six ERUs) is passing the same amount of stormwater to the City's stormwater system as six typical residential parcels would generate.