An equivalent residential unit is a normalising device. One ERU is defined as the demand, or the impervious area, of a typical single-family detached home in the service area, and every other property is expressed as some number of ERUs.
Where it is used
- Stormwater charges, where there is no meter and the standard basis is measured impervious area. One ERU is set at the median or mean impervious area of a single-family parcel, and a shopping centre with forty times that area is billed forty ERUs.
- Impact fees and capacity charges, where a new connection’s demand is expressed in ERUs against the capacity the system has to build for it.
- Wastewater charges for unmetered or irregular customers, where flow has to be estimated rather than measured.
Why the definition has to be published
The whole charge rests on two numbers: what one ERU is, and how many ERUs each parcel has. Both are contestable, and both are checkable by anyone who cares to. A utility that publishes the ERU definition, the measurement source for impervious area, and an appeal route for a parcel that was measured wrong has answered the dispute before it starts. One that publishes only the rate per ERU has not.
The choice between the median and the mean single-family parcel is not a technicality either. Where lot sizes are skewed, the two produce materially different ERU counts for every non-residential parcel in the system.
Sources
Water Environment Federation, User-Fee-Funded Stormwater Programs (2013), on ERU definition and impervious area measurement. American Water Works Association, Manual M1, 7th ed. (2017), on meter equivalents and capacity units.