What is an ERU in stormwater fee billing?
If you've worked a stormwater fee roll for more than one billing cycle, you already know the acronym even if nobody on council ever learns to say it right. ERU stands for equivalent residential unit, and it's the baseline measurement most stormwater utilities use to turn impervious surface into a dollar amount on a bill.
A typical single-family home has a predictable amount of hard surface on the lot: roof, driveway, walkway, maybe a patio. Your utility picks a number, say 2,500 square feet, as "one ERU," and that becomes the unit everybody else gets measured against. A commercial parcel with 25,000 square feet of rooftop and parking isn't billed as one flat rate. It gets billed as 10 ERUs. A small bungalow with a short driveway might come in under one ERU and get billed as a fraction of the unit, depending on how your ordinance handles it.
It's a tiered approach to volumetric billing without the meter. Water and sewer bills run off consumption data from a meter. Stormwater runoff doesn't pass through a pipe you can meter at each address, so the utility needs a proxy for how much rain a parcel sheds into the system. Impervious area is that proxy, and the ERU is how you turn square footage into a billing unit homeowners and councils can understand.
How utilities set the ERU in the first place
Setting the ERU value is a rate study exercise, usually done by an engineering consultant working off a sample of parcels. They pull impervious area for a representative set of single-family homes, average it, and round to a number that's defensible in a council meeting. That's your one ERU. Everything else in the fee roll gets priced as a multiple or fraction of it, with non-residential parcels billed based on their measured impervious area divided by the ERU figure.
This works fine the year it's set. The problem is that an ERU calculation stormwater utility teams rely on is a snapshot. It's accurate for the parcels and the impervious footprint that existed when someone flew the imagery or pulled the GIS layer for the study. Houses don't stay the same after that. Driveways get poured wider, additions go on, sheds and patios show up, old lots get subdivided. None of that updates itself in the billing database.
Where the ERU calculation starts drifting
Two things erode an ERU-based fee roll over time. First, the baseline residential unit itself can shift as housing stock changes, though most utilities only revisit that in a full rate study every several years. Second, and more immediately, individual parcels drift away from whatever impervious figure was on file when they were last measured. A property owner pours a new driveway, and the fee roll doesn't know it happened until someone appeals a bill, flags it during a resale, or your staff happens to drive by.
That second problem is the one that actually costs money and staff time. Every appeal where a homeowner says "that's not how much impervious area I have" turns into someone on your team pulling up old aerials or scheduling a site visit to re-measure a single lot. Multiply that across a county-sized service area and it's a standing line item nobody budgeted for.
Keeping parcel-level impervious area current
Refreshing the impervious area measurement underneath the fee roll on a regular cycle keeps individual parcel appeals tied to actual site conditions instead of a figure from the last rate study. The ERU formula itself rarely needs touching between studies; it's the per-parcel square footage feeding it that goes stale. Stormwater Fee does that refresh: a parcel-level impervious area layer, built from high-resolution satellite and aerial imagery, updated on a two-year cycle, sized to hand straight to your billing system instead of a one-off flyover you pay for every time someone contests a number.
If your fee roll is still running on impervious area from the last rate study, it's worth seeing what a current layer would change on the appeals list.