What is an ERU?

An ERU is term used for water system planning to represent the water use (and pattern) of
an average residential home. So the average home in a community uses 1 ERU. The definition of an ERU can vary from community to community when supported by measured data. But each single family residence in the community counts as one ERU whether it is retired individual or a house with half a dozen teenagers. Special cases such as hotels or restaurants may be assigned a number of ERUs as a way of counting a hotel as the equivalent of say 10 average residences. For the small number of exceptions we have, at this time we are counting each B&Bs and restaurants as one ERU apiece.

When a member applies for a building permit for a house they are required to provide "A Water Availability Number." The San Juan County Health Dept. issues the certificate number. This counts as one connection. For a single family home we must count this as one ERU towards our  current state permitted system capacity of 165. System capacity analysis considers the water source, treatment, storage, transmission, and distribution  for each ERU:

Average Day Demand (ADD)
Maximum Day Demand (MDD
Peak Hour Demand (PHD).

With sufficient metering data (typically 2 years) it may be possible to have a guest home count as a fraction of an ERU. Other communities on Orcas Island have been successful at this. Please keep in mind that over time seasonally- occupied homes tend to convert to full time occupancy. If we start counting guest homes as a fraction of an ERU, then we must plan carefully such that we are able to develop the capacity to serve the increase if they convert to full time occupancy.

For detailed [residential and nonresidential] definitions of MDD; ADD refer to:
WATER SYSTEM PHYSICAL CAPACITY ANALYSIS (ERUs)
See Section 6.4 for greater detail regarding the ERU

Here is a state DOH sample of a very diverse system accounting of ERUs:

Since the general population served by OWU is single family residences we count each connection as one ERU. The metering data now being gathered will show if there are too many exceptions, or if the simple assumption is close enough.



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