Meters on each home tell Irish Water exactly how much water is used by that household. Irish Water has no way of knowing how many people live in individual households at any given time so individual consumption estimated.
In order to calculate estimated individual consumption Irish Water works off household occupancy figures. These are based on original registration data and a sample survey Irish Water took of metered households in 2014 which provided a snapshot in time. The registration data is incomplete as it was reliant on self-declaration from householders and also becomes increasingly out of date. Based on that original data and survey, Irish Water report quarterly to the CRU on estimated domestic water consumption. The purpose of the quarterly domestic consumption reports is mainly to monitor quarterly trends in domestic consumption against those estimates used in the tariff setting process in 2014.
It is important to note that the quarterly domestic consumption reports remove very high users or outliers from the dataset as these may indicate a leak. Households using more than 800 litres per day are treated as outliers and removed from the dataset. This is in order to replicate the methodology used in the 2014 tariff setting process. This average usage metric is called ‘Regulated PCC (Per Capita Consumption)’. It is used to reflect that it omits high users and to distinguish it from actual per capita consumption (PCC) i.e. actual PCC includes outliers or high usage, making it higher.
These reports provided to the CRU show the estimated consumption per occupant for each occupancy group. It should be noted that doubling or trebling the number of occupants does not necessarily double or treble the amount of water used. For example, you do not put on the washing machine or dishwasher or fill the sink that multiple of times.
We have to make the assumption that the occupancy data is accurate in order to compile these estimates. The meter reading on individual households will always be accurate so a household consumption figure is always up to date, exact and accurate. Meter readings are the most reliable source of information we have for water consumption.