The ESRI is recommending that Irish Water and the CER "urgently revisit" assumptions on water consumption.
Last week the Commission for Energy Regulation announced the amount was to be 21 thousand litres per year for each child.
The original figure announced by the Department of the Environment last May was 38 thousand litres.
The ERSI said that the sample of metered houses used to research how much water a child would use may be unrepresentative.
Fianna Fail Senator Averil Power says consumers are deeply unhappy at the way the introduction of water charges has been handled:
