People who have smart meters installed are expected to save an average of £11 annually on their energy bills, much less than originally hoped.
A report from a parliamentary group now predicts a dual fuel saving of £26.
In a critical report, the 92 MPs and peers also said the government was likely to miss its own deadline to have the £11bn switchover completed.
Up to 53 million smart meters were to be installed in 30 million homes and businesses by the end of 2020.
A smart meter is designed to replace traditional gas and electricity meters.
It automatically sends usage data to suppliers via the mobile phone network, and comes with a display showing users how much energy they are using – and the cost in pounds and pence.
Grant Shapps, the chair of the British Infrastructure Group of Parliamentarians (BIG), said the programme had been “plagued by repeated delays and cost increases, with suppliers now almost certain to miss the 2020 deadline, and programme benefits likely to be slashed even further.”
Speaking on BBC Radio 4’s Today programme, Mr Shapps said one of the reasons for the “mess” was that “first generation” smart meters, which do not always work when a customer switches supplier, will continue to be rolled out until next year.