The major energy suppliers are preparing to install 53 million smart meters by the end of 2019.
Consumer watchdog Which? is warning that the installation of smart meters could be hijacked by "stealth salespeople disguised as installers".
It is pushing for all suppliers to sign up to its "No selling, just installing" smart meter promise that it launched in July.
Seven smaller energy companies, including Co-operative Energy and Spark, have signed up to the initiative alongside Npower.
By signing up to the programme, the suppliers are agreeing not to incentivise its installers to sell while fitting the smart meter, or ask customers to enter new contracts or change tariffs.
The initiative comes as suppliers continue to hike prices during a tough economic climate.
This month Scottish Power and British Gas raised their gas tariffs by 19% and 18% respectively.
Scottish & Southern Energy and E.ON will also raise their prices this month and Npower has revealed it will raise bills on 1 October.