Thames Water say its not possible to generalise about whether a family would be better or worse off on a meter.
The company says that customers won't be moved on to a metered account until two years after the new system has been activated. During that time they will still be able to monitor their usage on a daily basis and get price comparison bills before switch-over.
Around 30% of Thames Water customers are on meters and the company wants that to be 100% by 2030, starting in Bexley.
Thames Water says it hopes the scheme will encourage its customers to be more water-efficient. It says the new technology allows customers to monitor what they use and research shows metered customers tend to use water more sparingly.
It says London has less rainfall than Istanbul, only half as much as Sydney, yet one of the highest individual levels of water consumption in the UK.