Household water bills in England and Wales will Fall by between £15 and £25 a year from 2020 to 2025, the regulator Ofwat has pledged.
Ofwat was criticised by an influential government committee in 2016 for overestimating water firms' costs.
Water UK, said it was a "tough challenge from Ofwat".
Consumers can look forward to a real terms fall in water bills, Ofwat said.
Since privatisation in 1989, water bills have risen above inflation by about 40%, leading to a debate about whether privatisation works for that industry.
The regulator's chief executive, Cathryn Ross, told the BBC: "We have an early view on the financing costs that we're going to enable companies to recover from their customers.
"That's the biggest single driver of the bill. Financing costs are about a third of the average bill."