The average bill for 2014-15 is £440 but by 2019-20 will fall to £416.
Ofwat wants all water companies to cut household bills in real terms by 5%.
Severn Trent, which covers parts of mid Wales, will see average bills fall from £333 to £316 over five years, while Dee Valley Water, covering parts of north-east Wales will fall 2% to £149.
The new charges will come into effect in April 2015.
However, consumers will still pay more, as the companies are allowed to add on inflation, as measured by RPI.
The cost of bills across the UK will fall by an average £20 from £396 to £376 over five years.
It means the not-for-profit Welsh Water still remains one of the most expensive companies in Britain for combined water and sewerage but it says it is because of the extensive coastline it deals with and having to invest more in its assets per customer than other regions.
Chris Jones, chief executive of Welsh Water, said: "During this challenging price review, we needed to balance a number of priorities between making service improvements, safeguarding the environment whilst also keeping bills affordable by ensuring that we deliver a decade of below inflation price increases by 2020.
"Our customers have played a key part in shaping a very ambitious business plan that sets challenging performance targets for us between 2015 and 2020."
Welsh Water is cutting household bills next year and the cost will continue to drop for five years, after a watchdog ordered price cuts.