The electricity supply deal is for five years with an option to extend it for further 10 years.
Haven Power sources its energy from Drax’s power facility in Selby, Yorkshire, which is being converted from coal to biomass.
Two of the six units at Drax’s 4GW plant have been converted to biomass and the company intends to convert another later this year.
Thames Water already sources around 20% of its electricity via self-generated renewable technologies like solar, wind, hydro and biogas.
The deal will allow the water firm to meet all of its energy needs from renewables.
Thames Water energy manager Angus Berry said: "Our energy and carbon strategy centres around reducing costs for customers and minimising our impact on the environment. This deal with Haven puts downward pressure on bills and means we will now be using 100% renewable electricity.
"We look forward to growing our relationship with Haven to exploit further opportunities to minimise energy costs and emissions, as well as continuing to work towards our ambitious target of self-generating 30% of our own electricity by 2020."
Thames Water supplies about 2,600 million liters of tap water to 9 million customers throughout London and the Thames Valley. It also removes and treats over 4 billion liters of sewage for 15 million customers.
