Thames Water’s Desalination Plant at Beckton is to be part of Britain’s biggest on-site solar power producer and industrial user.

A ground-breaking deal has been signed to install photovoltaic systems large enough to cover 15 football pitches at three key sites around the capital.

Under a �7m, 25-year contract, Ennoviga Solar Ltd, a specialist photovoltaic developer, has created an investment company.

The company will own and maintain the solar arrays, repaying the investment by selling all the clean electricity produced to the water company at a market-competitive price.

The scheme will shave �100,000 a year off Thames Water’s electricity bill.

It will also be providing some of the energy required for pumping 2.6bn litres of water a day and 2.8bn litres of sewage a day to its giant treatment works also at Beckton.

As well as the desalination plant solar arrays are being fitted at Crossness sewage works in Bexley, and on redundant sand filters at Walton water treatment works in Sunbury.

When fully completed, the three sites will provide an annual output of more than 4,500 megawatt hours (MWh) of electricity – enough to run around 970 average-size homes.

Up to 100 further photovoltaic systems at smaller sites are planned.

Dr Piers Clark, commercial director at Thames Water, said: “With the price of energy forecast to increase above inflation, the way we’ve structured this agreement will give us cheaper, renewable source of power from a secure source over the long term,”

“We think this is the right thing to for our 14m customers and to help move Britain that little bit closer to becoming a low-carbon economy.”

Stefano Gambro, director at Ennoviga, said: “By working with us, Thames Water has led the industry in exploiting otherwise unusable space to generate clean electricity.

“When energy prices rise and carbon charging starts, the impact on Thames Water customers’ bills will now be that little bit less.”

Thames Water has set a voluntary target of reducing its greenhouse emissions by 20 per cent in real-terms on 1990 levels by 2015.