The waste is processed and made into blocks which are burned
A water firm says it saved £15m last year - by using human waste to make electricity.
In 2008/09, Thames Water generated 14% of its power from either burning sludge or methane derived from the 2.8 billion litres of sewage it treats every day.
The company hopes the move will ultimately help it cut greenhouse gases by 20%.
More than 13 million people in London, Berkshire, Oxfordshire, Hertfordshire and Surrey are served by Thames Water.
Dr Keith Colquhoun, Thames Water's climate change strategy manager, said: "There's no polite way of saying this but what we produce - our poo - isn't simply waste, it's a great source of energy.
"The solids in sewage have a high calorific content that we use to generate electricity.
"This isn't a gimmick. As well as helping us to be more sustainable as a company, it also saves money - £15m less of customers' cash spent on National Grid energy last year alone, which ultimately has a downward pressure of bills.
"Our goal is to cut greenhouse emissions by 20% on 1990 levels by 2020 - that's about 200,000 tonnes less CO2.
"By using poo power and other renewable energy sources, we're making significant progress towards this target."