Thames Water is generating almost a quarter of its power, much of it though sewage.
Posting half year results, the water company said its own generation portfolio, including wind and solar, was now producing 23% of its needs, reducing annual energy bills by £12m.
The company produced 146GWh of electricity during the first six months, up 17% year on year. The rest is supplied by Drax-owned Haven Power.
The company said it would miss its leakage targets.
Total post-tax profits for the period stood at £138m from total revenue of £1.04bn.