For one thing, the valves are in the middle of the streets. Electricity generated there would have to make its way to the grid, unless there was some use for it in the street.

Mr. Zammataro says that with power near at hand, the city would be able to place electronic monitors in the system that would track leaks and test water quality.

THE city’s water stewards have said that space is already very tight below the streets, with its honeycomb of subway tunnels, water pipes, sewage lines, electrical and fiber optic and other utility cables, transformers and assorted gizmos.

“It does not seem prudent to encroach upon and deplete this valuable underground real estate for unproven benefit,” Anthony J. Fiore, a deputy commissioner, testified last year.

Mr. Zammataro, 54, who grew up in the Bensonhurst neighborhood of Brooklyn, began thinking about water power after the Sept. 11 attacks, when the small wireless company he was working for downtown had to move temporarily to Times Square.

Looking out the windows from their office on the 40th floor, he and a partner noticed water tanks below them, on the roof of the building next door. They began to think out loud: maybe that water could be used to power an emergency lift during a catastrophic event. An engineering friend said that it wasn’t likely to be enough, but that maybe water mains could be a source of hydropower. They created Rentricity.

The company’s project in Keene captures water flowing into a treatment plant, generates up to 62 kilowatts of electricity and is enough to keep the plant running, he said. A generator due to come on line this year in California will provide 350 kilowatts. About 20 percent of all the electricity consumed in California is used either to clean water for drinking, or to clean the sewage.