When a cell phone or credit-card bill arrives, each call or purchase is itemized, making it possible to track trends in calling or spending, which is especially helpful if you use a phone plan with limited minutes or are trying to stick to a budget. Within the next few years, household utilities could be itemized as well, allowing residents to track their usage and see which devices utilize the most electricity, water, or gas. New sensor technology that consists of a single device for each utility, which builds a picture of household activity by tracing electrical wiring, plumbing, and gas lines back to specific devices or fixtures, could make this far simpler to implement.

Shwetak Patel, a professor of computer science and electrical engineering at the University of Washington, in Seattle, developed the sensors, which plug directly into existing infrastructure in buildings, thereby eliminating the need for an elaborate set of networked sensors throughout a structure. For example, an electrical sensor plugs into a single outlet and monitors characteristic "noise" in electrical lines that are linked to specific devices, such as cell-phone chargers, refrigerators, DVD players, and light switches. And a gas sensor attaches to a gas line and monitors pressure changes that can be correlated to turning on a stove or furnace, for instance.

Now, Patel and his colleagues have developed a pressure sensor that fits around a water pipe. The technology, called Hydrosense, can detect leaks and trace them back to their source, and can recognize characteristic pressure changes that indicate that a specific fixture or appliance is in use.

Patel hopes to incorporate electrical, gas, and water sensors into a unified technology and has cofounded a soon-to-be-named startup that he hopes will start offering combined smart meters to utility companies within the next year or so. The goal, says Patel, is to make a "smart home" universally deployable. "I looked at the existing infrastructures," he says, "and saw that they could be retrofitted."

Smart sensors have become increasingly popular over the past few years as more people have become interested in cutting their utility bills and minimizing the resources that they consume. A number of startups offer to connect utility providers and consumers so that resource use can be tracked over the Internet. So far, however, no company or utility has been able to provide the sort of fine-grain resource usage that Patel hopes to offer with his startup.

The idea behind the water sensor has its origins in Patel's original work with electrical lines. Rather than simply looking at the amount of power consumed by all the devices in a house, he decided to look at noise patterns--irregularities in the electrical signal--that propagated over household power lines as a result of electrical consumption. "Let's say you turn on a light switch in the bathroom and kitchen," he says. "We can tell the difference between the two" due to electrical impulses that resonate at a high frequency. "So if you have two different impulses you see originate from two different locations inside the home, you can trace them back to a particular device," Patel says, noting that location can be determined by the amount of time that it takes for a signal to reach the sensor, which is usually just plugged into a spare wall outlet.