The smart grid will use automated meters, two-way communications and advanced sensors to improve electricity efficiency and reliability. The nation's utilities have embraced the concept and are installing millions of automated meters on homes across the country, the first phase in Smart Grid's deployment. President Obama has championed Smart Grid, and the recent stimulus bill allocated $4.5 billion for the high-tech program.

But cybersecurity experts said some types of meters can be hacked, as can other points in the Smart Grid's communications systems. IOActive, a professional security services firm, determined that an attacker with $500 of equipment and materials and a background in electronics and software engineering could "take command and control of the [advanced meter infrastructure] allowing for the en masse manipulation of service to homes and businesses."

Experts said that once in the system, a hacker could gain control of thousands, even millions, of meters and shut them off simultaneously. A hacker also might be able to dramatically increase or decrease the demand for power, disrupting the load balance on the local power grid and causing a blackout. These experts said such a localized power outage would cascade to other parts of the grid, expanding the blackout. No one knows how big it could get.

More than 50 million smart meters are expected to be deployed by electric utilities by 2015, according to a list of publicly announced projects kept by The Edison Foundation. More than 8 million have already been deployed.

Spies have broken into parts of the electric grid and left behind programs that would allow them to disrupt service, government officials revealed this spring. The intrusions were discovered only after some electric companies opened their doors to audits. The full scope of the attacks is unknown, though, because the government doesn't have blanket authority to examine other electric systems