Government officials declined to comment on the report.
Congressional investigators and intelligence officials have warned before that electric utilities are vulnerable to cyber attacks. CIA analyst Tom Donahue told utility engineers at a conference last year that in other countries, hackers had broken into electric utilities and demanded payments before disrupting power - in one case turning off the lights in multiple cities.
Stewart Baker, the former assistant secretary for policy at the Homeland Security Department, said Wednesday that electric grids have been hacked for years, and that he would not be surprised if China, Russia and other countries had taken part.
"We were certainly aware that there were intrusions into the electrical grid," said Baker, now a senior fellow at the Center for Strategic and International Studies.
Security of these systems is not regulated, so the industry is under no mandate on how it should secure its computer networks.
"What I think we're seeing, as time goes on, is much more careful, much more intentional planned intrusions that have gone beyond hacking into (seeing) what can be found," Baker said. "The intruders are carrying out comprehensive surveillance with a view to actually taking action."
The story in the Journal said Russian and Chinese officials denied any involvement in spying on U.S. infrastructure.
Associated Press Writer Eileen Sullivan contributed to this report from Washington.