The building was reopened about an hour later after electricity was restored. The House and Senate were not in session at the time of the outage.
Both convened at 2 p.m., the House quickly recessing for the day and the Senate resuming work on immigration legislation.
Mary-Beth Hutchinson, a spokeswoman for Potomac Electric Power Co., said the electricity shut off automatically after there was "a momentary drop in voltage due to customer operations up the lines" away from the Capitol.
Nearby office buildings were not affected by the outage. Before the evacuation sirens went off, more than 100 visitors sat in the darkened House gallery. They exited with everyone else when the alarm sounded.
